Index: doc/bibliography/pl.bib =================================================================== --- doc/bibliography/pl.bib (revision a5873bd9568f82230e97ba50cf4e33da0f932488) +++ doc/bibliography/pl.bib (revision 97392b6998d47706885e39064f5f23e53dbe7496) @@ -1001,9 +1001,9 @@ } -@misc{CforallBenchMarks, +@misc{CforallConcurrentBenchmarks, contributer = {pabuhr@plg}, key = {Cforall Benchmarks}, author = {{\textsf{C}{$\mathbf{\forall}$} Benchmarks}}, - howpublished= {\href{https://plg.uwaterloo.ca/~cforall/benchmark.tar}{https://\-plg.uwaterloo.ca/\-$\sim$cforall/\-benchmark.tar}}, + howpublished= {\href{https://plg.uwaterloo.ca/~cforall/doc/CforallConcurrentBenchmarks.tar}{https://\-plg.uwaterloo.ca/\-$\sim$cforall/\-doc/\-CforallConcurrentBenchmarks.tar}}, } @@ -1982,5 +1982,5 @@ author = {Adya, Atul and Howell, Jon and Theimer, Marvin and Bolosky, William J. and Douceur, John R.}, title = {Cooperative Task Management Without Manual Stack Management}, - booktitle = {Proceedings of the General Track of the Annual Conference on USENIX Annual Technical Conference}, + booktitle = {Proc. of the General Track USENIX Tech. Conf.}, series = {ATEC '02}, year = {2002}, Index: doc/papers/AMA/AMA-stix/ama/WileyNJD-v2.cls =================================================================== --- doc/papers/AMA/AMA-stix/ama/WileyNJD-v2.cls (revision a5873bd9568f82230e97ba50cf4e33da0f932488) +++ doc/papers/AMA/AMA-stix/ama/WileyNJD-v2.cls (revision 97392b6998d47706885e39064f5f23e53dbe7496) @@ -2444,7 +2444,7 @@ \@afterheading} -\renewcommand\section{\@startsection{section}{1}{\z@}{-25pt \@plus -2pt \@minus -2pt}{12\p@}{\sectionfont}}% -\renewcommand\subsection{\@startsection{subsection}{2}{\z@}{-22pt \@plus -2pt \@minus -2pt}{5\p@}{\subsectionfont}}% -\renewcommand\subsubsection{\@startsection{subsubsection}{3}{\z@}{-20pt \@plus -2pt \@minus -2pt}{2\p@}{\subsubsectionfont}}% +\renewcommand\section{\@startsection{section}{1}{\z@}{-20pt \@plus -2pt \@minus -2pt}{7\p@}{\sectionfont}}% +\renewcommand\subsection{\@startsection{subsection}{2}{\z@}{-18pt \@plus -2pt \@minus -2pt}{5\p@}{\subsectionfont}}% +\renewcommand\subsubsection{\@startsection{subsubsection}{3}{\z@}{-16pt \@plus -2pt \@minus -2pt}{2\p@}{\subsubsectionfont}}% % \newskip\secruleskip\secruleskip8.5\p@% Index: doc/papers/concurrency/Paper.tex =================================================================== --- doc/papers/concurrency/Paper.tex (revision a5873bd9568f82230e97ba50cf4e33da0f932488) +++ doc/papers/concurrency/Paper.tex (revision 97392b6998d47706885e39064f5f23e53dbe7496) @@ -99,6 +99,6 @@ \newcommand{\CRT}{\global\columnposn=\gcolumnposn} -% Denote newterms in particular font and index them without particular font and in lowercase, e.g., \newterm{abc}. -% The option parameter provides an index term different from the new term, e.g., \newterm[\texttt{abc}]{abc} +% Denote newterms in particular font and index them without particular font and in lowercase, \eg \newterm{abc}. +% The option parameter provides an index term different from the new term, \eg \newterm[\texttt{abc}]{abc} % The star version does not lowercase the index information, e.g., \newterm*{IBM}. \newcommand{\newtermFontInline}{\emph} @@ -235,4 +235,7 @@ {\lstset{language=python,moredelim=**[is][\protect\color{red}]{`}{`},#1}\lstset{#1}} {} +\lstnewenvironment{java}[1][] +{\lstset{language=java,moredelim=**[is][\protect\color{red}]{`}{`},#1}\lstset{#1}} +{} % inline code @...@ @@ -266,5 +269,5 @@ \abstract[Summary]{ -\CFA is a polymorphic, non-object-oriented, concurrent, backwards-compatible extension of the C programming language. +\CFA is a polymorphic, non-object-oriented, concurrent, backwards compatible extension of the C programming language. This paper discusses the design philosophy and implementation of its advanced control-flow and concurrent/parallel features, along with the supporting runtime written in \CFA. These features are created from scratch as ISO C has only low-level and/or unimplemented concurrency, so C programmers continue to rely on library approaches like pthreads. @@ -272,5 +275,5 @@ % Library extension for executors, futures, and actors are built on these basic mechanisms. The runtime provides significant programmer simplification and safety by eliminating spurious wakeup and monitor barging. -The runtime also ensures multiple monitors can be safely acquired \emph{simultaneously} (deadlock free), and this feature is fully integrated with all monitor synchronization mechanisms. +The runtime also ensures multiple monitors can be safely acquired in a deadlock-free way, and this feature is fully integrated with all monitor synchronization mechanisms. All control-flow features integrate with the \CFA polymorphic type-system and exception handling, while respecting the expectations and style of C programmers. Experimental results show comparable performance of the new features with similar mechanisms in other concurrent programming languages. @@ -293,27 +296,27 @@ % "Trait inheritance" works for me. "Interface inheritance" might also be a good choice, and distinguish clearly from implementation inheritance. % You'll want to be a little bit careful with terms like "structural" and "nominal" inheritance as well. CFA has structural inheritance (I think Go as well) -- it's inferred based on the structure of the code. Java, Rust, and Haskell (not sure about Swift) have nominal inheritance, where there needs to be a specific statement that "this type inherits from this type". -However, functions \emph{cannot} be nested in structures, so there is no lexical binding between a structure and set of functions (member/method) implemented by an implicit \lstinline@this@ (receiver) parameter.}, +However, functions \emph{cannot} be nested in structures, so there is no lexical binding between a structure and set of functions implemented by an implicit \lstinline@this@ (receiver) parameter.}, backwards-compatible extension of the C programming language. -In many ways, \CFA is to C as Scala~\cite{Scala} is to Java, providing a \emph{research vehicle} for new typing and control-flow capabilities on top of a highly popular programming language\footnote{ -The TIOBE index~\cite{TIOBE} for December 2019 ranks the top five \emph{popular} programming languages as Java 17\%, C 16\%, Python 10\%, and \CC 6\%, \Csharp 5\% = 54\%, and over the past 30 years, C has always ranked either first or second in popularity.} +In many ways, \CFA is to C as Scala~\cite{Scala} is to Java, providing a vehicle for new typing and control-flow capabilities on top of a highly popular programming language\footnote{ +The TIOBE index~\cite{TIOBE} for May 2020 ranks the top five \emph{popular} programming languages as C 17\%, Java 16\%, Python 9\%, \CC 6\%, and \Csharp 4\% = 52\%, and over the past 30 years, C has always ranked either first or second in popularity.} allowing immediate dissemination. -This paper discusses the design philosophy and implementation of advanced language-level control-flow and concurrent/parallel features in \CFA and its runtime, which is written entirely in \CFA. -The \CFA control-flow framework extends ISO \Celeven~\cite{C11} with new call/return and concurrent/parallel control-flow. +This paper discusses the design philosophy and implementation of \CFA's advanced control-flow and concurrent/parallel features, along with the supporting runtime written in \CFA. % The call/return extensions retain state between callee and caller versus losing the callee's state on return; % the concurrency extensions allow high-level management of threads. -Call/return control-flow with argument/parameter passing appeared in the first programming languages. -Over the past 50 years, call/return has been augmented with features like static/dynamic call, exceptions (multi-level return) and generators/coroutines (retain state between calls). -While \CFA has mechanisms for dynamic call (algebraic effects) and exceptions\footnote{ +The \CFA control-flow framework extends ISO \Celeven~\cite{C11} with new call/return and concurrent/parallel control-flow. +Call/return control-flow with argument and parameter passing appeared in the first programming languages. +Over the past 50 years, call/return has been augmented with features like static and dynamic call, exceptions (multi-level return) and generators/coroutines (see Section~\ref{s:StatefulFunction}). +While \CFA has mechanisms for dynamic call (algebraic effects~\cite{Zhang19}) and exceptions\footnote{ \CFA exception handling will be presented in a separate paper. -The key feature that dovetails with this paper is nonlocal exceptions allowing exceptions to be raised across stacks, with synchronous exceptions raised among coroutines and asynchronous exceptions raised among threads, similar to that in \uC~\cite[\S~5]{uC++}}, this work only discusses retaining state between calls via generators/coroutines. +The key feature that dovetails with this paper is nonlocal exceptions allowing exceptions to be raised across stacks, with synchronous exceptions raised among coroutines and asynchronous exceptions raised among threads, similar to that in \uC~\cite[\S~5]{uC++}}, this work only discusses retaining state between calls via generators and coroutines. \newterm{Coroutining} was introduced by Conway~\cite{Conway63} (1963), discussed by Knuth~\cite[\S~1.4.2]{Knuth73V1}, implemented in Simula67~\cite{Simula67}, formalized by Marlin~\cite{Marlin80}, and is now popular and appears in old and new programming languages: CLU~\cite{CLU}, \Csharp~\cite{Csharp}, Ruby~\cite{Ruby}, Python~\cite{Python}, JavaScript~\cite{JavaScript}, Lua~\cite{Lua}, \CCtwenty~\cite{C++20Coroutine19}. Coroutining is sequential execution requiring direct handoff among coroutines, \ie only the programmer is controlling execution order. -If coroutines transfer to an internal event-engine for scheduling the next coroutines, the program transitions into the realm of concurrency~\cite[\S~3]{Buhr05a}. +If coroutines transfer to an internal event-engine for scheduling the next coroutines (as in async-await), the program transitions into the realm of concurrency~\cite[\S~3]{Buhr05a}. Coroutines are only a stepping stone towards concurrency where the commonality is that coroutines and threads retain state between calls. -\Celeven/\CCeleven define concurrency~\cite[\S~7.26]{C11}, but it is largely wrappers for a subset of the pthreads library~\cite{Pthreads}.\footnote{Pthreads concurrency is based on simple thread fork/join in a function and mutex/condition locks, which is low-level and error-prone} -Interestingly, almost a decade after the \Celeven standard, neither gcc-9, clang-9 nor msvc-19 (most recent versions) support the \Celeven include @threads.h@, indicating no interest in the C11 concurrency approach (possibly because of the recent effort to add concurrency to \CC). +\Celeven/\CCeleven define concurrency~\cite[\S~7.26]{C11}, but it is largely wrappers for a subset of the pthreads library~\cite{Pthreads}.\footnote{Pthreads concurrency is based on simple thread fork and join in a function and mutex or condition locks, which is low-level and error-prone} +Interestingly, almost a decade after the \Celeven standard, the most recent versions of gcc, clang, and msvc do not support the \Celeven include @threads.h@, indicating no interest in the C11 concurrency approach (possibly because of the recent effort to add concurrency to \CC). While the \Celeven standard does not state a threading model, the historical association with pthreads suggests implementations would adopt kernel-level threading (1:1)~\cite{ThreadModel}, as for \CC. In contrast, there has been a renewed interest during the past decade in user-level (M:N, green) threading in old and new programming languages. @@ -321,22 +324,22 @@ Kernel threading was chosen, largely because of its simplicity and fit with the simpler operating systems and hardware architectures at the time, which gave it a performance advantage~\cite{Drepper03}. Libraries like pthreads were developed for C, and the Solaris operating-system switched from user (JDK 1.1~\cite{JDK1.1}) to kernel threads. -As a result, many current languages implementations adopt the 1:1 kernel-threading model, like Java (Scala), Objective-C~\cite{obj-c-book}, \CCeleven~\cite{C11}, C\#~\cite{Csharp} and Rust~\cite{Rust}, with a variety of presentation mechanisms. +As a result, many languages adopt the 1:1 kernel-threading model, like Java (Scala), Objective-C~\cite{obj-c-book}, \CCeleven~\cite{C11}, C\#~\cite{Csharp} and Rust~\cite{Rust}, with a variety of presentation mechanisms. From 2000 onwards, several language implementations have championed the M:N user-threading model, like Go~\cite{Go}, Erlang~\cite{Erlang}, Haskell~\cite{Haskell}, D~\cite{D}, and \uC~\cite{uC++,uC++book}, including putting green threads back into Java~\cite{Quasar}, and many user-threading libraries have appeared~\cite{Qthreads,MPC,Marcel}. -The main argument for user-level threading is that it is lighter weight than kernel threading (locking and context switching do not cross the kernel boundary), so there is less restriction on programming styles that encourages large numbers of threads performing medium-sized work to facilitate load balancing by the runtime~\cite{Verch12}. +The main argument for user-level threading is that it is lighter weight than kernel threading because locking and context switching do not cross the kernel boundary, so there is less restriction on programming styles that encourages large numbers of threads performing medium-sized work to facilitate load balancing by the runtime~\cite{Verch12}. As well, user-threading facilitates a simpler concurrency approach using thread objects that leverage sequential patterns versus events with call-backs~\cite{Adya02,vonBehren03}. -Finally, performant user-threading implementations (both time and space) meet or exceed direct kernel-threading implementations, while achieving the programming advantages of high concurrency levels and safety. +Finally, performant user-threading implementations, both in time and space, meet or exceed direct kernel-threading implementations, while achieving the programming advantages of high concurrency levels and safety. A further effort over the past two decades is the development of language memory models to deal with the conflict between language features and compiler/hardware optimizations, \eg some language features are unsafe in the presence of aggressive sequential optimizations~\cite{Buhr95a,Boehm05}. -The consequence is that a language must provide sufficient tools to program around safety issues, as inline and library code is all sequential to the compiler. -One solution is low-level qualifiers and functions (\eg @volatile@ and atomics) allowing \emph{programmers} to explicitly write safe (race-free~\cite{Boehm12}) programs. -A safer solution is high-level language constructs so the \emph{compiler} knows the concurrency boundaries (where mutual exclusion and synchronization are acquired/released) and provide implicit safety at and across these boundaries. +The consequence is that a language must provide sufficient tools to program around safety issues, as inline and library code is compiled as sequential without any explicit concurrent directive. +One solution is low-level qualifiers and functions, \eg @volatile@ and atomics, allowing \emph{programmers} to explicitly write safe, race-free~\cite{Boehm12} programs. +A safer solution is high-level language constructs so the \emph{compiler} knows the concurrency boundaries, \ie where mutual exclusion and synchronization are acquired and released, and provide implicit safety at and across these boundaries. While the optimization problem is best known with respect to concurrency, it applies to other complex control-flow, like exceptions and coroutines. As well, language solutions allow matching the language paradigm with the approach, \eg matching the functional paradigm with data-flow programming or the imperative paradigm with thread programming. -Finally, it is important for a language to provide safety over performance \emph{as the default}, allowing careful reduction of safety (unsafe code) for performance when necessary. -Two concurrency violations of this philosophy are \emph{spurious wakeup} (random wakeup~\cite[\S~9]{Buhr05a}) and \emph{barging}\footnote{ +Finally, it is important for a language to provide safety over performance \emph{as the default}, allowing careful reduction of safety for performance when necessary. +Two concurrency violations of this philosophy are \emph{spurious} or \emph{random wakeup}~\cite[\S~9]{Buhr05a}, and \emph{barging}\footnote{ Barging is competitive succession instead of direct handoff, \ie after a lock is released both arriving and preexisting waiter threads compete to acquire the lock. Hence, an arriving thread can temporally \emph{barge} ahead of threads already waiting for an event, which can repeat indefinitely leading to starvation of waiter threads. -} (signals-as-hints~\cite[\S~8]{Buhr05a}), where one is a consequence of the other, \ie once there is spurious wakeup, signals-as-hints follow. +} or signals-as-hints~\cite[\S~8]{Buhr05a}, where one is a consequence of the other, \ie once there is spurious wakeup, barging follows. (Author experience teaching concurrency is that students are confused by these semantics.) However, spurious wakeup is \emph{not} a foundational concurrency property~\cite[\S~9]{Buhr05a}; @@ -356,5 +359,5 @@ \item -monitor synchronization without barging, and the ability to safely acquiring multiple monitors \emph{simultaneously} (deadlock free), while seamlessly integrating these capabilities with all monitor synchronization mechanisms, +monitor synchronization without barging, and the ability to safely acquiring multiple monitors in a deadlock-free way, while seamlessly integrating these capabilities with all monitor synchronization mechanisms, \item @@ -368,5 +371,5 @@ \item -a dynamic partitioning mechanism to segregate groups of executing user and kernel threads performing specialized work (\eg web-server or compute engine) or requiring different scheduling (\eg NUMA or real-time). +a dynamic partitioning mechanism to segregate groups of executing user and kernel threads performing specialized work, \eg web-server or compute engine, or requiring different scheduling, \eg NUMA or real-time. % \item @@ -380,8 +383,8 @@ Section~\ref{s:StatefulFunction} begins advanced control by introducing sequential functions that retain data and execution state between calls producing constructs @generator@ and @coroutine@. Section~\ref{s:Concurrency} begins concurrency, or how to create (fork) and destroy (join) a thread producing the @thread@ construct. -Section~\ref{s:MutualExclusionSynchronization} discusses the two mechanisms to restricted nondeterminism when controlling shared access to resources (mutual exclusion) and timing relationships among threads (synchronization). +Section~\ref{s:MutualExclusionSynchronization} discusses the two mechanisms to restricted nondeterminism when controlling shared access to resources, called mutual exclusion, and timing relationships among threads, called synchronization. Section~\ref{s:Monitor} shows how both mutual exclusion and synchronization are safely embedded in the @monitor@ and @thread@ constructs. -Section~\ref{s:CFARuntimeStructure} describes the large-scale mechanism to structure (cluster) threads and virtual processors (kernel threads). -Section~\ref{s:Performance} uses a series of microbenchmarks to compare \CFA threading with pthreads, Java 11.0.6, Go 1.12.6, Rust 1.37.0, Python 3.7.6, Node.js 12.14.1, and \uC 7.0.0. +Section~\ref{s:CFARuntimeStructure} describes the large-scale mechanism to structure threads and virtual processors (kernel threads). +Section~\ref{s:Performance} uses microbenchmarks to compare \CFA threading with pthreads, Java 11.0.6, Go 1.12.6, Rust 1.37.0, Python 3.7.6, Node.js v12.18.0, and \uC 7.0.0. @@ -392,51 +395,45 @@ To this end, the control-flow features created for \CFA are based on the fundamental properties of any language with function-stack control-flow (see also \uC~\cite[pp.~140-142]{uC++}). The fundamental properties are execution state, thread, and mutual-exclusion/synchronization (MES). -These independent properties can be used alone, in pairs, or in triplets to compose different language features, forming a compositional hierarchy where the most advanced feature has all the properties (state/thread/MES). -While it is possible for a language to only support the most advanced feature~\cite{Hermes90}, this unnecessarily complicates and makes inefficient solutions to certain classes of problems. -As is shown, each of the (non-rejected) composed features solves a particular set of problems, and hence, has a defensible position in a programming language. -If a compositional feature is missing, a programmer has too few/many fundamental properties resulting in a complex and/or is inefficient solution. +These independent properties can be used to compose different language features, forming a compositional hierarchy, where the combination of all three is the most advanced feature, called a thread. +While it is possible for a language to only provide threads for composing programs~\cite{Hermes90}, this unnecessarily complicates and makes inefficient solutions to certain classes of problems. +As is shown, each of the non-rejected composed language features solves a particular set of problems, and hence, has a defensible position in a programming language. +If a compositional feature is missing, a programmer has too few fundamental properties resulting in a complex and/or is inefficient solution. In detail, the fundamental properties are: \begin{description}[leftmargin=\parindent,topsep=3pt,parsep=0pt] \item[\newterm{execution state}:] -is the state information needed by a control-flow feature to initialize, manage compute data and execution location(s), and de-initialize. -State is retained in fixed-sized aggregate structures and dynamic-sized stack(s), often allocated in the heap(s) managed by the runtime system. -The lifetime of the state varies with the control-flow feature, where longer life-time and dynamic size provide greater power but also increase usage complexity and cost. -Control-flow transfers among execution states occurs in multiple ways, such as function call, context switch, asynchronous await, etc. +is the state information needed by a control-flow feature to initialize, manage compute data and execution location(s), and de-initialize, \eg calling a function initializes a stack frame including contained objects with constructors, manages local data in blocks and return locations during calls, and de-initializes the frame by running any object destructors and management operations. +State is retained in fixed-sized aggregate structures (objects) and dynamic-sized stack(s), often allocated in the heap(s) managed by the runtime system. +The lifetime of state varies with the control-flow feature, where longer life-time and dynamic size provide greater power but also increase usage complexity and cost. +Control-flow transfers among execution states in multiple ways, such as function call, context switch, asynchronous await, etc. Because the programming language determines what constitutes an execution state, implicitly manages this state, and defines movement mechanisms among states, execution state is an elementary property of the semantics of a programming language. % An execution-state is related to the notion of a process continuation \cite{Hieb90}. \item[\newterm{threading}:] -is execution of code that occurs independently of other execution, \ie the execution resulting from a thread is sequential. +is execution of code that occurs independently of other execution, where an individual thread's execution is sequential. Multiple threads provide \emph{concurrent execution}; -concurrent execution becomes parallel when run on multiple processing units (hyper-threading, cores, sockets). -There must be language mechanisms to create, block/unblock, and join with a thread. +concurrent execution becomes parallel when run on multiple processing units, \eg hyper-threading, cores, or sockets. +There must be language mechanisms to create, block and unblock, and join with a thread, even if the mechanism is indirect. \item[\newterm{MES}:] is the concurrency mechanisms to perform an action without interruption and establish timing relationships among multiple threads. -These two properties are independent, \ie mutual exclusion cannot provide synchronization and vice versa without introducing additional threads~\cite[\S~4]{Buhr05a}. +We contented these two properties are independent, \ie mutual exclusion cannot provide synchronization and vice versa without introducing additional threads~\cite[\S~4]{Buhr05a}. Limiting MES, \eg no access to shared data, results in contrived solutions and inefficiency on multi-core von Neumann computers where shared memory is a foundational aspect of its design. \end{description} -These properties are fundamental because they cannot be built from existing language features, \eg a basic programming language like C99~\cite{C99} cannot create new control-flow features, concurrency, or provide MES using atomic hardware mechanisms. - - -\subsection{Execution Properties} - -Table~\ref{t:ExecutionPropertyComposition} shows how the three fundamental execution properties: state, thread, and mutual exclusion compose a hierarchy of control-flow features needed in a programming language. -(When doing case analysis, not all combinations are meaningful.) -Note, basic von Neumann execution requires at least one thread and an execution state providing some form of call stack. +These properties are fundamental because they cannot be built from existing language features, \eg a basic programming language like C99~\cite{C99} cannot create new control-flow features, concurrency, or provide MES without atomic hardware mechanisms. + + +\subsection{Structuring Execution Properties} + +Programming languages seldom present the fundamental execution properties directly to programmers. +Instead, the properties are packaged into higher-level constructs that encapsulate details and provide safety to these low-level mechanisms. +Interestingly, language designers often pick and choose among these execution properties proving a varying subset of constructs. + +Table~\ref{t:ExecutionPropertyComposition} shows all combinations of the three fundamental execution properties available to language designers. +(When doing combination case-analysis, not all combinations are meaningful.) +The combinations of state, thread, and MES compose a hierarchy of control-flow features all of which have appeared in prior programming languages, where each of these languages have found the feature useful. +To understand the table, it is important to review the basic von Neumann execution requirement of at least one thread and execution state providing some form of call stack. For table entries missing these minimal components, the property is borrowed from the invoker (caller). - -Case 1 is a function that borrows storage for its state (stack frame/activation) and a thread from its invoker and retains this state across \emph{callees}, \ie function local-variables are retained on the stack across calls. -Case 2 is case 1 with access to shared state so callers are restricted during update (mutual exclusion) and scheduling for other threads (synchronization). -Case 3 is a stateful function supporting resume/suspend along with call/return to retain state across \emph{callers}, but has some restrictions because the function's state is stackless. -Note, stackless functions still borrow the caller's stack and thread, where the stack is used to preserve state across its callees. -Case 4 is cases 2 and 3 with protection to shared state for stackless functions. -Cases 5 and 6 are the same as 3 and 4 but only the thread is borrowed as the function state is stackful, so resume/suspend is a context switch from the caller's to the function's stack. -Cases 7 and 8 are rejected because a function that is given a new thread must have its own stack where the thread begins and stack frames are stored for calls, \ie there is no stack to borrow. -Cases 9 and 10 are rejected because a thread with a fixed state (no stack) cannot accept calls, make calls, block, or be preempted, all of which require an unknown amount of additional dynamic state. -Hence, once started, this kind of thread must execute to completion, \ie computation only, which severely restricts runtime management. -Cases 11 and 12 have a stackful thread with and without safe access to shared state. -Execution properties increase the cost of creation and execution along with complexity of usage. +Each entry in the table, numbered \textbf{1}--\textbf{12}, is discussed with respect to how the execution properties combine to generate a high-level language construct. \begin{table} @@ -452,9 +449,9 @@ \hline \hline -No & No & \textbf{1}\ \ \ function & \textbf{2}\ \ \ @monitor@ function \\ +No & No & \textbf{1}\ \ \ @struct@ & \textbf{2}\ \ \ @mutex@ @struct@ \\ \hline -Yes (stackless) & No & \textbf{3}\ \ \ @generator@ & \textbf{4}\ \ \ @monitor@ @generator@ \\ +Yes (stackless) & No & \textbf{3}\ \ \ @generator@ & \textbf{4}\ \ \ @mutex@ @generator@ \\ \hline -Yes (stackful) & No & \textbf{5}\ \ \ @coroutine@ & \textbf{6}\ \ \ @monitor@ @coroutine@ \\ +Yes (stackful) & No & \textbf{5}\ \ \ @coroutine@ & \textbf{6}\ \ \ @mutex@ @coroutine@ \\ \hline No & Yes & \textbf{7}\ \ \ {\color{red}rejected} & \textbf{8}\ \ \ {\color{red}rejected} \\ @@ -462,11 +459,30 @@ Yes (stackless) & Yes & \textbf{9}\ \ \ {\color{red}rejected} & \textbf{10}\ \ \ {\color{red}rejected} \\ \hline -Yes (stackful) & Yes & \textbf{11}\ \ \ @thread@ & \textbf{12}\ \ @monitor@ @thread@ \\ +Yes (stackful) & Yes & \textbf{11}\ \ \ @thread@ & \textbf{12}\ \ @mutex@ @thread@ \\ \end{tabular} \end{table} -Given the execution-properties taxonomy, programmers can now answer three basic questions: is state necessary across calls and how much, is a separate thread necessary, is access to shared state necessary. -The answers define the optimal language feature need for implementing a programming problem. -The next sections discusses how \CFA fills in the table with language features, while other programming languages may only provide a subset of the table. +Case 1 is a structure where access functions borrow local state (stack frame/activation) and thread from the invoker and retain this state across \emph{callees}, \ie function local-variables are retained on the borrowed stack during calls. +Structures are a foundational mechanism for data organization, and access functions provide interface abstraction and code sharing in all programming languages. +Case 2 is case 1 with thread safety to a structure's state where access functions provide serialization (mutual exclusion) and scheduling among calling threads (synchronization). +A @mutex@ structure, often called a \newterm{monitor}, provides a high-level interface for race-free access of shared data in concurrent programming-languages. +Case 3 is case 1 where the structure can implicitly retain execution state and access functions use this execution state to resume/suspend across \emph{callers}, but resume/suspend does not retain a function's local state. +A stackless structure, often called a \newterm{generator} or \emph{iterator}, is \newterm{stackless} because it still borrow the caller's stack and thread, but the stack is used only to preserve state across its callees not callers. +Generators provide the first step toward directly solving problems like finite-state machines that retain data and execution state between calls, whereas normal functions restart on each call. +Case 4 is cases 2 and 3 with thread safety during execution of the generator's access functions. +A @mutex@ generator extends generators into the concurrent domain. +Cases 5 and 6 are like cases 3 and 4 where the structure is extended with an implicit separate stack, so only the thread is borrowed by access functions. +A stackful generator, often called a \newterm{coroutine}, is \newterm{stackful} because resume/suspend now context switch to/from the caller's and coroutine's stack. +A coroutine extends the state retained between calls beyond the generator's structure to arbitrary call depth in the access functions. +Cases 7 and 8 are rejected because a new thread must have its own stack, where the thread begins and stack frames are stored for calls, \ie it is unrealistic for a thread to borrow a stack. +Cases 9 and 10 are rejected because a thread needs a growable stack to accept calls, make calls, block, or be preempted, all of which compound to require an unknown amount of execution state. +If this kind of thread exists, it must execute to completion, \ie computation only, which severely restricts runtime management. +Cases 11 and 12 are a stackful thread with and without safe access to shared state. +A thread is the language mechanism to start another thread of control in a program with growable execution state for call/return execution. +In general, language constructs with more execution properties increase the cost of creation and execution along with complexity of usage. + +Given the execution-properties taxonomy, programmers now ask three basic questions: is state necessary across callers and how much, is a separate thread necessary, is thread safety necessary. +Table~\ref{t:ExecutionPropertyComposition} then suggests the optimal language feature needed for implementing a programming problem. +The following sections describe how \CFA fills in \emph{all} the non-rejected table entries with language features, while other programming languages may only provide a subset of the table. @@ -481,6 +497,6 @@ \item Direct interaction among language features must be possible allowing any feature to be selected without restricting comm\-unication. -For example, many concurrent languages do not provide direct communication (calls) among threads, \ie threads only communicate indirectly through monitors, channels, messages, and/or futures. -Indirect communication increases the number of objects, consuming more resources, and require additional synchronization and possibly data transfer. +For example, many concurrent languages do not provide direct communication calls among threads, \ie threads only communicate indirectly through monitors, channels, messages, and/or futures. +Indirect communication increases the number of objects, consuming more resources, and requires additional synchronization and possibly data transfer. \item @@ -493,5 +509,5 @@ \item -MES must be available implicitly in language constructs as well as explicitly for specialized requirements, because requiring programmers to build MES using low-level locks often leads to incorrect programs. +MES must be available implicitly in language constructs, \eg Java built-in monitors, as well as explicitly for specialized requirements, \eg @java.util.concurrent@, because requiring programmers to build MES using low-level locks often leads to incorrect programs. Furthermore, reducing synchronization scope by encapsulating it within language constructs further reduces errors in concurrent programs. @@ -502,5 +518,5 @@ \item Synchronization must be able to control the service order of requests including prioritizing selection from different kinds of outstanding requests, and postponing a request for an unspecified time while continuing to accept new requests. -Otherwise, certain concurrency problems are difficult, e.g.\ web server, disk scheduling, and the amount of concurrency is inhibited~\cite{Gentleman81}. +Otherwise, certain concurrency problems are difficult, \eg web server, disk scheduling, and the amount of concurrency is inhibited~\cite{Gentleman81}. \end{itemize} We have satisfied these requirements in \CFA while maintaining backwards compatibility with the huge body of legacy C programs. @@ -511,5 +527,5 @@ Asynchronous await/call is a caller mechanism for structuring programs and/or increasing concurrency, where the caller (client) postpones an action into the future, which is subsequently executed by a callee (server). -The caller detects the action's completion through a \newterm{future}/\newterm{promise}. +The caller detects the action's completion through a \newterm{future} or \newterm{promise}. The benefit is asynchronous caller execution with respect to the callee until future resolution. For single-threaded languages like JavaScript, an asynchronous call passes a callee action, which is queued in the event-engine, and continues execution with a promise. @@ -517,9 +533,9 @@ A promise-completion call-back can be part of the callee action or the caller is rescheduled; in either case, the call back is executed after the promise is fulfilled. -While asynchronous calls generate new callee (server) events, we content this mechanism is insufficient for advanced control-flow mechanisms like generators or coroutines (which are discussed next). +While asynchronous calls generate new callee (server) events, we contend this mechanism is insufficient for advanced control-flow mechanisms like generators or coroutines, which are discussed next. Specifically, control between caller and callee occurs indirectly through the event-engine precluding direct handoff and cycling among events, and requires complex resolution of a control promise and data. Note, @async-await@ is just syntactic-sugar over the event engine so it does not solve these deficiencies. For multi-threaded languages like Java, the asynchronous call queues a callee action with an executor (server), which subsequently executes the work by a thread in the executor thread-pool. -The problem is when concurrent work-units need to interact and/or block as this effects the executor, \eg stops threads. +The problem is when concurrent work-units need to interact and/or block as this effects the executor by stopping threads. While it is possible to extend this approach to support the necessary mechanisms, \eg message passing in Actors, we show monitors and threads provide an equally competitive approach that does not deviate from normal call communication and can be used to build asynchronous call, as is done in Java. @@ -540,6 +556,6 @@ There are two styles of activating a stateful function, \emph{asymmetric} or \emph{symmetric}, identified by resume/suspend (no cycles) and resume/resume (cycles). These styles \emph{do not} cause incremental stack growth, \eg a million resume/suspend or resume/resume cycles do not remember each cycle just the last resumer for each cycle. -Selecting between stackless/stackful semantics and asymmetric/symmetric style is a tradeoff between programming requirements, performance, and design, where stackless is faster and smaller (modified call/return between closures), stackful is more general but slower and larger (context switching between distinct stacks), and asymmetric is simpler control-flow than symmetric. -Additionally, storage management for the closure/stack (especially in unmanaged languages, \ie no garbage collection) must be factored into design and performance. +Selecting between stackless/stackful semantics and asymmetric/symmetric style is a tradeoff between programming requirements, performance, and design, where stackless is faster and smaller using modified call/return between closures, stackful is more general but slower and larger using context switching between distinct stacks, and asymmetric is simpler control-flow than symmetric. +Additionally, storage management for the closure/stack must be factored into design and performance, especially in unmanaged languages without garbage collection. Note, creation cost (closure/stack) is amortized across usage, so activation cost (resume/suspend) is usually the dominant factor. @@ -578,5 +594,5 @@ & \begin{cfa} -void * rtn( void * arg ) { ... } +void * `rtn`( void * arg ) { ... } int i = 3, rc; pthread_t t; $\C{// thread id}$ @@ -690,5 +706,5 @@ \hspace{3pt} \subfloat[C generated code for \CFA version]{\label{f:CFibonacciSim}\usebox\myboxC} -\caption{Fibonacci (output) asymmetric generator} +\caption{Fibonacci output asymmetric generator} \label{f:FibonacciAsymmetricGenerator} @@ -765,12 +781,12 @@ \subfloat[C generated code for \CFA version]{\label{f:CFormatGenImpl}\usebox\myboxB} \hspace{3pt} -\caption{Formatter (input) asymmetric generator} +\caption{Formatter input asymmetric generator} \label{f:FormatterAsymmetricGenerator} \end{figure} -Figure~\ref{f:FibonacciAsymmetricGenerator} shows an unbounded asymmetric generator for an infinite sequence of Fibonacci numbers written (left to right) in C, \CFA, and showing the underlying C implementation for the \CFA version. +Figure~\ref{f:FibonacciAsymmetricGenerator} shows an unbounded asymmetric generator for an infinite sequence of Fibonacci numbers written left to right in C, \CFA, and showing the underlying C implementation for the \CFA version. This generator is an \emph{output generator}, producing a new result on each resumption. To compute Fibonacci, the previous two values in the sequence are retained to generate the next value, \ie @fn1@ and @fn@, plus the execution location where control restarts when the generator is resumed, \ie top or middle. -An additional requirement is the ability to create an arbitrary number of generators (of any kind), \ie retaining one state in global variables is insufficient; +An additional requirement is the ability to create an arbitrary number of generators of any kind, \ie retaining one state in global variables is insufficient; hence, state is retained in a closure between calls. Figure~\ref{f:CFibonacci} shows the C approach of manually creating the closure in structure @Fib@, and multiple instances of this closure provide multiple Fibonacci generators. @@ -794,7 +810,7 @@ Figure~\ref{f:CFibonacciSim} shows the C implementation of the \CFA asymmetric generator. Only one execution-state field, @restart@, is needed to subscript the suspension points in the generator. -At the start of the generator main, the @static@ declaration, @states@, is initialized to the N suspend points in the generator (where operator @&&@ dereferences/references a label~\cite{gccValueLabels}). +At the start of the generator main, the @static@ declaration, @states@, is initialized to the N suspend points in the generator, where operator @&&@ dereferences or references a label~\cite{gccValueLabels}. Next, the computed @goto@ selects the last suspend point and branches to it. -The cost of setting @restart@ and branching via the computed @goto@ adds very little cost to the suspend/resume calls. +The cost of setting @restart@ and branching via the computed @goto@ adds very little cost to the suspend and resume calls. An advantage of the \CFA explicit generator type is the ability to allow multiple type-safe interface functions taking and returning arbitrary types. @@ -877,5 +893,5 @@ With respect to safety, we believe static analysis can discriminate persistent generator state from temporary generator-main state and raise a compile-time error for temporary usage spanning suspend points. Our experience using generators is that the problems have simple data state, including local state, but complex execution state, so the burden of creating the generator type is small. -As well, C programmers are not afraid of this kind of semantic programming requirement, if it results in very small, fast generators. +As well, C programmers are not afraid of this kind of semantic programming requirement, if it results in very small and fast generators. Figure~\ref{f:CFAFormatGen} shows an asymmetric \newterm{input generator}, @Fmt@, for restructuring text into groups of characters of fixed-size blocks, \ie the input on the left is reformatted into the output on the right, where newlines are ignored. @@ -899,9 +915,9 @@ The destructor provides a newline, if formatted text ends with a full line. Figure~\ref{f:CFormatGenImpl} shows the C implementation of the \CFA input generator with one additional field and the computed @goto@. -For contrast, Figure~\ref{f:PythonFormatter} shows the equivalent Python format generator with the same properties as the format generator. +For contrast, Figure~\ref{f:PythonFormatter} shows the equivalent Python format generator with the same properties as the \CFA format generator. % https://dl-acm-org.proxy.lib.uwaterloo.ca/ -Figure~\ref{f:DeviceDriverGen} shows an important application for an asymmetric generator, a device-driver, because device drivers are a significant source of operating-system errors: 85\% in Windows XP~\cite[p.~78]{Swift05} and 51.6\% in Linux~\cite[p.~1358,]{Xiao19}. %\cite{Palix11} +An important application for the asymmetric generator is a device-driver, because device drivers are a significant source of operating-system errors: 85\% in Windows XP~\cite[p.~78]{Swift05} and 51.6\% in Linux~\cite[p.~1358,]{Xiao19}. %\cite{Palix11} Swift \etal~\cite[p.~86]{Swift05} restructure device drivers using the Extension Procedure Call (XPC) within the kernel via functions @nooks_driver_call@ and @nooks_kernel_call@, which have coroutine properties context switching to separate stacks with explicit hand-off calls; however, the calls do not retain execution state, and hence always start from the top. @@ -909,15 +925,15 @@ However, Adya \etal~\cite{Adya02} argue against stack ripping in Section 3.2 and suggest a hybrid approach in Section 4 using cooperatively scheduled \emph{fibers}, which is coroutining. -As an example, the following protocol: +Figure~\ref{f:DeviceDriverGen} shows the generator advantages in implementing a simple network device-driver with the following protocol: \begin{center} \ldots\, STX \ldots\, message \ldots\, ESC ETX \ldots\, message \ldots\, ETX 2-byte crc \ldots \end{center} -is for a simple network message beginning with the control character STX, ending with an ETX, and followed by a 2-byte cyclic-redundancy check. +where the network message begins with the control character STX, ends with an ETX, and is followed by a 2-byte cyclic-redundancy check. Control characters may appear in a message if preceded by an ESC. When a message byte arrives, it triggers an interrupt, and the operating system services the interrupt by calling the device driver with the byte read from a hardware register. -The device driver returns a status code of its current state, and when a complete message is obtained, the operating system read the message accumulated in the supplied buffer. -Hence, the device driver is an input/output generator, where the cost of resuming the device-driver generator is the same as call/return, so performance in an operating-system kernel is excellent. +The device driver returns a status code of its current state, and when a complete message is obtained, the operating system reads the message accumulated in the supplied buffer. +Hence, the device driver is an input/output generator, where the cost of resuming the device-driver generator is the same as call and return, so performance in an operating-system kernel is excellent. The key benefits of using a generator are correctness, safety, and maintenance because the execution states are transcribed directly into the programming language rather than table lookup or stack ripping. -The conclusion is that FSMs are complex and occur in important domains, so direct generator support is important in a system programming language. +% The conclusion is that FSMs are complex and occur in important domains, so direct generator support is important in a system programming language. \begin{figure} @@ -976,13 +992,13 @@ \end{figure} -Figure~\ref{f:CFAPingPongGen} shows a symmetric generator, where the generator resumes another generator, forming a resume/resume cycle. +Generators can also have symmetric activation using resume/resume to create control-flow cycles among generators. (The trivial cycle is a generator resuming itself.) This control flow is similar to recursion for functions but without stack growth. -Figure~\ref{f:PingPongFullCoroutineSteps} shows the steps for symmetric control-flow are creating, executing, and terminating the cycle. +Figure~\ref{f:PingPongFullCoroutineSteps} shows the steps for symmetric control-flow using for the ping/pong program in Figure~\ref{f:CFAPingPongGen}. +The program starts by creating the generators, @ping@ and @pong@, and then assigns the partners that form the cycle. Constructing the cycle must deal with definition-before-use to close the cycle, \ie, the first generator must know about the last generator, which is not within scope. (This issue occurs for any cyclic data structure.) -The example creates the generators, @ping@/@pong@, and then assigns the partners that form the cycle. % (Alternatively, the constructor can assign the partners as they are declared, except the first, and the first-generator partner is set after the last generator declaration to close the cycle.) -Once the cycle is formed, the program main resumes one of the generators, @ping@, and the generators can then traverse an arbitrary cycle using @resume@ to activate partner generator(s). +Once the cycle is formed, the program main resumes one of the generators, @ping@, and the generators can then traverse an arbitrary number of cycles using @resume@ to activate partner generator(s). Terminating the cycle is accomplished by @suspend@ or @return@, both of which go back to the stack frame that started the cycle (program main in the example). Note, the creator and starter may be different, \eg if the creator calls another function that starts the cycle. @@ -990,4 +1006,12 @@ Also, since local variables are not retained in the generator function, there are no objects with destructors to be called, so the cost is the same as a function return. Destructor cost occurs when the generator instance is deallocated by the creator. + +\begin{figure} +\centering +\input{FullCoroutinePhases.pstex_t} +\vspace*{-10pt} +\caption{Symmetric coroutine steps: Ping / Pong} +\label{f:PingPongFullCoroutineSteps} +\end{figure} \begin{figure} @@ -1053,12 +1077,4 @@ \end{figure} -\begin{figure} -\centering -\input{FullCoroutinePhases.pstex_t} -\vspace*{-10pt} -\caption{Symmetric coroutine steps: Ping / Pong} -\label{f:PingPongFullCoroutineSteps} -\end{figure} - Figure~\ref{f:CPingPongSim} shows the C implementation of the \CFA symmetric generator, where there is still only one additional field, @restart@, but @resume@ is more complex because it does a forward rather than backward jump. Before the jump, the parameter for the next call @partner@ is placed into the register used for the first parameter, @rdi@, and the remaining registers are reset for a return. @@ -1066,6 +1082,6 @@ While the semantics of call forward is a tail-call optimization, which compilers perform, the generator state is different on each call rather a common state for a tail-recursive function (i.e., the parameter to the function never changes during the forward calls. However, this assembler code depends on what entry code is generated, specifically if there are local variables and the level of optimization. -Hence, internal compiler support is necessary for any forward call (or backwards return), \eg LLVM has various coroutine support~\cite{CoroutineTS}, and \CFA can leverage this support should it eventually fork @clang@. -For this reason, \CFA does not support general symmetric generators at this time, but, it is possible to hand generate any symmetric generators (as in Figure~\ref{f:CPingPongSim}) for proof of concept and performance testing. +Hence, internal compiler support is necessary for any forward call or backwards return, \eg LLVM has various coroutine support~\cite{CoroutineTS}, and \CFA can leverage this support should it eventually fork @clang@. +For this reason, \CFA does not support general symmetric generators at this time, but, it is possible to hand generate any symmetric generators, as in Figure~\ref{f:CPingPongSim}, for proof of concept and performance testing. Finally, part of this generator work was inspired by the recent \CCtwenty coroutine proposal~\cite{C++20Coroutine19}, which uses the general term coroutine to mean generator. @@ -1084,10 +1100,15 @@ \label{s:Coroutine} -Stackful coroutines (Table~\ref{t:ExecutionPropertyComposition} case 5) extend generator semantics, \ie there is an implicit closure and @suspend@ may appear in a helper function called from the coroutine main. -A coroutine is specified by replacing @generator@ with @coroutine@ for the type. +Stackful coroutines (Table~\ref{t:ExecutionPropertyComposition} case 5) extend generator semantics with an implicit closure and @suspend@ may appear in a helper function called from the coroutine main because of the separate stack. +Note, simulating coroutines with stacks of generators, \eg Python with @yield from@ cannot handle symmetric control-flow. +Furthermore, all stack components must be of generators, so it is impossible to call a library function passing a generator that yields. +Creating a generator copy of the library function maybe impossible because the library function is opaque. + +A \CFA coroutine is specified by replacing @generator@ with @coroutine@ for the type. Coroutine generality results in higher cost for creation, due to dynamic stack allocation, for execution, due to context switching among stacks, and for terminating, due to possible stack unwinding and dynamic stack deallocation. A series of different kinds of coroutines and their implementations demonstrate how coroutines extend generators. First, the previous generator examples are converted to their coroutine counterparts, allowing local-state variables to be moved from the generator type into the coroutine main. +Now the coroutine type only contains communication variables between interface functions and the coroutine main. \begin{center} \begin{tabular}{@{}l|l|l|l@{}} @@ -1126,12 +1147,10 @@ \begin{cfa} int Crc() { - `suspend;` - short int crc = byte << 8; - `suspend;` - status = (crc | byte) == sum ? MSG : ECRC; + `suspend;` short int crc = byte << 8; + `suspend;` status = (crc | byte) == sum ? MSG : ECRC; return crc; } \end{cfa} -A call to this function is placed at the end of the driver's coroutine-main. +A call to this function is placed at the end of the device driver's coroutine-main. For complex finite-state machines, refactoring is part of normal program abstraction, especially when code is used in multiple places. Again, this complexity is usually associated with execution state rather than data state. @@ -1139,5 +1158,5 @@ \begin{comment} Figure~\ref{f:Coroutine3States} creates a @coroutine@ type, @`coroutine` Fib { int fn; }@, which provides communication, @fn@, for the \newterm{coroutine main}, @main@, which runs on the coroutine stack, and possibly multiple interface functions, \eg @restart@. -Like the structure in Figure~\ref{f:ExternalState}, the coroutine type allows multiple instances, where instances of this type are passed to the (overloaded) coroutine main. +Like the structure in Figure~\ref{f:ExternalState}, the coroutine type allows multiple instances, where instances of this type are passed to the overloaded coroutine main. The coroutine main's stack holds the state for the next generation, @f1@ and @f2@, and the code represents the three states in the Fibonacci formula via the three suspend points, to context switch back to the caller's @resume@. The interface function @restart@, takes a Fibonacci instance and context switches to it using @resume@; @@ -1373,5 +1392,5 @@ Figure~\ref{f:ProdCons} shows the ping-pong example in Figure~\ref{f:CFAPingPongGen} extended into a producer/consumer symmetric-coroutine performing bidirectional communication. -This example is illustrative because both producer/consumer have two interface functions with @resume@s that suspend execution in these interface (helper) functions. +This example is illustrative because both producer and consumer have two interface functions with @resume@s that suspend execution in these interface functions. The program main creates the producer coroutine, passes it to the consumer coroutine in its initialization, and closes the cycle at the call to @start@ along with the number of items to be produced. The call to @start@ is the first @resume@ of @prod@, which remembers the program main as the starter and creates @prod@'s stack with a frame for @prod@'s coroutine main at the top, and context switches to it. @@ -1401,5 +1420,5 @@ The question now is where does control continue? -The na\"{i}ve semantics for coroutine-cycle termination is to context switch to the last resumer, like executing a @suspend@/@return@ in a generator. +The na\"{i}ve semantics for coroutine-cycle termination is to context switch to the last resumer, like executing a @suspend@ or @return@ in a generator. However, for coroutines, the last resumer is \emph{not} implicitly below the current stack frame, as for generators, because each coroutine's stack is independent. Unfortunately, it is impossible to determine statically if a coroutine is in a cycle and unrealistic to check dynamically (graph-cycle problem). @@ -1410,10 +1429,10 @@ For asymmetric coroutines, it is common for the first resumer (starter) coroutine to be the only resumer; for symmetric coroutines, it is common for the cycle creator to persist for the lifetime of the cycle. -For other scenarios, it is always possible to devise a solution with additional programming effort, such as forcing the cycle forward (backward) to a safe point before starting termination. +For other scenarios, it is always possible to devise a solution with additional programming effort, such as forcing the cycle forward or backward to a safe point before starting termination. Note, the producer/consumer example does not illustrate the full power of the starter semantics because @cons@ always ends first. Assume generator @PingPong@ in Figure~\ref{f:PingPongSymmetricGenerator} is converted to a coroutine. Unlike generators, coroutines have a starter structure with multiple levels, where the program main starts @ping@ and @ping@ starts @pong@. -By adjusting $N$ for either @ping@/@pong@, it is possible to have either finish first. +By adjusting $N$ for either @ping@ or @pong@, it is possible to have either finish first. If @pong@ ends first, it resumes its starter @ping@ in its coroutine main, then @ping@ ends and resumes its starter the program main on return; if @ping@ ends first, it resumes its starter the program main on return. @@ -1425,5 +1444,5 @@ \subsection{Generator / Coroutine Implementation} -A significant implementation challenge for generators/coroutines (and threads in Section~\ref{s:threads}) is adding extra fields to the custom types and related functions, \eg inserting code after/before the coroutine constructor/destructor and @main@ to create/initialize/de-initialize/destroy any extra fields, \eg stack. +A significant implementation challenge for generators and coroutines (and threads in Section~\ref{s:threads}) is adding extra fields to the custom types and related functions, \eg inserting code after/before the coroutine constructor/destructor and @main@ to create/initialize/de-initialize/destroy any extra fields, \eg the coroutine stack. There are several solutions to these problem, which follow from the object-oriented flavour of adopting custom types. @@ -1433,5 +1452,5 @@ \end{cfa} % The problem is that the programming language and its tool chain, \eg debugger, @valgrind@, need to understand @baseCoroutine@ because it infers special property, so type @baseCoroutine@ becomes a de facto keyword and all types inheriting from it are implicitly custom types. -The problem is that some special properties are not handled by existing language semantics, \eg the execution of constructors/destructors is in the wrong order to implicitly start threads because the thread must start \emph{after} all constructors as it relies on a completely initialized object, but the inherited constructor runs \emph{before} the derived. +The problem is that some special properties are not handled by existing language semantics, \eg the execution of constructors and destructors is in the wrong order to implicitly start threads because the thread must start \emph{after} all constructors as it relies on a completely initialized object, but the inherited constructor runs \emph{before} the derived. Alternatives, such as explicitly starting threads as in Java, are repetitive and forgetting to call start is a common source of errors. An alternative is composition: @@ -1461,8 +1480,8 @@ forall( `dtype` T | is_coroutine(T) ) void $suspend$( T & ), resume( T & ); \end{cfa} -Note, copying generators/coroutines/threads is undefined because muliple objects cannot execute on a shared stack and stack copying does not work in unmanaged languages (no garbage collection), like C, because the stack may contain pointers to objects within it that require updating for the copy. -The \CFA @dtype@ property provides no \emph{implicit} copying operations and the @is_coroutine@ trait provides no \emph{explicit} copying operations, so all coroutines must be passed by reference (pointer). -The function definitions ensure there is a statically typed @main@ function that is the starting point (first stack frame) of a coroutine, and a mechanism to get (read) the coroutine descriptor from its handle. -The @main@ function has no return value or additional parameters because the coroutine type allows an arbitrary number of interface functions with corresponding arbitrary typed input/output values versus fixed ones. +Note, copying generators, coroutines, and threads is undefined because muliple objects cannot execute on a shared stack and stack copying does not work in unmanaged languages (no garbage collection), like C, because the stack may contain pointers to objects within it that require updating for the copy. +The \CFA @dtype@ property provides no \emph{implicit} copying operations and the @is_coroutine@ trait provides no \emph{explicit} copying operations, so all coroutines must be passed by reference or pointer. +The function definitions ensure there is a statically typed @main@ function that is the starting point (first stack frame) of a coroutine, and a mechanism to read the coroutine descriptor from its handle. +The @main@ function has no return value or additional parameters because the coroutine type allows an arbitrary number of interface functions with arbitrary typed input and output values versus fixed ones. The advantage of this approach is that users can easily create different types of coroutines, \eg changing the memory layout of a coroutine is trivial when implementing the @get_coroutine@ function, and possibly redefining \textsf{suspend} and @resume@. @@ -1506,15 +1525,16 @@ Figure~\ref{f:CoroutineMemoryLayout} shows different memory-layout options for a coroutine (where a thread is similar). -The coroutine handle is the @coroutine@ instance containing programmer specified type global/communication variables across interface functions. +The coroutine handle is the @coroutine@ instance containing programmer specified type global and communication variables across interface functions. The coroutine descriptor contains all implicit declarations needed by the runtime, \eg @suspend@/@resume@, and can be part of the coroutine handle or separate. The coroutine stack can appear in a number of locations and be fixed or variable sized. -Hence, the coroutine's stack could be a variable-length structure (VLS)\footnote{ -We are examining VLSs, where fields can be variable-sized structures or arrays. -Once allocated, a VLS is fixed sized.} +Hence, the coroutine's stack could be a variable-length structure (VLS) +% \footnote{ +% We are examining VLSs, where fields can be variable-sized structures or arrays. +% Once allocated, a VLS is fixed sized.} on the allocating stack, provided the allocating stack is large enough. -For a VLS stack allocation/deallocation is an inexpensive adjustment of the stack pointer, modulo any stack constructor costs (\eg initial frame setup). -For stack allocation in the heap, allocation/deallocation is an expensive allocation, where the heap can be a shared resource, modulo any stack constructor costs. -It is also possible to use a split (segmented) stack calling convention, available with gcc and clang, allowing a variable-sized stack via a set of connected blocks in the heap. -Currently, \CFA supports stack/heap allocated descriptors but only fixed-sized heap allocated stacks. +For a VLS stack allocation and deallocation is an inexpensive adjustment of the stack pointer, modulo any stack constructor costs to initial frame setup. +For stack allocation in the heap, allocation and deallocation is an expensive allocation, where the heap can be a shared resource, modulo any stack constructor costs. +It is also possible to use a split or segmented stack calling convention, available with gcc and clang, allowing a variable-sized stack via a set of connected blocks in the heap. +Currently, \CFA supports stack and heap allocated descriptors but only fixed-sized heap allocated stacks. In \CFA debug-mode, the fixed-sized stack is terminated with a write-only page, which catches most stack overflows. Experience teaching concurrency with \uC~\cite{CS343} shows fixed-sized stacks are rarely an issue for students. @@ -1539,8 +1559,8 @@ The transition to concurrency, even for a single thread with multiple stacks, occurs when coroutines context switch to a \newterm{scheduling coroutine}, introducing non-determinism from the coroutine perspective~\cite[\S~3]{Buhr05a}. Therefore, a minimal concurrency system requires coroutines \emph{in conjunction with a nondeterministic scheduler}. -The resulting execution system now follows a cooperative threading-model~\cite{Adya02,libdill} because context-switching points to the scheduler (blocking) are known, but the next unblocking point is unknown due to the scheduler. +The resulting execution system now follows a cooperative threading-model~\cite{Adya02,libdill} because context-switching points to the scheduler are known, but the next unblocking point is unknown due to the scheduler. Adding \newterm{preemption} introduces \newterm{non-cooperative} or \newterm{preemptive} scheduling, where context switching points to the scheduler are unknown as they can occur randomly between any two instructions often based on a timer interrupt. Uncertainty gives the illusion of parallelism on a single processor and provides a mechanism to access and increase performance on multiple processors. -The reason is that the scheduler/runtime have complete knowledge about resources and how to best utilized them. +The reason is that the scheduler and runtime have complete knowledge about resources and how to best utilized them. However, the introduction of unrestricted nondeterminism results in the need for \newterm{mutual exclusion} and \newterm{synchronization}~\cite[\S~4]{Buhr05a}, which restrict nondeterminism for correctness; otherwise, it is impossible to write meaningful concurrent programs. @@ -1556,6 +1576,5 @@ \label{s:threads} -Threading (Table~\ref{t:ExecutionPropertyComposition} case 11) needs the ability to start a thread and wait for its completion. -A common API for this ability is @fork@ and @join@. +Threading (Table~\ref{t:ExecutionPropertyComposition} case 11) needs the ability to start a thread and wait for its completion, where a common API is @fork@ and @join@. \vspace{4pt} \par\noindent @@ -1589,5 +1608,5 @@ \vspace{1pt} \par\noindent -\CFA has a simpler approach using a custom @thread@ type and leveraging declaration semantics (allocation/deallocation), where threads implicitly @fork@ after construction and @join@ before destruction. +\CFA has a simpler approach using a custom @thread@ type and leveraging declaration semantics, allocation and deallocation, where threads implicitly @fork@ after construction and @join@ before destruction. \begin{cfa} thread MyThread {}; @@ -1598,6 +1617,6 @@ } $\C{// deallocate stack-based threads, implicit joins before destruction}$ \end{cfa} -This semantic ensures a thread is started and stopped exactly once, eliminating some programming error, and scales to multiple threads for basic (termination) synchronization. -For block allocation to arbitrary depth, including recursion, threads are created/destroyed in a lattice structure (tree with top and bottom). +This semantic ensures a thread is started and stopped exactly once, eliminating some programming error, and scales to multiple threads for basic termination synchronization. +For block allocation to arbitrary depth, including recursion, threads are created and destroyed in a lattice structure (tree with top and bottom). Arbitrary topologies are possible using dynamic allocation, allowing threads to outlive their declaration scope, identical to normal dynamic allocation. \begin{cfa} @@ -1670,10 +1689,10 @@ \end{tabular} \end{cquote} -Like coroutines, the @dtype@ property prevents \emph{implicit} copy operations and the @is_thread@ trait provides no \emph{explicit} copy operations, so threads must be passed by reference (pointer). -Similarly, the function definitions ensure there is a statically typed @main@ function that is the thread starting point (first stack frame), a mechanism to get (read) the thread descriptor from its handle, and a special destructor to prevent deallocation while the thread is executing. +Like coroutines, the @dtype@ property prevents \emph{implicit} copy operations and the @is_thread@ trait provides no \emph{explicit} copy operations, so threads must be passed by reference or pointer. +Similarly, the function definitions ensure there is a statically typed @main@ function that is the thread starting point (first stack frame), a mechanism to read the thread descriptor from its handle, and a special destructor to prevent deallocation while the thread is executing. (The qualifier @mutex@ for the destructor parameter is discussed in Section~\ref{s:Monitor}.) The difference between the coroutine and thread is that a coroutine borrows a thread from its caller, so the first thread resuming a coroutine creates the coroutine's stack and starts running the coroutine main on the stack; whereas, a thread is scheduling for execution in @main@ immediately after its constructor is run. -No return value or additional parameters are necessary for this function because the @thread@ type allows an arbitrary number of interface functions with corresponding arbitrary typed input/output values. +No return value or additional parameters are necessary for this function because the @thread@ type allows an arbitrary number of interface functions with corresponding arbitrary typed input and output values. @@ -1683,6 +1702,6 @@ Unrestricted nondeterminism is meaningless as there is no way to know when a result is completed and safe to access. To produce meaningful execution requires clawing back some determinism using mutual exclusion and synchronization, where mutual exclusion provides access control for threads using shared data, and synchronization is a timing relationship among threads~\cite[\S~4]{Buhr05a}. -The shared data protected by mutual exlusion is called a \newterm{critical section}~\cite{Dijkstra65}, and the protection can be simple (only 1 thread) or complex (only N kinds of threads, \eg group~\cite{Joung00} or readers/writer~\cite{Courtois71}). -Without synchronization control in a critical section, an arriving thread can barge ahead of preexisting waiter threads resulting in short/long-term starvation, staleness/freshness problems, and/or incorrect transfer of data. +The shared data protected by mutual exlusion is called a \newterm{critical section}~\cite{Dijkstra65}, and the protection can be simple, only 1 thread, or complex, only N kinds of threads, \eg group~\cite{Joung00} or readers/writer~\cite{Courtois71} problems. +Without synchronization control in a critical section, an arriving thread can barge ahead of preexisting waiter threads resulting in short/long-term starvation, staleness and freshness problems, and incorrect transfer of data. Preventing or detecting barging is a challenge with low-level locks, but made easier through higher-level constructs. This challenge is often split into two different approaches: barging \emph{avoidance} and \emph{prevention}. @@ -1696,7 +1715,7 @@ Some concurrent systems eliminate mutable shared-state by switching to non-shared communication like message passing~\cite{Thoth,Harmony,V-Kernel,MPI} (Erlang, MPI), channels~\cite{CSP} (CSP,Go), actors~\cite{Akka} (Akka, Scala), or functional techniques (Haskell). However, these approaches introduce a new communication mechanism for concurrency different from the standard communication using function call/return. -Hence, a programmer must learn and manipulate two sets of design/programming patterns. +Hence, a programmer must learn and manipulate two sets of design and programming patterns. While this distinction can be hidden away in library code, effective use of the library still has to take both paradigms into account. -In contrast, approaches based on shared-state models more closely resemble the standard call/return programming model, resulting in a single programming paradigm. +In contrast, approaches based on shared-state models more closely resemble the standard call and return programming model, resulting in a single programming paradigm. Finally, a newer approach for restricting non-determinism is transactional memory~\cite{Herlihy93}. While this approach is pursued in hardware~\cite{Nakaike15} and system languages, like \CC~\cite{Cpp-Transactions}, the performance and feature set is still too restrictive~\cite{Cascaval08,Boehm09} to be the main concurrency paradigm for system languages. @@ -1711,42 +1730,77 @@ For these reasons, \CFA selected monitors as the core high-level concurrency construct, upon which higher-level approaches can be easily constructed. -Specifically, a \textbf{monitor} is a set of functions that ensure mutual exclusion when accessing shared state. -More precisely, a monitor is a programming technique that implicitly binds mutual exclusion to static function scope by call/return, as opposed to locks, where mutual-exclusion is defined by acquire/release calls, independent of lexical context (analogous to block and heap storage allocation). -Restricting acquire/release points eases programming, comprehension, and maintenance, at a slight cost in flexibility and efficiency. -\CFA uses a custom @monitor@ type and leverages declaration semantics (deallocation) to protect active or waiting threads in a monitor. - -The following is a \CFA monitor implementation of an atomic counter. -\begin{cfa} -`monitor` Aint { int cnt; }; $\C[4.25in]{// atomic integer counter}$ -int ++?( Aint & `mutex` this ) with( this ) { return ++cnt; } $\C{// increment}$ -int ?=?( Aint & `mutex` lhs, int rhs ) with( lhs ) { cnt = rhs; } $\C{// conversions with int, mutex optional}\CRT$ -int ?=?( int & lhs, Aint & `mutex` rhs ) with( rhs ) { lhs = cnt; } -\end{cfa} -The operators use the parameter-only declaration type-qualifier @mutex@ to mark which parameters require locking during function execution to protect from race conditions. -The assignment operators provide bidirectional conversion between an atomic and normal integer without accessing field @cnt@. -(These operations only need @mutex@, if reading/writing the implementation type is not atomic.) -The atomic counter is used without any explicit mutual-exclusion and provides thread-safe semantics. -\begin{cfa} +Figure~\ref{f:AtomicCounter} compares a \CFA and Java monitor implementing an atomic counter. +(Like other concurrent programming languages, \CFA and Java have performant specializations for the basic types using atomic instructions.) +A \newterm{monitor} is a set of functions that ensure mutual exclusion when accessing shared state. +(Note, in \CFA, @monitor@ is short-hand for @mutex struct@.) +More precisely, a monitor is a programming technique that implicitly binds mutual exclusion to static function scope by call and return, as opposed to locks, where mutual exclusion is defined by acquire/release calls, independent of lexical context (analogous to block and heap storage allocation). +Restricting acquire and release points eases programming, comprehension, and maintenance, at a slight cost in flexibility and efficiency. +As for other special types, \CFA has a custom @monitor@ type. + +\begin{figure} +\centering + +\begin{lrbox}{\myboxA} +\begin{cfa}[aboveskip=0pt,belowskip=0pt] +`monitor` Aint { // atomic integer counter + int cnt; +}; +int ++?( Aint & `mutex` this ) with(this) { return ++cnt; } +int ?=?( Aint & `mutex` lhs, int rhs ) with(lhs) { cnt = rhs; } +int ?=?(int & lhs, Aint & rhs) with(rhs) { lhs = cnt; } + int i = 0, j = 0, k = 5; -Aint x = { 0 }, y = { 0 }, z = { 5 }; $\C{// no mutex required}$ -++x; ++y; ++z; $\C{// safe increment by multiple threads}$ -x = 2; y = i; z = k; $\C{// conversions}$ -i = x; j = y; k = z; -\end{cfa} -Note, like other concurrent programming languages, \CFA has specializations for the basic types using atomic instructions for performance and a general trait similar to the \CC template @std::atomic@. - -\CFA monitors have \newterm{multi-acquire} semantics so the thread in the monitor may acquire it multiple times without deadlock, allowing recursion and calling other interface functions. -\newpage -\begin{cfa} -monitor M { ... } m; -void foo( M & mutex m ) { ... } $\C{// acquire mutual exclusion}$ -void bar( M & mutex m ) { $\C{// acquire mutual exclusion}$ - ... `bar( m );` ... `foo( m );` ... $\C{// reacquire mutual exclusion}$ -} -\end{cfa} -\CFA monitors also ensure the monitor lock is released regardless of how an acquiring function ends (normal or exceptional), and returning a shared variable is safe via copying before the lock is released. +Aint x = { 0 }, y = { 0 }, z = { 5 }; // no mutex +++x; ++y; ++z; // mutex +x = 2; y = i; z = k; // mutex +i = x; j = y; k = z; // no mutex +\end{cfa} +\end{lrbox} + +\begin{lrbox}{\myboxB} +\begin{java}[aboveskip=0pt,belowskip=0pt] +class Aint { + private int cnt; + public Aint( int init ) { cnt = init; } + `synchronized` public int inc() { return ++cnt; } + `synchronized` public void set( int rhs ) {cnt=rhs;} + public int get() { return cnt; } +} +int i = 0, j = 0, k = 5; +Aint x=new Aint(0), y=new Aint(0), z=new Aint(5); +x.inc(); y.inc(); z.inc(); +x.set( 2 ); y.set( i ); z.set( k ); +i = x.get(); j = y.get(); k = z.get(); +\end{java} +\end{lrbox} + +\subfloat[\CFA]{\label{f:AtomicCounterCFA}\usebox\myboxA} +\hspace{3pt} +\vrule +\hspace{3pt} +\subfloat[Java]{\label{f:AtomicCounterJava}\usebox\myboxB} +\caption{Atomic counter} +\label{f:AtomicCounter} +\end{figure} + +Like Java, \CFA monitors have \newterm{multi-acquire} semantics so the thread in the monitor may acquire it multiple times without deadlock, allowing recursion and calling other interface functions. +% \begin{cfa} +% monitor M { ... } m; +% void foo( M & mutex m ) { ... } $\C{// acquire mutual exclusion}$ +% void bar( M & mutex m ) { $\C{// acquire mutual exclusion}$ +% ... `bar( m );` ... `foo( m );` ... $\C{// reacquire mutual exclusion}$ +% } +% \end{cfa} +\CFA monitors also ensure the monitor lock is released regardless of how an acquiring function ends, normal or exceptional, and returning a shared variable is safe via copying before the lock is released. Similar safety is offered by \emph{explicit} opt-in disciplines like \CC RAII versus the monitor \emph{implicit} language-enforced safety guarantee ensuring no programmer usage errors. -Furthermore, RAII mechanisms cannot handle complex synchronization within a monitor, where the monitor lock may not be released on function exit because it is passed to an unblocking thread; +However, RAII mechanisms cannot handle complex synchronization within a monitor, where the monitor lock may not be released on function exit because it is passed to an unblocking thread; RAII is purely a mutual-exclusion mechanism (see Section~\ref{s:Scheduling}). + +Both Java and \CFA use a keyword @mutex@/\lstinline[language=java]|synchronized| to designate functions that implicitly acquire/release the monitor lock on call/return providing mutual exclusion to the stared data. +Non-designated functions provide no mutual exclusion for read-only access or as an interface to a multi-step protocol requiring several steps of acquiring and releasing the monitor. +Monitor objects can be passed through multiple helper functions without acquiring mutual exclusion, until a designated function associated with the object is called. +\CFA designated functions are marked by an explicitly parameter-only pointer/reference qualifier @mutex@ (discussed further in Section\ref{s:MutexAcquisition}). +Whereas, Java designated members are marked with \lstinline[language=java]|synchronized| that applies to the implicit reference parameter @this@. +In the example, the increment and setter operations need mutual exclusion while the read-only getter operation can be non-mutex if reading the implementation is atomic. @@ -1771,7 +1825,8 @@ \end{tabular} \end{cquote} -The @dtype@ property prevents \emph{implicit} copy operations and the @is_monitor@ trait provides no \emph{explicit} copy operations, so monitors must be passed by reference (pointer). -Similarly, the function definitions ensures there is a mechanism to get (read) the monitor descriptor from its handle, and a special destructor to prevent deallocation if a thread using the shared data. +The @dtype@ property prevents \emph{implicit} copy operations and the @is_monitor@ trait provides no \emph{explicit} copy operations, so monitors must be passed by reference or pointer. +Similarly, the function definitions ensures there is a mechanism to read the monitor descriptor from its handle, and a special destructor to prevent deallocation if a thread is using the shared data. The custom monitor type also inserts any locks needed to implement the mutual exclusion semantics. +\CFA relies heavily on traits as an abstraction mechanism, so the @mutex@ qualifier prevents coincidentally matching of a monitor trait with a type that is not a monitor, similar to coincidental inheritance where a shape and playing card can both be drawable. @@ -1779,16 +1834,8 @@ \label{s:MutexAcquisition} -While the monitor lock provides mutual exclusion for shared data, there are implementation options for when and where the locking/unlocking occurs. -(Much of this discussion also applies to basic locks.) -For example, a monitor may be passed through multiple helper functions before it is necessary to acquire the monitor's mutual exclusion. - -\CFA requires programmers to identify the kind of parameter with the @mutex@ keyword and uses no keyword to mean \lstinline[morekeywords=nomutex]@nomutex@, because @mutex@ parameters are rare and no keyword is the \emph{normal} parameter semantics. -Hence, @mutex@ parameters are documentation, at the function and its prototype, to both programmer and compiler, without other redundant keywords. -Furthermore, \CFA relies heavily on traits as an abstraction mechanism, so the @mutex@ qualifier prevents coincidentally matching of a monitor trait with a type that is not a monitor, similar to coincidental inheritance where a shape and playing card can both be drawable. - -The next semantic decision is establishing which parameter \emph{types} may be qualified with @mutex@. -The following has monitor parameter types that are composed of multiple objects. -\begin{cfa} -monitor M { ... } +For object-oriented programming languages, the mutex property applies to one object, the implicit pointer/reference to the monitor type. +Because \CFA uses a pointer qualifier, other possibilities exist, \eg: +\begin{cfa} +monitor M { ... }; int f1( M & mutex m ); $\C{// single parameter object}$ int f2( M * mutex m ); $\C{// single or multiple parameter object}$ @@ -1796,14 +1843,10 @@ int f4( stack( M * ) & mutex m ); $\C{// multiple parameters object}$ \end{cfa} -Function @f1@ has a single parameter object, while @f2@'s indirection could be a single or multi-element array, where static array size is often unknown in C. -Function @f3@ has a multiple object matrix, and @f4@ a multiple object data structure. -While shown shortly, multiple object acquisition is possible, but the number of objects must be statically known. -Therefore, \CFA only acquires one monitor per parameter with exactly one level of indirection, and exclude pointer types to unknown sized arrays. - -For object-oriented monitors, \eg Java, calling a mutex member \emph{implicitly} acquires mutual exclusion of the receiver object, @`rec`.foo(...)@. -\CFA has no receiver, and hence, the explicit @mutex@ qualifier is used to specify which objects acquire mutual exclusion. -A positive consequence of this design decision is the ability to support multi-monitor functions,\footnote{ +Function @f1@ has a single object parameter, while functions @f2@ to @f4@ can be a single or multi-element parameter with statically unknown size. +Because of the statically unknown size, \CFA only supports a single reference @mutex@ parameter, @f1@. + +The \CFA @mutex@ qualifier does allow the ability to support multi-monitor functions,\footnote{ While object-oriented monitors can be extended with a mutex qualifier for multiple-monitor members, no prior example of this feature could be found.} -called \newterm{bulk acquire}. +where the number of acquisitions is statically known, called \newterm{bulk acquire}. \CFA guarantees bulk acquisition order is consistent across calls to @mutex@ functions using the same monitors as arguments, so acquiring multiple monitors in a bulk acquire is safe from deadlock. Figure~\ref{f:BankTransfer} shows a trivial solution to the bank transfer problem~\cite{BankTransfer}, where two resources must be locked simultaneously, using \CFA monitors with implicit locking and \CC with explicit locking. @@ -1933,5 +1976,5 @@ % There are many aspects of scheduling in a concurrency system, all related to resource utilization by waiting threads, \ie which thread gets the resource next. % Different forms of scheduling include access to processors by threads (see Section~\ref{s:RuntimeStructureCluster}), another is access to a shared resource by a lock or monitor. -This section discusses scheduling for waiting threads eligible for monitor entry, \ie which user thread gets the shared resource next. (See Section~\ref{s:RuntimeStructureCluster} for scheduling kernel threads on virtual processors.) +This section discusses scheduling for waiting threads eligible for monitor entry~\cite{Buhr95b}, \ie which user thread gets the shared resource next. (See Section~\ref{s:RuntimeStructureCluster} for scheduling kernel threads on virtual processors.) While monitor mutual-exclusion provides safe access to its shared data, the data may indicate a thread cannot proceed, \eg a bounded buffer may be full/\-empty so produce/consumer threads must block. Leaving the monitor and retrying (busy waiting) is impractical for high-level programming. @@ -1939,10 +1982,10 @@ Monitors eliminate busy waiting by providing synchronization within the monitor critical-section to schedule threads needing access to the shared data, where threads block versus spin. Synchronization is generally achieved with internal~\cite{Hoare74} or external~\cite[\S~2.9.2]{uC++} scheduling. -\newterm{Internal} (largely) schedules threads located \emph{inside} the monitor and is accomplished using condition variables with signal and wait. -\newterm{External} (largely) schedules threads located \emph{outside} the monitor and is accomplished with the @waitfor@ statement. -Note, internal scheduling has a small amount of external scheduling and vice versus, so the naming denotes where the majority of the block threads reside (inside or outside) for scheduling. -For complex scheduling, the approaches can be combined, so there can be an equal number of threads waiting inside and outside. - -\CFA monitors do not allow calling threads to barge ahead of signalled threads (via barging prevention), which simplifies synchronization among threads in the monitor and increases correctness. +\newterm{Internal} largely schedules threads located \emph{inside} the monitor and is accomplished using condition variables with signal and wait. +\newterm{External} largely schedules threads located \emph{outside} the monitor and is accomplished with the @waitfor@ statement. +Note, internal scheduling has a small amount of external scheduling and vice versa, so the naming denotes where the majority of the block threads reside (inside or outside) for scheduling. +For complex scheduling, the approaches can be combined, so there are threads waiting inside and outside. + +\CFA monitors do not allow calling threads to barge ahead of signalled threads via barging prevention, which simplifies synchronization among threads in the monitor and increases correctness. A direct consequence of this semantics is that unblocked waiting threads are not required to recheck the waiting condition, \ie waits are not in a starvation-prone busy-loop as required by the signals-as-hints style with barging. Preventing barging comes directly from Hoare's semantics in the seminal paper on monitors~\cite[p.~550]{Hoare74}. @@ -1953,15 +1996,15 @@ Furthermore, \CFA concurrency has no spurious wakeup~\cite[\S~9]{Buhr05a}, which eliminates an implicit self barging. -Monitor mutual-exclusion means signalling cannot have the signaller and signalled thread in the monitor simultaneously, so only the signaller or signallee can proceed. -Figure~\ref{f:MonitorScheduling} shows internal/external scheduling for the bounded-buffer examples in Figure~\ref{f:GenericBoundedBuffer}. -For internal scheduling in Figure~\ref{f:BBInt}, the @signal@ moves the signallee (front thread of the specified condition queue) to urgent and the signaller continues (solid line). +Monitor mutual-exclusion means signalling cannot have the signaller and signalled thread in the monitor simultaneously, so only the signaller or signallee can proceed and the other waits on an implicit urgent list~\cite[p.~551]{Hoare74}. +Figure~\ref{f:MonitorScheduling} shows internal and external scheduling for the bounded-buffer examples in Figure~\ref{f:GenericBoundedBuffer}. +For internal scheduling in Figure~\ref{f:BBInt}, the @signal@ moves the signallee, front thread of the specified condition queue, to the urgent list (see Figure~\ref{f:MonitorScheduling}) and the signaller continues (solid line). Multiple signals move multiple signallees to urgent until the condition queue is empty. -When the signaller exits or waits, a thread is implicitly unblocked from urgent (if available) before unblocking a calling thread to prevent barging. +When the signaller exits or waits, a thread is implicitly unblocked from urgent, if available, before unblocking a calling thread to prevent barging. (Java conceptually moves the signalled thread to the calling queue, and hence, allows barging.) -Signal is used when the signaller is providing the cooperation needed by the signallee (\eg creating an empty slot in a buffer for a producer) and the signaller immediately exits the monitor to run concurrently (consume the buffer element) and passes control of the monitor to the signalled thread, which can immediately take advantage of the state change. +Signal is used when the signaller is providing the cooperation needed by the signallee, \eg creating an empty slot in a buffer for a producer, and the signaller immediately exits the monitor to run concurrently consuming the buffer element, and passes control of the monitor to the signalled thread, which can immediately take advantage of the state change. Specifically, the @wait@ function atomically blocks the calling thread and implicitly releases the monitor lock(s) for all monitors in the function's parameter list. Signalling is unconditional because signalling an empty condition queue does nothing. It is common to declare condition queues as monitor fields to prevent shared access, hence no locking is required for access as the queues are protected by the monitor lock. -In \CFA, a condition queue can be created/stored independently. +In \CFA, a condition queue can be created and stored independently. \begin{figure} @@ -2049,9 +2092,9 @@ \end{figure} -The @signal_block@ provides the opposite unblocking order, where the signaller is moved to urgent and the signallee continues and a thread is implicitly unblocked from urgent when the signallee exits or waits (dashed line). -Signal block is used when the signallee is providing the cooperation needed by the signaller (\eg if the buffer is removed and a producer hands off an item to a consumer, as in Figure~\ref{f:DatingSignalBlock}) so the signaller must wait until the signallee unblocks, provides the cooperation, exits the monitor to run concurrently, and passes control of the monitor to the signaller, which can immediately take advantage of the state change. +The @signal_block@ provides the opposite unblocking order, where the signaller is moved to urgent and the signallee continues and a thread is implicitly unblocked from urgent when the signallee exits or waits (dashed line)~\cite[p.~551]{Hoare74}. +Signal block is used when the signallee is providing the cooperation needed by the signaller, \eg if the buffer is removed and a producer hands off an item to a consumer as in Figure~\ref{f:DatingSignalBlock}, so the signaller must wait until the signallee unblocks, provides the cooperation, exits the monitor to run concurrently, and passes control of the monitor to the signaller, which can immediately take advantage of the state change. Using @signal@ or @signal_block@ can be a dynamic decision based on whether the thread providing the cooperation arrives before or after the thread needing the cooperation. -External scheduling in Figure~\ref{f:BBExt} simplifies internal scheduling by eliminating condition queues and @signal@/@wait@ (cases where it cannot are discussed shortly), and has existed in the programming language Ada for almost 40 years with variants in other languages~\cite{SR,ConcurrentC++,uC++}. +For external scheduling in Figure~\ref{f:BBExt}, the internal scheduling is replaced, eliminating condition queues and @signal@/@wait@ (cases where it cannot are discussed shortly), and has existed in the programming language Ada for almost 40 years with variants in other languages~\cite{SR,ConcurrentC++,uC++}. While prior languages use external scheduling solely for thread interaction, \CFA generalizes it to both monitors and threads. External scheduling allows waiting for events from other threads while restricting unrelated events, that would otherwise have to wait on condition queues in the monitor. @@ -2062,8 +2105,8 @@ Now when a producer/consumer detects a full/empty buffer, the necessary cooperation for continuation is specified by indicating the next function call that can occur. For example, a producer detecting a full buffer must have cooperation from a consumer to remove an item so function @remove@ is accepted, which prevents producers from entering the monitor, and after a consumer calls @remove@, the producer waiting on urgent is \emph{implicitly} unblocked because it can now continue its insert operation. -Hence, this mechanism is done in terms of control flow, next call, versus in terms of data, channels, as in Go/Rust @select@. +Hence, this mechanism is done in terms of control flow, next call, versus in terms of data, channels, as in Go and Rust @select@. While both mechanisms have strengths and weaknesses, \CFA uses the control-flow mechanism to be consistent with other language features. -Figure~\ref{f:ReadersWriterLock} shows internal/external scheduling for a readers/writer lock with no barging and threads are serviced in FIFO order to eliminate staleness/freshness among the reader/writer threads. +Figure~\ref{f:ReadersWriterLock} shows internal and external scheduling for a readers/writer lock with no barging and threads are serviced in FIFO order to eliminate staleness and freshness among the reader/writer threads. For internal scheduling in Figure~\ref{f:RWInt}, the readers and writers wait on the same condition queue in FIFO order, making it impossible to tell if a waiting thread is a reader or writer. To clawback the kind of thread, a \CFA condition can store user data in the node for a blocking thread at the @wait@, \ie whether the thread is a @READER@ or @WRITER@. @@ -2229,14 +2272,21 @@ For signal scheduling, the @exchange@ condition is necessary to block the thread finding the match, while the matcher unblocks to take the opposite number, post its phone number, and unblock the partner. For signal-block scheduling, the implicit urgent-queue replaces the explicit @exchange@-condition and @signal_block@ puts the finding thread on the urgent stack and unblocks the matcher. - -The dating service is an important example of a monitor that cannot be written using external scheduling. -First, because scheduling requires knowledge of calling parameters to make matching decisions, and parameters of calling threads are unavailable within the monitor. -For example, a girl thread within the monitor cannot examine the @ccode@ of boy threads waiting on the calling queue to determine if there is a matching partner. -Second, because a scheduling decision may be delayed when there is no immediate match, which requires a condition queue for waiting, and condition queues imply internal scheduling. -For example, if a girl thread could determine there is no calling boy with the same @ccode@, it must wait until a matching boy arrives. -Finally, barging corrupts the dating service during an exchange because a barger may also match and change the phone numbers, invalidating the previous exchange phone number. +Note, barging corrupts the dating service during an exchange because a barger may also match and change the phone numbers, invalidating the previous exchange phone number. This situation shows rechecking the waiting condition and waiting again (signals-as-hints) fails, requiring significant restructured to account for barging. -Both internal and external scheduling extend to multiple monitors in a natural way. +Given external and internal scheduling, what guidelines can a programmer use to select between them? +In general, external scheduling is easier to understand and code because only the next logical action (mutex function(s)) is stated, and the monitor implicitly handles all the details. +Therefore, there are no condition variables, and hence, no wait and signal, which reduces coding complexity and synchronization errors. +If external scheduling is simpler than internal, why not use it all the time? +Unfortunately, external scheduling cannot be used if: scheduling depends on parameter value(s) or scheduling must block across an unknown series of calls on a condition variable, \ie internal scheduling. +For example, the dating service cannot be written using external scheduling. +First, scheduling requires knowledge of calling parameters to make matching decisions and parameters of calling threads are unavailable within the monitor. +Specifically, a thread within the monitor cannot examine the @ccode@ of threads waiting on the calling queue to determine if there is a matching partner. +(Similarly, if the bounded buffer or readers/writer are restructured with a single interface function with a parameter denoting producer/consumer or reader/write, they cannot be solved with external scheduling.) +Second, a scheduling decision may be delayed across an unknown number of calls when there is no immediate match so the thread in the monitor must block on a condition. +Specifically, if a thread determines there is no opposite calling thread with the same @ccode@, it must wait an unknown period until a matching thread arrives. +For complex synchronization, both external and internal scheduling can be used to take advantage of best of properties of each. + +Finally, both internal and external scheduling extend to multiple monitors in a natural way. \begin{cquote} \begin{tabular}{@{}l@{\hspace{2\parindentlnth}}l@{}} @@ -2274,7 +2324,7 @@ Similarly, for @waitfor( rtn )@, the default semantics is to atomically block the acceptor and release all acquired mutex parameters, \ie @waitfor( rtn : m1, m2 )@. To override the implicit multi-monitor wait, specific mutex parameter(s) can be specified, \eg @waitfor( rtn : m1 )@. -@waitfor@ does statically verify the monitor types passed are the same as the acquired mutex-parameters of the given function or function pointer, hence the function (pointer) prototype must be accessible. +@waitfor@ does statically verify the monitor types passed are the same as the acquired mutex-parameters of the given function or function pointer, hence the prototype must be accessible. % When an overloaded function appears in an @waitfor@ statement, calls to any function with that name are accepted. -% The rationale is that members with the same name should perform a similar function, and therefore, all should be eligible to accept a call. +% The rationale is that functions with the same name should perform a similar actions, and therefore, all should be eligible to accept a call. Overloaded functions can be disambiguated using a cast \begin{cfa} @@ -2285,5 +2335,4 @@ The ability to release a subset of acquired monitors can result in a \newterm{nested monitor}~\cite{Lister77} deadlock (see Section~\ref{s:MutexAcquisition}). -\newpage \begin{cfa} void foo( M & mutex m1, M & mutex m2 ) { @@ -2300,13 +2349,13 @@ Figure~\ref{f:ExtendedWaitfor} shows the extended form of the @waitfor@ statement to conditionally accept one of a group of mutex functions, with an optional statement to be performed \emph{after} the mutex function finishes. -For a @waitfor@ clause to be executed, its @when@ must be true and an outstanding call to its corresponding member(s) must exist. +For a @waitfor@ clause to be executed, its @when@ must be true and an outstanding call to its corresponding function(s) must exist. The \emph{conditional-expression} of a @when@ may call a function, but the function must not block or context switch. -If there are multiple acceptable mutex calls, selection occurs top-to-bottom (prioritized) among the @waitfor@ clauses, whereas some programming languages with similar mechanisms accept nondeterministically for this case, \eg Go \lstinline[morekeywords=select]@select@. -If some accept guards are true and there are no outstanding calls to these members, the acceptor is blocked until a call to one of these members is made. +If there are multiple acceptable mutex calls, selection is prioritized top-to-bottom among the @waitfor@ clauses, whereas some programming languages with similar mechanisms accept nondeterministically for this case, \eg Go \lstinline[morekeywords=select]@select@. +If some accept guards are true and there are no outstanding calls to these functions, the acceptor is blocked until a call to one of these functions is made. If there is a @timeout@ clause, it provides an upper bound on waiting. If all the accept guards are false, the statement does nothing, unless there is a terminating @else@ clause with a true guard, which is executed instead. Hence, the terminating @else@ clause allows a conditional attempt to accept a call without blocking. If both @timeout@ and @else@ clause are present, the @else@ must be conditional, or the @timeout@ is never triggered. -There is also a traditional future wait queue (not shown) (\eg Microsoft @WaitForMultipleObjects@), to wait for a specified number of future elements in the queue. +% There is also a traditional future wait queue (not shown) (\eg Microsoft @WaitForMultipleObjects@), to wait for a specified number of future elements in the queue. Finally, there is a shorthand for specifying multiple functions using the same set of monitors: @waitfor( f, g, h : m1, m2, m3 )@. @@ -2315,7 +2364,7 @@ \begin{cfa} `when` ( $\emph{conditional-expression}$ ) $\C{// optional guard}$ - waitfor( $\emph{mutex-member-name}$ ) $\emph{statement}$ $\C{// action after call}$ + waitfor( $\emph{mutex-function-name}$ ) $\emph{statement}$ $\C{// action after call}$ `or` `when` ( $\emph{conditional-expression}$ ) $\C{// any number of functions}$ - waitfor( $\emph{mutex-member-name}$ ) $\emph{statement}$ + waitfor( $\emph{mutex-function-name}$ ) $\emph{statement}$ `or` ... `when` ( $\emph{conditional-expression}$ ) $\C{// optional guard}$ @@ -2335,4 +2384,5 @@ The left example only accepts @mem1@ if @C1@ is true or only @mem2@ if @C2@ is true. The right example accepts either @mem1@ or @mem2@ if @C1@ and @C2@ are true. +Hence, the @waitfor@ has parallel semantics, accepting any true @when@ clause. An interesting use of @waitfor@ is accepting the @mutex@ destructor to know when an object is deallocated, \eg assume the bounded buffer is restructured from a monitor to a thread with the following @main@. @@ -2432,6 +2482,6 @@ If W1 waited first, the signaller must retain @m1@ amd @m2@ until completion of the outer mutex statement and then pass both to W1. % Furthermore, there is an execution sequence where the signaller always finds waiter W2, and hence, waiter W1 starves. -To support this efficient semantics (and prevent barging), the implementation maintains a list of monitors acquired for each blocked thread. -When a signaller exits or waits in a monitor function/statement, the front waiter on urgent is unblocked if all its monitors are released. +To support this efficient semantics and prevent barging, the implementation maintains a list of monitors acquired for each blocked thread. +When a signaller exits or waits in a mutex function or statement, the front waiter on urgent is unblocked if all its monitors are released. Implementing a fast subset check for the necessary released monitors is important and discussed in the following sections. % The benefit is encapsulating complexity into only two actions: passing monitors to the next owner when they should be released and conditionally waking threads if all conditions are met. @@ -2442,5 +2492,5 @@ In a statically-typed object-oriented programming language, a class has an exhaustive list of members, even when members are added via static inheritance (see Figure~\ref{f:uCinheritance}). -Knowing all members at compilation (even separate compilation) allows uniquely numbered them so the accept-statement implementation can use a fast/compact bit mask with $O(1)$ compare. +Knowing all members at compilation, even separate compilation, allows uniquely numbered them so the accept-statement implementation can use a fast and compact bit mask with $O(1)$ compare. \begin{figure} @@ -2493,5 +2543,5 @@ Hence, function pointers are used to identify the functions listed in the @waitfor@ statement, stored in a variable-sized array. Then, the same implementation approach used for the urgent stack (see Section~\ref{s:Scheduling}) is used for the calling queue. -Each caller has a list of monitors acquired, and the @waitfor@ statement performs a (short) linear search matching functions in the @waitfor@ list with called functions, and then verifying the associated mutex locks can be transfers. +Each caller has a list of monitors acquired, and the @waitfor@ statement performs a short linear search matching functions in the @waitfor@ list with called functions, and then verifying the associated mutex locks can be transfers. @@ -2571,5 +2621,5 @@ struct Msg { int i, j; }; -monitor thread GoRtn { int i; float f; Msg m; }; +mutex thread GoRtn { int i; float f; Msg m; }; void mem1( GoRtn & mutex gortn, int i ) { gortn.i = i; } void mem2( GoRtn & mutex gortn, float f ) { gortn.f = f; } @@ -2577,5 +2627,5 @@ void ^?{}( GoRtn & mutex ) {} -void main( GoRtn & gortn ) with( gortn ) { // thread starts +void main( GoRtn & mutex gortn ) with(gortn) { // thread starts for () { @@ -2644,5 +2694,5 @@ \begin{cfa} -monitor thread DatingService { +mutex thread DatingService { condition Girls[CompCodes], Boys[CompCodes]; int girlPhoneNo, boyPhoneNo, ccode; @@ -2708,13 +2758,13 @@ % \label{f:pingpong} % \end{figure} -Note, the ping/pong threads are globally declared, @pi@/@po@, and hence, start (and possibly complete) before the program main starts. +Note, the ping/pong threads are globally declared, @pi@/@po@, and hence, start and possibly complete before the program main starts. \end{comment} -\subsection{\texorpdfstring{\protect\lstinline@monitor@ Generators / Coroutines / Threads}{monitor Generators / Coroutines / Threads}} - -\CFA generators, coroutines, and threads can also be monitors (Table~\ref{t:ExecutionPropertyComposition} cases 4, 6, 12) allowing safe \emph{direct communication} with threads, \ie the custom types can have mutex functions that are called by other threads. +\subsection{\texorpdfstring{\protect\lstinline@mutex@ Generators / Coroutines / Threads}{monitor Generators / Coroutines / Threads}} + +\CFA generators, coroutines, and threads can also be @mutex@ (Table~\ref{t:ExecutionPropertyComposition} cases 4, 6, 12) allowing safe \emph{direct communication} with threads, \ie the custom types can have mutex functions that are called by other threads. All monitor features are available within these mutex functions. -For example, if the formatter generator (or coroutine equivalent) in Figure~\ref{f:CFAFormatGen} is extended with the monitor property and this interface function is used to communicate with the formatter: +For example, if the formatter generator or coroutine equivalent in Figure~\ref{f:CFAFormatGen} is extended with the monitor property and this interface function is used to communicate with the formatter: \begin{cfa} void fmt( Fmt & mutex fmt, char ch ) { fmt.ch = ch; resume( fmt ) } @@ -2724,11 +2774,15 @@ Figure~\ref{f:DirectCommunicationComparison} shows a comparison of direct call-communication in \CFA versus indirect channel-communication in Go. (Ada has a similar mechanism to \CFA direct communication.) -The program thread in \CFA @main@ uses the call/return paradigm to directly communicate with the @GoRtn main@, whereas Go switches to the channel paradigm to indirectly communicate with the goroutine. +% The thread main function is by default @mutex@, so the @mutex@ qualifier for the thread parameter is optional. +% The reason is that the thread logically starts instantaneously in the thread main acquiring its mutual exclusion, so it starts before any calls to prepare for synchronizing these calls. +The \CFA program @main@ uses the call/return paradigm to directly communicate with the @GoRtn main@, whereas Go switches to the unbuffered channel paradigm to indirectly communicate with the goroutine. Communication by multiple threads is safe for the @gortn@ thread via mutex calls in \CFA or channel assignment in Go. +The different between call and channel send occurs for buffered channels making the send asynchronous. +In \CFA, asynchronous call and multiple buffers is provided using an administrator and worker threads~\cite{Gentleman81} and/or futures (not discussed). Figure~\ref{f:DirectCommunicationDatingService} shows the dating-service problem in Figure~\ref{f:DatingServiceMonitor} extended from indirect monitor communication to direct thread communication. -When converting a monitor to a thread (server), the coding pattern is to move as much code as possible from the accepted members into the thread main so it does an much work as possible. +When converting a monitor to a thread (server), the coding pattern is to move as much code as possible from the accepted functions into the thread main so it does an much work as possible. Notice, the dating server is postponing requests for an unspecified time while continuing to accept new requests. -For complex servers (web-servers), there can be hundreds of lines of code in the thread main and safe interaction with clients can be complex. +For complex servers, \eg web-servers, there can be hundreds of lines of code in the thread main and safe interaction with clients can be complex. @@ -2768,5 +2822,5 @@ In contrast to direct threading is indirect \newterm{thread pools}, \eg Java @executor@, where small jobs (work units) are inserted into a work pool for execution. -If the jobs are dependent, \ie interact, there is an implicit/explicit dependency graph that ties them together. +If the jobs are dependent, \ie interact, there is an implicit dependency graph that ties them together. While removing direct concurrency, and hence the amount of context switching, thread pools significantly limit the interaction that can occur among jobs. Indeed, jobs should not block because that also blocks the underlying thread, which effectively means the CPU utilization, and therefore throughput, suffers. @@ -2857,5 +2911,5 @@ \label{s:RuntimeStructureProcessor} -A virtual processor is implemented by a kernel thread (\eg UNIX process), which are scheduled for execution on a hardware processor by the underlying operating system. +A virtual processor is implemented by a kernel thread, \eg UNIX process, which are scheduled for execution on a hardware processor by the underlying operating system. Programs may use more virtual processors than hardware processors. On a multiprocessor, kernel threads are distributed across the hardware processors resulting in virtual processors executing in parallel. @@ -2872,5 +2926,5 @@ \label{s:Implementation} -A primary implementation challenge is avoiding contention from dynamically allocating memory because of bulk acquire, \eg the internal-scheduling design is (almost) free of allocations. +A primary implementation challenge is avoiding contention from dynamically allocating memory because of bulk acquire, \eg the internal-scheduling design is almost free of allocations. All blocking operations are made by parking threads onto queues, therefore all queues are designed with intrusive nodes, where each node has preallocated link fields for chaining. Furthermore, several bulk-acquire operations need a variable amount of memory. @@ -2918,5 +2972,5 @@ There are two versions of the \CFA runtime kernel: debug and non-debug. -The debugging version has many runtime checks and internal assertions, \eg stack (non-writable) guard page, and checks for stack overflow whenever context switches occur among coroutines and threads, which catches most stack overflows. +The debugging version has many runtime checks and internal assertions, \eg stack non-writable guard page, and checks for stack overflow whenever context switches occur among coroutines and threads, which catches most stack overflows. After a program is debugged, the non-debugging version can be used to significantly decrease space and increase performance. @@ -2926,5 +2980,5 @@ To test the performance of the \CFA runtime, a series of microbenchmarks are used to compare \CFA with pthreads, Java 11.0.6, Go 1.12.6, Rust 1.37.0, Python 3.7.6, Node.js 12.14.1, and \uC 7.0.0. -For comparison, the package must be multi-processor (M:N), which excludes libdill/libmil~\cite{libdill} (M:1)), and use a shared-memory programming model, \eg not message passing. +For comparison, the package must be multi-processor (M:N), which excludes libdil and /libmil~\cite{libdill} (M:1)), and use a shared-memory programming model, \eg not message passing. The benchmark computer is an AMD Opteron\texttrademark\ 6380 NUMA 64-core, 8 socket, 2.5 GHz processor, running Ubuntu 16.04.6 LTS, and pthreads/\CFA/\uC are compiled with gcc 9.2.1. @@ -2937,72 +2991,57 @@ The total time is divided by @N@ to obtain the average time for a benchmark. Each benchmark experiment is run 13 times and the average appears in the table. -All omitted tests for other languages are functionally identical to the \CFA tests and available online~\cite{CforallBenchMarks}. +All omitted tests for other languages are functionally identical to the \CFA tests and available online~\cite{CforallConcurrentBenchmarks}. % tar --exclude-ignore=exclude -cvhf benchmark.tar benchmark - -\paragraph{Context Switching} - -In procedural programming, the cost of a function call is important as modularization (refactoring) increases. -(In many cases, a compiler inlines function calls to increase the size and number of basic blocks for optimizing.) -Similarly, when modularization extends to coroutines/threads, the time for a context switch becomes a relevant factor. -The coroutine test is from resumer to suspender and from suspender to resumer, which is two context switches. -%For async-await systems, the test is scheduling and fulfilling @N@ empty promises, where all promises are allocated before versus interleaved with fulfillment to avoid garbage collection. -For async-await systems, the test measures the cost of the @await@ expression entering the event engine by awaiting @N@ promises, where each created promise is resolved by an immediate event in the engine (using Node.js @setImmediate@). -The thread test is using yield to enter and return from the runtime kernel, which is two context switches. -The difference in performance between coroutine and thread context-switch is the cost of scheduling for threads, whereas coroutines are self-scheduling. -Figure~\ref{f:ctx-switch} shows the \CFA code for a coroutine/thread with results in Table~\ref{t:ctx-switch}. - -% From: Gregor Richards -% To: "Peter A. Buhr" -% Date: Fri, 24 Jan 2020 13:49:18 -0500 -% -% I can also verify that the previous version, which just tied a bunch of promises together, *does not* go back to the -% event loop at all in the current version of Node. Presumably they're taking advantage of the fact that the ordering of -% events is intentionally undefined to just jump right to the next 'then' in the chain, bypassing event queueing -% entirely. That's perfectly correct behavior insofar as its difference from the specified behavior isn't observable, but -% it isn't typical or representative of much anything useful, because most programs wouldn't have whole chains of eager -% promises. Also, it's not representative of *anything* you can do with async/await, as there's no way to encode such an -% eager chain that way. +% cp -p benchmark.tar /u/cforall/public_html/doc/concurrent_benchmark.tar + +\paragraph{Creation} + +Creation is measured by creating and deleting a specific kind of control-flow object. +Figure~\ref{f:creation} shows the code for \CFA with results in Table~\ref{t:creation}. +Note, the call stacks of \CFA coroutines are lazily created on the first resume, therefore the cost of creation with and without a stack are presented. \begin{multicols}{2} \lstset{language=CFA,moredelim=**[is][\color{red}]{@}{@},deletedelim=**[is][]{`}{`}} -\begin{cfa}[aboveskip=0pt,belowskip=0pt] -@coroutine@ C {}; -void main( C & ) { for () { @suspend;@ } } -int main() { // coroutine test - C c; - BENCH( for ( N ) { @resume( c );@ } ) +\begin{cfa} +@coroutine@ MyCoroutine {}; +void ?{}( MyCoroutine & this ) { +#ifdef EAGER + resume( this ); +#endif +} +void main( MyCoroutine & ) {} +int main() { + BENCH( for ( N ) { @MyCoroutine c;@ } ) sout | result; } -int main() { // thread test - BENCH( for ( N ) { @yield();@ } ) - sout | result; -} -\end{cfa} -\captionof{figure}{\CFA context-switch benchmark} -\label{f:ctx-switch} +\end{cfa} +\captionof{figure}{\CFA creation benchmark} +\label{f:creation} \columnbreak \vspace*{-16pt} -\captionof{table}{Context switch comparison (nanoseconds)} -\label{t:ctx-switch} -\begin{tabular}{@{}r*{3}{D{.}{.}{3.2}}@{}} -\multicolumn{1}{@{}c}{} & \multicolumn{1}{c}{Median} &\multicolumn{1}{c}{Average} & \multicolumn{1}{c@{}}{Std Dev} \\ -C function & 1.8 & 1.8 & 0.0 \\ -\CFA generator & 1.8 & 2.0 & 0.3 \\ -\CFA coroutine & 32.5 & 32.9 & 0.8 \\ -\CFA thread & 93.8 & 93.6 & 2.2 \\ -\uC coroutine & 50.3 & 50.3 & 0.2 \\ -\uC thread & 97.3 & 97.4 & 1.0 \\ -Python generator & 40.9 & 41.3 & 1.5 \\ -Node.js generator & 32.6 & 32.2 & 1.0 \\ -Node.js await & 1852.2 & 1854.7 & 16.4 \\ -Goroutine thread & 143.0 & 143.3 & 1.1 \\ -Rust thread & 332.0 & 331.4 & 2.4 \\ -Java thread & 405.0 & 415.0 & 17.6 \\ -Pthreads thread & 334.3 & 335.2 & 3.9 +\captionof{table}{Creation comparison (nanoseconds)} +\label{t:creation} + +\begin{tabular}[t]{@{}r*{3}{D{.}{.}{5.2}}@{}} +\multicolumn{1}{@{}c}{} & \multicolumn{1}{c}{Median} & \multicolumn{1}{c}{Average} & \multicolumn{1}{c@{}}{Std Dev} \\ +\CFA generator & 0.6 & 0.6 & 0.0 \\ +\CFA coroutine lazy & 13.4 & 13.1 & 0.5 \\ +\CFA coroutine eager & 144.7 & 143.9 & 1.5 \\ +\CFA thread & 466.4 & 468.0 & 11.3 \\ +\uC coroutine & 155.6 & 155.7 & 1.7 \\ +\uC thread & 523.4 & 523.9 & 7.7 \\ +Python generator & 123.2 & 124.3 & 4.1 \\ +Node.js generator & 33.4 & 33.5 & 0.3 \\ +Goroutine thread & 751.0 & 750.5 & 3.1 \\ +Rust tokio thread & 1860.0 & 1881.1 & 37.6 \\ +Rust thread & 53801.0 & 53896.8 & 274.9 \\ +Java thread & 120274.0 & 120722.9 & 2356.7 \\ +Pthreads thread & 31465.5 & 31419.5 & 140.4 \end{tabular} \end{multicols} +\vspace*{-10pt} \paragraph{Internal Scheduling} @@ -3036,4 +3075,5 @@ } \end{cfa} +\vspace*{-8pt} \captionof{figure}{\CFA Internal-scheduling benchmark} \label{f:schedint} @@ -3104,6 +3144,6 @@ \paragraph{Mutual-Exclusion} -Uncontented mutual exclusion, which frequently occurs, is measured by entering/leaving a critical section. -For monitors, entering and leaving a monitor function is measured, otherwise the language-appropriate mutex-lock is measured. +Uncontented mutual exclusion, which frequently occurs, is measured by entering and leaving a critical section. +For monitors, entering and leaving a mutex function is measured, otherwise the language-appropriate mutex-lock is measured. For comparison, a spinning (versus blocking) test-and-test-set lock is presented. Figure~\ref{f:mutex} shows the code for \CFA with results in Table~\ref{t:mutex}. @@ -3142,48 +3182,68 @@ \end{multicols} -\paragraph{Creation} - -Creation is measured by creating/deleting a specific kind of control-flow object. -Figure~\ref{f:creation} shows the code for \CFA with results in Table~\ref{t:creation}. -Note, the call stacks of \CFA coroutines are lazily created on the first resume, therefore the cost of creation with and without a stack are presented. +\paragraph{Context Switching} + +In procedural programming, the cost of a function call is important as modularization (refactoring) increases. +(In many cases, a compiler inlines function calls to increase the size and number of basic blocks for optimizing.) +Similarly, when modularization extends to coroutines and threads, the time for a context switch becomes a relevant factor. +The coroutine test is from resumer to suspender and from suspender to resumer, which is two context switches. +%For async-await systems, the test is scheduling and fulfilling @N@ empty promises, where all promises are allocated before versus interleaved with fulfillment to avoid garbage collection. +For async-await systems, the test measures the cost of the @await@ expression entering the event engine by awaiting @N@ promises, where each created promise is resolved by an immediate event in the engine (using Node.js @setImmediate@). +The thread test is using yield to enter and return from the runtime kernel, which is two context switches. +The difference in performance between coroutine and thread context-switch is the cost of scheduling for threads, whereas coroutines are self-scheduling. +Figure~\ref{f:ctx-switch} shows the \CFA code for a coroutine and thread with results in Table~\ref{t:ctx-switch}. + +% From: Gregor Richards +% To: "Peter A. Buhr" +% Date: Fri, 24 Jan 2020 13:49:18 -0500 +% +% I can also verify that the previous version, which just tied a bunch of promises together, *does not* go back to the +% event loop at all in the current version of Node. Presumably they're taking advantage of the fact that the ordering of +% events is intentionally undefined to just jump right to the next 'then' in the chain, bypassing event queueing +% entirely. That's perfectly correct behavior insofar as its difference from the specified behavior isn't observable, but +% it isn't typical or representative of much anything useful, because most programs wouldn't have whole chains of eager +% promises. Also, it's not representative of *anything* you can do with async/await, as there's no way to encode such an +% eager chain that way. \begin{multicols}{2} \lstset{language=CFA,moredelim=**[is][\color{red}]{@}{@},deletedelim=**[is][]{`}{`}} -\begin{cfa} -@coroutine@ MyCoroutine {}; -void ?{}( MyCoroutine & this ) { -#ifdef EAGER - resume( this ); -#endif -} -void main( MyCoroutine & ) {} -int main() { - BENCH( for ( N ) { @MyCoroutine c;@ } ) +\begin{cfa}[aboveskip=0pt,belowskip=0pt] +@coroutine@ C {}; +void main( C & ) { for () { @suspend;@ } } +int main() { // coroutine test + C c; + BENCH( for ( N ) { @resume( c );@ } ) sout | result; } -\end{cfa} -\captionof{figure}{\CFA creation benchmark} -\label{f:creation} +int main() { // thread test + BENCH( for ( N ) { @yield();@ } ) + sout | result; +} +\end{cfa} +\captionof{figure}{\CFA context-switch benchmark} +\label{f:ctx-switch} \columnbreak \vspace*{-16pt} -\captionof{table}{Creation comparison (nanoseconds)} -\label{t:creation} - -\begin{tabular}[t]{@{}r*{3}{D{.}{.}{5.2}}@{}} -\multicolumn{1}{@{}c}{} & \multicolumn{1}{c}{Median} & \multicolumn{1}{c}{Average} & \multicolumn{1}{c@{}}{Std Dev} \\ -\CFA generator & 0.6 & 0.6 & 0.0 \\ -\CFA coroutine lazy & 13.4 & 13.1 & 0.5 \\ -\CFA coroutine eager & 144.7 & 143.9 & 1.5 \\ -\CFA thread & 466.4 & 468.0 & 11.3 \\ -\uC coroutine & 155.6 & 155.7 & 1.7 \\ -\uC thread & 523.4 & 523.9 & 7.7 \\ -Python generator & 123.2 & 124.3 & 4.1 \\ -Node.js generator & 32.3 & 32.2 & 0.3 \\ -Goroutine thread & 751.0 & 750.5 & 3.1 \\ -Rust thread & 53801.0 & 53896.8 & 274.9 \\ -Java thread & 120274.0 & 120722.9 & 2356.7 \\ -Pthreads thread & 31465.5 & 31419.5 & 140.4 +\captionof{table}{Context switch comparison (nanoseconds)} +\label{t:ctx-switch} +\begin{tabular}{@{}r*{3}{D{.}{.}{3.2}}@{}} +\multicolumn{1}{@{}c}{} & \multicolumn{1}{c}{Median} &\multicolumn{1}{c}{Average} & \multicolumn{1}{c@{}}{Std Dev} \\ +C function & 1.8 & 1.8 & 0.0 \\ +\CFA generator & 1.8 & 2.0 & 0.3 \\ +\CFA coroutine & 32.5 & 32.9 & 0.8 \\ +\CFA thread & 93.8 & 93.6 & 2.2 \\ +\uC coroutine & 50.3 & 50.3 & 0.2 \\ +\uC thread & 97.3 & 97.4 & 1.0 \\ +Python generator & 40.9 & 41.3 & 1.5 \\ +Node.js await & 1852.2 & 1854.7 & 16.4 \\ +Node.js generator & 33.3 & 33.4 & 0.3 \\ +Goroutine thread & 143.0 & 143.3 & 1.1 \\ +Rust async await & 32.0 & 32.0 & 0.0 \\ +Rust tokio thread & 143.0 & 143.0 & 1.7 \\ +Rust thread & 332.0 & 331.4 & 2.4 \\ +Java thread & 405.0 & 415.0 & 17.6 \\ +Pthreads thread & 334.3 & 335.2 & 3.9 \end{tabular} \end{multicols} @@ -3192,5 +3252,5 @@ \subsection{Discussion} -Languages using 1:1 threading based on pthreads can at best meet or exceed (due to language overhead) the pthread results. +Languages using 1:1 threading based on pthreads can at best meet or exceed, due to language overhead, the pthread results. Note, pthreads has a fast zero-contention mutex lock checked in user space. Languages with M:N threading have better performance than 1:1 because there is no operating-system interactions. @@ -3200,5 +3260,5 @@ -\section{Conclusion} +\section{Conclusions and Future Work} Advanced control-flow will always be difficult, especially when there is temporal ordering and nondeterminism. @@ -3207,22 +3267,18 @@ Combining these properties creates a number of high-level, efficient, and maintainable control-flow types: generator, coroutine, thread, each of which can be a monitor. Eliminated from \CFA are barging and spurious wakeup, which are nonintuitive and lead to errors, and having to work with a bewildering set of low-level locks and acquisition techniques. -\CFA high-level race-free monitors and threads provide the core mechanisms for mutual exclusion and synchronization, without having to resort to magic qualifiers like @volatile@/@atomic@. +\CFA high-level race-free monitors and threads, when used with mutex access function, provide the core mechanisms for mutual exclusion and synchronization, without having to resort to magic qualifiers like @volatile@ or @atomic@. Extending these mechanisms to handle high-level deadlock-free bulk acquire across both mutual exclusion and synchronization is a unique contribution. The \CFA runtime provides concurrency based on a preemptive M:N user-level threading-system, executing in clusters, which encapsulate scheduling of work on multiple kernel threads providing parallelism. The M:N model is judged to be efficient and provide greater flexibility than a 1:1 threading model. These concepts and the \CFA runtime-system are written in the \CFA language, extensively leveraging the \CFA type-system, which demonstrates the expressiveness of the \CFA language. -Performance comparisons with other concurrent systems/languages show the \CFA approach is competitive across all basic operations, which translates directly into good performance in well-written applications with advanced control-flow. +Performance comparisons with other concurrent systems and languages show the \CFA approach is competitive across all basic operations, which translates directly into good performance in well-written applications with advanced control-flow. C programmers should feel comfortable using these mechanisms for developing complex control-flow in applications, with the ability to obtain maximum available performance by selecting mechanisms at the appropriate level of need using only calling communication. - -\section{Future Work} - While control flow in \CFA has a strong start, development is still underway to complete a number of missing features. -\paragraph{Flexible Scheduling} -\label{futur:sched} - +\medskip +\textbf{Flexible Scheduling:} An important part of concurrency is scheduling. -Different scheduling algorithms can affect performance (both in terms of average and variation). +Different scheduling algorithms can affect performance, both in terms of average and variation. However, no single scheduler is optimal for all workloads and therefore there is value in being able to change the scheduler for given programs. One solution is to offer various tuning options, allowing the scheduler to be adjusted to the requirements of the workload. @@ -3230,7 +3286,6 @@ Currently, the \CFA pluggable scheduler is too simple to handle complex scheduling, \eg quality of service and real-time, where the scheduler must interact with mutex objects to deal with issues like priority inversion~\cite{Buhr00b}. -\paragraph{Non-Blocking I/O} -\label{futur:nbio} - +\smallskip +\textbf{Non-Blocking I/O:} Many modern workloads are not bound by computation but IO operations, common cases being web servers and XaaS~\cite{XaaS} (anything as a service). These types of workloads require significant engineering to amortizing costs of blocking IO-operations. @@ -3241,7 +3296,6 @@ A non-blocking I/O library is currently under development for \CFA. -\paragraph{Other Concurrency Tools} -\label{futur:tools} - +\smallskip +\textbf{Other Concurrency Tools:} While monitors offer flexible and powerful concurrency for \CFA, other concurrency tools are also necessary for a complete multi-paradigm concurrency package. Examples of such tools can include futures and promises~\cite{promises}, executors and actors. @@ -3249,8 +3303,7 @@ As well, new \CFA extensions should make it possible to create a uniform interface for virtually all mutual exclusion, including monitors and low-level locks. -\paragraph{Implicit Threading} -\label{futur:implcit} - -Basic concurrent (embarrassingly parallel) applications can benefit greatly from implicit concurrency, where sequential programs are converted to concurrent, possibly with some help from pragmas to guide the conversion. +\smallskip +\textbf{Implicit Threading:} +Basic \emph{embarrassingly parallel} applications can benefit greatly from implicit concurrency, where sequential programs are converted to concurrent, with some help from pragmas to guide the conversion. This type of concurrency can be achieved both at the language level and at the library level. The canonical example of implicit concurrency is concurrent nested @for@ loops, which are amenable to divide and conquer algorithms~\cite{uC++book}. Index: doc/papers/concurrency/mail2 =================================================================== --- doc/papers/concurrency/mail2 (revision a5873bd9568f82230e97ba50cf4e33da0f932488) +++ doc/papers/concurrency/mail2 (revision 97392b6998d47706885e39064f5f23e53dbe7496) @@ -512,2 +512,424 @@ Software: Practice and Experience Editorial Office + + +Date: Sat, 18 Apr 2020 10:42:13 +0000 +From: Richard Jones +Reply-To: R.E.Jones@kent.ac.uk +To: tdelisle@uwaterloo.ca, pabuhr@uwaterloo.ca +Subject: Software: Practice and Experience - Decision on Manuscript ID + SPE-19-0219.R1 + +18-Apr-2020 + +Dear Dr Buhr, + +Many thanks for submitting SPE-19-0219.R1 entitled "Advanced Control-flow and Concurrency in Cforall" to Software: Practice and Experience. The paper has now been reviewed and the comments of the referees are included at the bottom of this letter. + +I believe that we are making progress here towards a paper that can be published in Software: Practice and Experience. However the referees still have significant concerns about the paper. The journal's focus is on practice and experience, and one of the the reviewers' concerns remains that your submission should focus the narrative more on the perspective of the programmer than the language designer. I agree that this would strengthen your submission, and I ask you to address this as well as the referees' other comments. + +A revised version of your manuscript that takes into account the comments of the referee(s) will be reconsidered for publication. + +Please note that submitting a revision of your manuscript does not guarantee eventual acceptance, and that your revision may be subject to re-review by the referees before a decision is rendered. + +You have 90 days from the date of this email to submit your revision. If you are unable to complete the revision within this time, please contact me to request a short extension. + +You can upload your revised manuscript and submit it through your Author Center. Log into https://mc.manuscriptcentral.com/spe and enter your Author Center, where you will find your manuscript title listed under "Manuscripts with Decisions". + +When submitting your revised manuscript, you will be able to respond to the comments made by the referee(s) in the space provided. You can use this space to document any changes you make to the original manuscript. + +If you would like help with English language editing, or other article preparation support, Wiley Editing Services offers expert help with English Language Editing, as well as translation, manuscript formatting, and figure formatting at www.wileyauthors.com/eeo/preparation. You can also check out our resources for Preparing Your Article for general guidance about writing and preparing your manuscript at www.wileyauthors.com/eeo/prepresources. + +Once again, thank you for submitting your manuscript to Software: Practice and Experience and I look forward to receiving your revision. + +Sincerely, +Richard + +Prof. Richard Jones +Software: Practice and Experience +R.E.Jones@kent.ac.uk + + +Referee(s)' Comments to Author: + +Reviewing: 1 + +Comments to the Author +(A relatively short second review) + +I thank the authors for their revisions and comprehensive response to +reviewers' comments --- many of my comments have been successfully +addressed by the revisions. Here I'll structure my comments around +the main salient points in that response which I consider would +benefit from further explanation. + +> Table 1 is moved to the start and explained in detail. + +I consider this change makes a significant improvement to the paper, +laying out the landscape of language features at the start, and thus +addresses my main concerns about the paper. + +I still have a couple of issues --- perhaps the largest is that it's +still not clear at this point in the paper what some of these options +are, or crucially how they would be used. I don't know if it's +possbile to give high-level examples or use cases to be clear about +these up front - or if that would duplicate too much information from +later in the paper - either way expanding out the discussion - even if +just two a couple of sentences for each row - would help me more. The +point is not just to define these categories but to ensure the +readers' understanding of these definitons agrees with that used in +the paper. + +in a little more detail: + + * 1st para section 2 begs the question: why not support each + dimension independently, and let the programmer or library designer + combiine features? + + * "execution state" seems a relatively low-level description here. + I don't think of e.g. the lambda calculus that way. Perhaps it's as + good a term as any. + + * Why must there "be language mechanisms to create, block/unblock, + and join with a thread"? There aren't in Smalltalk (although there + are in the runtime). Especially given in Cforall those mechanisms + are *implicit* on thread creation and destruction? + + * "Case 1 is a function that borrows storage for its state (stack + frame/activation) and a thread from its invoker" + + this much makes perfect sense to me, but I don't understand how a + non-stateful, non-theaded function can then retain + + "this state across callees, ie, function local-variables are + retained on the stack across calls." + + how can it retain function-local values *across calls* when it + doesn't have any functional-local state? + + I'm not sure if I see two separate cases here - rougly equivalent + to C functions without static storage, and then C functions *with* + static storage. I assumed that was the distinction between cases 1 + & 3; but perhpas the actual distinction is that 3 has a + suspend/resume point, and so the "state" in figure 1 is this + component of execution state (viz figs 1 & 2), not the state + representing the cross-call variables? + +> but such evaluation isn't appropriate for garbage-collected or JITTed + languages like Java or Go. + +For JITTed languages in particular, reporting peak performance needs +to "warm up" the JIT with a number of iterators before beginning +measurement. Actually for JIT's its even worse: see Edd Barrett et al +OOPSLA 2017. + + + +minor issues: + + * footnote A - I've looked at various other papers & the website to + try to understand how "object-oriented" Cforall is - I'm still not + sure. This footnote says Cforall has "virtuals" - presumably + virtual functions, i.e. dynamic dispatch - and inheritance: that + really is OO as far as I (and most OO people) are concerned. For + example Haskell doesn't have inheritance, so it's not OO; while + CLOS (the Common Lisp *Object* System) or things like Cecil and + Dylan are considered OO even though they have "multiple function + parameters as receivers", lack "lexical binding between a structure + and set of functions", and don't have explicit receiver invocation + syntax. Python has receiver syntax, but unlike Java or Smalltalk + or C++, method declarations still need to have an explicit "self" + receiver parameter. Seems to me that Go, for example, is + more-or-less OO with interfaces, methods, and dynamic dispatch (yes + also and an explicit receiver syntax but that's not + determiniative); while Rust lacks dynamic dispatch built-in. C is + not OO as a language, but as you say given it supports function + pointers with structures, it does support an OO programm style. + + This is why I again recommend just not buying into this fight: not + making any claims about whether Cforall is OO or is not - because + as I see it, the rest of the paper doesn't depend on whether + Cforall is OO or not. That said: this is just a recommendation, + and I won't quibble over this any further. + + * is a "monitor function" the same as a "mutex function"? + if so the paper should pick one term; if not, make the distinction clear. + + + * "As stated on line 1 because state declarations from the generator + type can be moved out of the coroutine type into the coroutine main" + + OK sure, but again: *why* would a programmer want to do that? + (Other than, I guess, to show the difference between coroutines & + generators?) Perhaps another way to put this is that the first + para of 3.2 gives the disadvantages of coroutines vs-a-vs + generators, briefly describes the extended semantics, but never + actualy says why a programmer may want those extended semantics, + or how they would benefit. I don't mean to belabour the point, + but (generalist?) readers like me would generally benefit from + those kinds of discussions about each feature throughout the + paper: why might a programmer want to use them? + + +> p17 if the multiple-monitor entry procedure really is novel, write a paper +> about that, and only about that. + +> We do not believe this is a practical suggestion. + + * I'm honestly not trying to be snide here: I'm not an expert on + monitor or concurrent implementations. Brinch Hansen's original + monitors were single acquire; this draft does not cite any other + previous work that I could see. I'm not suggesting that the brief + mention of this mechanism necessarily be removed from this paper, + but if this is novel (and a clear advance over a classical OO + monitor a-la Java which only acquires the distinguished reciever) + then that would be worth another paper in itself. + +> * conclusion should conclude the paper, not the related. +> We do not understand this comment.if ithis + +My typo: the paper's conclusion should come at the end, after the +future work section. + + + + +To encourage accountability, I'm signing my reviews in 2020. +For the record, I am James Noble, kjx@ecs.vuw.ac.nz. + + +Reviewing: 2 + +Comments to the Author +I thank the authors for their detailed response. To respond to a couple of points raised in response to my review (number 2): + +- on the Boehm paper and whether code is "all sequential to the compiler": I now understand the authors' position better and suspect we are in violent agreement, except for whether it's appropriate to use the rather breezy phrase "all sequential to the compiler". It would be straightforward to clarify that code not using the atomics features is optimized *as if* it were sequential, i.e. on the assumption of a lack of data races. + +- on the distinction between "mutual exclusion" and "synchronization": the added citation does help, in that it makes a coherent case for the definition the authors prefer. However, the text could usefully clarify that this is a matter of definition not of fact, given especially that in my assessment the authors' preferred definition is not the most common one. (Although the mention of Hoare's apparent use of this definition is one data point, countervailing ones are found in many contemporaneous or later papers, e.g. Habermann's 1972 "Synchronization of Communicating Processes" (CACM 15(3)), Reed & Kanodia's 1979 "Synchronization with eventcounts and sequencers" (CACM (22(2)) and so on.) + +I am glad to see that the authors have taken on board most of the straightforward improvements I suggested. + +However, a recurring problem of unclear writing still remains through many parts of the paper, including much of sections 2, 3 and 6. To highlight a couple of problem patches (by no means exhaustive): + +- section 2 (an expanded version of what was previously section 5.9) lacks examples and is generally obscure and allusory ("the most advanced feature" -- name it! "in triplets" -- there is only one triplet!; what are "execution locations"? "initialize" and "de-initialize" what? "borrowed from the invoker" is a concept in need of explaining or at least a fully explained example -- in what sense does a plain function borrow" its stack frame? "computation only" as opposed to what? in 2.2, in what way is a "request" fundamental to "synchronization"? and the "implicitly" versus "explicitly" point needs stating as elsewhere, with a concrete example e.g. Java built-in mutexes versus java.util.concurrent). + +- section 6: 6.2 omits the most important facts in preference for otherwise inscrutable detail: "identify the kind of parameter" (first say *that there are* kinds of parameter, and what "kinds" means!); "mutex parameters are documentation" is misleading (they are also semantically significant!) and fails to say *what* they mean; the most important thing is surely that 'mutex' is a language feature for performing lock/unlock operations at function entry/exit. So say it! The meanings of examples f3 and f4 remain unclear. Meanwhile in 6.3, "urgent" is not introduced (we are supposed to infer its meaning from Figure 12, but that Figure is incomprehensible to me), and we are told of "external scheduling"'s long history in Ada but not clearly what it actually means; 6.4's description of "waitfor" tells us it is different from an if-else chain but tries to use two *different* inputs to tell us that the behavior is different; tell us an instance where *the same* values of C1 and C2 give different behavior (I even wrote out a truth table and still don't see the semantic difference) + +The authors frequently use bracketed phrases, and sometimes slashes "/", in ways that are confusing and/or detrimental to readability. Page 13 line 2's "forward (backward)" is one particularly egregious example. In general I would recommend the the authors try to limit their use of parentheses and slashes as a means of forcing a clearer wording to emerge. Also, the use of "eg." is often cursory and does not explain the examples given, which are frequently a one- or two-word phrase of unclear referent. + +Considering the revision more broadly, none of the more extensive or creative rewrites I suggested in my previous review have been attempted, nor any equivalent efforts to improve its readability. The hoisting of the former section 5.9 is a good idea, but the newly added material accompanying it (around Table 1) suffers fresh deficiencies in clarity. Overall the paper is longer than before, even though (as my previous review stated), I believe a shorter paper is required in order to serve the likely purpose of publication. (Indeed, the authors' letter implies that a key goal of publication is to build community and gain external users.) + +Given this trajectory, I no longer see a path to an acceptable revision of the present submission. Instead I suggest the authors consider splitting the paper in two: one half about coroutines and stack management, the other about mutexes, monitors and the runtime. (A briefer presentation of the runtime may be helpful in the first paper also, and a brief recap of the generator and coroutine support is obviously needed in the second too.) Both of these new papers would need to be written with a strong emphasis on clarity, paying great care to issues of structure, wording, choices of example, and restraint (saying what's important, not everything that could be said). I am confident the authors could benefit from getting early feedback from others at their institution. For the performance experiments, of course these do not split evenly -- most (but not all) belong in the second of these two hypothetical papers. But the first of them would still have plenty of meat to it; for me, a clear and thorough study of the design space around coroutines is the most interesting and tantalizing prospect. + +I do not buy the authors' defense of the limited practical experience or "non-micro" benchmarking presented. Yes, gaining external users is hard and I am sympathetic on that point. But building something at least *somewhat* substantial with your own system should be within reach, and without it the "practice and experience" aspects of the work have not been explored. Clearly C\/ is the product of a lot of work over an extended period, so it is a surprise that no such experience is readily available for inclusion. + +Some smaller points: + +It does not seem right to state that a stack is essential to Von Neumann architectures -- since the earliest Von Neumann machines (and indeed early Fortran) did not use one. + +To elaborate on something another reviewer commented on: it is a surprise to find a "Future work" section *after* the "Conclusion" section. A "Conclusions and future work" section often works well. + + +Reviewing: 3 + +Comments to the Author +This is the second round of reviewing. + +As in the first review, I found that the paper (and Cforall) contains +a lot of really interesting ideas, but it remains really difficult to +have a good sense of which idea I should use and when. This applies in +different ways to different features from the language: + +* coroutines/generators/threads: here there is + some discussion, but it can be improved. +* interal/external scheduling: I didn't find any direct comparison + between these features, except by way of example. + +I requested similar things in my previous review and I see that +content was added in response to those requests. Unfortunately, I'm +not sure that I can say it improved the paper's overall read. I think +in some sense the additions were "too much" -- I would have preferred +something more like a table or a few paragraphs highlighting the key +reasons one would pick one construct or the other. + +In general, I do wonder if the paper is just trying to do too much. +The discussion of clusters and pre-emption in particular feels quite +rushed. + +## Summary + +I make a number of suggestions below but the two most important +I think are: + +* Recommend to shorten the comparison on coroutine/generator/threads + in Section 2 to a paragraph with a few examples, or possibly a table + explaining the trade-offs between the constructs +* Recommend to clarify the relationship between internal/external + scheduling -- is one more general but more error-prone or low-level? + +## Coroutines/generators/threads + +There is obviously a lot of overlap between these features, and in +particular between coroutines and generators. As noted in the previous +review, many languages have chosen to offer *only* generators, and to +build coroutines by stacks of generators invoking one another. + +I believe the newly introduced Section 2 of the paper is trying to +motivate why each of these constructs exist, but I did not find it +effective. It was dense and difficult to understand. I think the +problem is that Section 2 seems to be trying to derive "from first +principles" why each construct exists, but I think that a more "top +down" approach would be easier to understand. + +In fact, the end of Section 2.1 (on page 5) contains a particular +paragraph that embodies this "top down" approach. It starts, +"programmers can now answer three basic questions", and thus gives +some practical advice for which construct you should use and when. I +think giving some examples of specific applications that this +paragraph, combined with some examples of cases where each construct +was needed, would be a better approach. + +I don't think this compariosn needs to be very long. It seems clear +enough that one would + +* prefer generators for simple computations that yield up many values, +* prefer coroutines for more complex processes that have significant + internal structure, +* prefer threads for cases where parallel execution is desired or + needed. + +I did appreciate the comparison in Section 2.3 between async-await in +JS/Java and generators/coroutines. I agree with its premise that those +mechanisms are a poor replacement for generators (and, indeed, JS has +a distinct generator mechanism, for example, in part for this reason). +I believe I may have asked for this in a previous review, but having +read it, I wonder if it is really necessary, since those mechanisms +are so different in purpose. + +## Internal vs external scheduling + +I find the motivation for supporting both internal and external +scheduling to be fairly implicit. After several reads through the +section, I came to the conclusion that internal scheduling is more +expressive than external scheduling, but sometimes less convenient or +clear. Is this correct? If not, it'd be useful to clarify where +external scheduling is more expressive. + +The same is true, I think, of the `signal_block` function, which I +have not encountered before; it seems like its behavior can be modeled +with multiple condition variables, but that's clearly more complex. + +One question I had about `signal_block`: what happens if one signals +but no other thread is waiting? Does it block until some other thread +waits? Or is that user error? + +I would find it very interesting to try and capture some of the +properties that make internal vs external scheduling the better +choice. + +For example, it seems to me that external scheduling works well if +there are only a few "key" operations, but that internal scheduling +might be better otherwise, simply because it would be useful to have +the ability to name a signal that can be referenced by many +methods. Consider the bounded buffer from Figure 13: if it had +multiple methods for removing elements, and not just `remove`, then +the `waitfor(remove)` call in `insert` might not be sufficient. + +## Comparison of external scheduling to messaging + +I did enjoy the section comparing external scheduling to Go's +messaging mechanism, which I believe is a new addition. + +I believe that one difference between the Go program and the Cforall +equivalent is that the Goroutine has an associated queue, so that +multiple messages could be enqueued, whereas the Cforall equivalent is +effectively a "bounded buffer" of length 1. Is that correct? I think +this should be stated explicitly. (Presumably, one could modify the +Cforall program to include an explicit vector of queued messages if +desired, but you would also be reimplementing the channel +abstraction.) + +Also, in Figure 20, I believe that there is a missing `mutex` keyword. +The fiugre states: + +``` +void main(GoRtn & gortn) with(gortn) { +``` + +but I think it should probably be as follows: + +``` +void main(GoRtn & mutex gortn) with(gortn) { +``` + +Unless there is some implicit `mutex` associated with being a main +function for a `monitor thread`. + +## Atomic operations and race freedom + +I was glad to see that the paper acknowledged that Cforall still had +low-level atomic operations, even if their use is discouraged in favor +of higher-level alternatives. + +However, I still feel that the conclusion overstates the value of the +contribution here when it says that "Cforall high-level race-free +monitors and threads provide the core mechanisms for mutual exclusion +and synchronization, without the need for volatile and atomics". I +feel confident that Java programmers, for example, would be advised to +stick with synchronized methods whenever possible, and it seems to me +that they offer similar advantages -- but they sometimes wind up using +volatiles for performance reasons. + +I was also confused by the term "race-free" in that sentence. In +particular, I don't think that Cforall has any mechanisms for +preventing *data races*, and it clearly doesn't prevent "race +conditions" (which would bar all sorts of useful programs). I suppose +that "race free" here might be referring to the improvements such as +removing barging behavior. + +## Performance comparisons + +In my previous review, I requested comparisons against Rust and +node.js, and I see that the new version of the paper includes both, +which is a good addition. + +One note on the Rust results: I believe that the results are comparing +against the threads found in Rust's standard library, which are +essentially a shallow wrapper around pthreads, and hence the +performance is quite close to pthread performance (as one would +expect). It would perhaps be more interesting to see a comparison +built using [tokio] or [async-std], two of the more prominent +user-space threading libraries that build on Rust's async-await +feature (which operates quite differently than Javascript's +async-await, in that it doesn't cause every aync function call to +schedule a distinct task). + +[tokio]: https://tokio.rs/ +[async-std]: https://async.rs/ + +That said, I am satisfied with the performance results as they are in +the current revision. + +## Minor notes and typos + +Several figures used the `with` keyword. I deduced that `with(foo)` +permits one to write `bar` instead of `foo.bar`. It seems worth +introducing. Apologies if this is stated in the paper, if so I missed +it. + +On page 20, section 6.3, "external scheduling and vice versus" should be +"external scheduling and vice versa". + +On page 5, section 2.3, the paper states "we content" but it should be +"we contend". + +Reviewing: Editor + +A few small comments in addition to those of the referees. + +Page 1. I don't believe that it s fair to imply that Scala is "research vehicle" as it is used by major players, Twitter being the most prominent example. + +Page 15. Must Cforall threads start after construction (e.g. see your example on page 15, line 21)? I can think of examples where it is not desirable that threads start immediately after construction, e.g. a game with N players, each of whom is expensive to create, but all of whom should be started at the same time. + +Page 18, line 17: is using + Index: doc/papers/concurrency/response2 =================================================================== --- doc/papers/concurrency/response2 (revision 97392b6998d47706885e39064f5f23e53dbe7496) +++ doc/papers/concurrency/response2 (revision 97392b6998d47706885e39064f5f23e53dbe7496) @@ -0,0 +1,1013 @@ +Reviewing: 1 + + I still have a couple of issues --- perhaps the largest is that it's + still not clear at this point in the paper what some of these options + are, or crucially how they would be used. I don't know if it's + possible to give high-level examples or use cases to be clear about + these up front - or if that would duplicate too much information from + later in the paper - either way expanding out the discussion - even if + just two a couple of sentences for each row - would help me more. + +Section 2.1 is changed to address this suggestion. + + + * 1st para section 2 begs the question: why not support each + dimension independently, and let the programmer or library designer + combine features? + +As seen in Table 1, not all of the combinations work, and having programmers +directly use these low-level mechanisms is error prone. Accessing these +fundamental mechanisms through higher-level constructs has always been the +purpose of a programming language. + + + * Why must there "be language mechanisms to create, block/unblock, and join + with a thread"? There aren't in Smalltalk (although there are in the + runtime). Especially given in Cforall those mechanisms are *implicit* on + thread creation and destruction? + +The best description of Smalltalk concurrency I can find is in J. Hunt, +Smalltalk and Object Orientation, Springer-Verlag London Limited, 1997, Chapter +31 Concurrency in Smalltalk. It states on page 332: + + For a process to be spawned from the current process there must be some way + of creating a new process. This is done using one of four messages to a + block. These messages are: + + aBlock fork: This creates and schedules a process which will execute the + block. The priority of this process is inherited from the parent process. + ... + + The Semaphore class provides facilities for achieving simple synchronization, + it is simple because it only allows for two forms of communication signal and + wait. + +Hence, "aBlock fork" creates, "Semaphore" blocks/unblocks (as does message send +to an aBlock object), and garbage collection of an aBlock joins with its +thread. The fact that a programmer *implicitly* does "fork", "block"/"unblock", +and "join", does not change their fundamental requirement. + + + * "Case 1 is a function that borrows storage for its state (stack + frame/activation) and a thread from its invoker" + + this much makes perfect sense to me, but I don't understand how a + non-stateful, non-threaded function can then retain + + "this state across callees, ie, function local-variables are + retained on the stack across calls." + + how can it retain function-local values *across calls* when it + doesn't have any functional-local state? + +In the following example: + + void foo() { + // local variables and code + } + void bar() { + // local variables + foo(); + } + +bar is the caller and foo is the callee. bar borrows the program stack and +thread to make the call to foo. When foo, the callee, is executing, bar's local +variables (state) is retained on the *borrowed* stack across the call. (Note, I +added *borrowed* to that sentence in the paper to help clarify.) Furthermore, +foo's local variables are also retain on the borrowed stack. When foo and bar +return, all of their local state is gone (not retained). This behaviour is +standard call/return semantics in an imperative language. + + + I'm not sure if I see two separate cases here - roughly equivalent + to C functions without static storage, and then C functions *with* + static storage. + +Yes, but there is only one instance of the static storage across all +activations of the C function. For generators and coroutines, each instance has +its own state, like an object in an OO language. + + + I assumed that was the distinction between cases 1 & 3; but perhaps the + actual distinction is that 3 has a suspend/resume point, and so the + "state" in figure 1 is this component of execution state (viz figs 1 & 2), + not the state representing the cross-call variables? + +So case 3 is like an object with the added ability to retain where it was last +executing. When a generator is resumed, the generator object (structure +instance) is passed as an explicit reference, and within this object is the +restart location in the generator's "main". When the generator main executes, +it uses the borrowed stack for its local variables and any functions it calls, +just like an object member borrows the stack for its local variables but also +has an implicit receiver to the object state. A generator can have static +storage, too, which is a single instance across all generator instances of that +type, as for static storage in an object type. All the kinds of storage are +at play with semantics that is virtually the same as in other languages. + + + > but such evaluation isn't appropriate for garbage-collected or JITTed + > languages like Java or Go. + For JITTed languages in particular, reporting peak performance needs to + "warm up" the JIT with a number of iterators before beginning + measurement. Actually for JIT's its even worse: see Edd Barrett et al + OOPSLA 2017. + +Of our testing languages, only Java and Node.js are JITTED. To ensure the Java +test-programs correctly measured the specific feature, we consulted with Dave +Dice at Oracle who works directly on the development of the Oracle JVM +Just-in-Time Compiler. We modified our test programs based on his advise, and +he validated our programs as correctly measuring the specified language +feature. Hence, we have taken into account all issues related to performing +benchmarks in JITTED languages. Dave's help is recognized in the +Acknowledgment section. Also, all the benchmark programs are publicly available +for independent verification. + +Similarly, we verified our Node.js programs with Gregor Richards, an expert in +just-in-time compilation for dynamic typing. + + + * footnote A - I've looked at various other papers & the website to try to + understand how "object-oriented" Cforall is - I'm still not sure. This + footnote says Cforall has "virtuals" - presumably virtual functions, + i.e. dynamic dispatch - and inheritance: that really is OO as far as I + (and most OO people) are concerned. For example Haskell doesn't have + inheritance, so it's not OO; while CLOS (the Common Lisp *Object* System) + or things like Cecil and Dylan are considered OO even though they have + "multiple function parameters as receivers", lack "lexical binding between + a structure and set of functions", and don't have explicit receiver + invocation syntax. Python has receiver syntax, but unlike Java or + Smalltalk or C++, method declarations still need to have an explicit + "self" receiver parameter. Seems to me that Go, for example, is + more-or-less OO with interfaces, methods, and dynamic dispatch (yes also + and an explicit receiver syntax but that's not determinative); while Rust + lacks dynamic dispatch built-in. C is not OO as a language, but as you + say given it supports function pointers with structures, it does support + an OO programming style. + + This is why I again recommend just not buying into this fight: not making + any claims about whether Cforall is OO or is not - because as I see it, + the rest of the paper doesn't depend on whether Cforall is OO or not. + That said: this is just a recommendation, and I won't quibble over this + any further. + +We believe it is important to identify Cforall as a non-OO language because it +heavily influences the syntax and semantics used to build its concurrency. +Since many aspects of Cforall are not OO, the rest of the paper *does* depend +on Cforall being identified as non-OO, otherwise readers would have +significantly different expectations for the design. We believe your definition +of OO is too broad, such as including C. Just because a programming language +can support aspects of the OO programming style, does not make it OO. (Just +because a duck can swim does not make it a fish.) + +Our definition of non-OO follows directly from the Wikipedia entry: + + Object-oriented programming (OOP) is a programming paradigm based on the + concept of "objects", which can contain data, in the form of fields (often + known as attributes or properties), and code, in the form of procedures + (often known as methods). A feature of objects is an object's procedures that + can access and often modify the data fields of the object with which they are + associated (objects have a notion of "this" or "self"). + https://en.wikipedia.org/wiki/Object-oriented_programming + +Cforall fails this definition as code cannot appear in an "object" and there is +no implicit receiver. As well, Cforall, Go, and Rust do not have nominal +inheritance and they not considered OO languages, e.g.: + + "**Is Go an object-oriented language?** Yes and no. Although Go has types and + methods and allows an object-oriented style of programming, there is no type + hierarchy. The concept of "interface" in Go provides a different approach + that we believe is easy to use and in some ways more general. There are also + ways to embed types in other types to provide something analogous-but not + identical-to subclassing. Moreover, methods in Go are more general than in + C++ or Java: they can be defined for any sort of data, even built-in types + such as plain, "unboxed" integers. They are not restricted to structs (classes). + https://golang.org/doc/faq#Is_Go_an_object-oriented_language + + + * is a "monitor function" the same as a "mutex function"? + if so the paper should pick one term; if not, make the distinction clear. + +Fixed. Picked "mutex". Changed the language and all places in the paper. + + + * "As stated on line 1 because state declarations from the generator + type can be moved out of the coroutine type into the coroutine main" + + OK sure, but again: *why* would a programmer want to do that? + (Other than, I guess, to show the difference between coroutines & + generators?) Perhaps another way to put this is that the first + para of 3.2 gives the disadvantages of coroutines vs-a-vs + generators, briefly describes the extended semantics, but never + actually says why a programmer may want those extended semantics, + or how they would benefit. I don't mean to belabour the point, + but (generalist?) readers like me would generally benefit from + those kinds of discussions about each feature throughout the + paper: why might a programmer want to use them? + +On page 8, it states: + + Having to manually create the generator closure by moving local-state + variables into the generator type is an additional programmer burden (removed + by the coroutine in Section 3.2). ... + +also these variables can now be refactored into helper function, where the +helper function suspends at arbitrary call depth. So imagine a coroutine helper +function that is only called occasionally within the coroutine but it has a +large array that is retained across suspends within the helper function. For a +generator, this large array has to be declared in the generator type enlarging +each generator instance even through the array is only used occasionally. +Whereas, the coroutine only needs the array allocated when needed. Now a +coroutine has a stack which occupies storage, but the maximum stack size only +needs to be the call chain allocating the most storage, where as the generator +has a maximum size of all variable that could be created. + + + > p17 if the multiple-monitor entry procedure really is novel, write a paper + > about that, and only about that. + > We do not believe this is a practical suggestion. + * I'm honestly not trying to be snide here: I'm not an expert on monitor or + concurrent implementations. Brinch Hansen's original monitors were single + acquire; this draft does not cite any other previous work that I could + see. I'm not suggesting that the brief mention of this mechanism + necessarily be removed from this paper, but if this is novel (and a clear + advance over a classical OO monitor a-la Java which only acquires the + distinguished receiver) then that would be worth another paper in itself. + +First, to explain multiple-monitor entry in Cforall as a separate paper would +require significant background on Cforall concurrency, which means repeating +large sections of this paper. Second, it feels like a paper just on +multiple-monitor entry would be a little thin, even if the capability is novel +to the best of our knowledge. Third, we feel multiple-monitor entry springs +naturally from the overarching tone in the paper that Cforall is a non-OO +programming language, allowing multiple-mutex receivers. + + + My typo: the paper's conclusion should come at the end, after the + future work section. + +Combined into a Conclusions and Future Work section. + + + +Reviewing: 2 + + on the Boehm paper and whether code is "all sequential to the compiler": I + now understand the authors' position better and suspect we are in violent + agreement, except for whether it's appropriate to use the rather breezy + phrase "all sequential to the compiler". It would be straightforward to + clarify that code not using the atomics features is optimized *as if* it + were sequential, i.e. on the assumption of a lack of data races. + +Fixed, "as inline and library code is compiled as sequential without any +explicit concurrent directive." + + + on the distinction between "mutual exclusion" and "synchronization": the + added citation does help, in that it makes a coherent case for the + definition the authors prefer. However, the text could usefully clarify + that this is a matter of definition not of fact, given especially that in + my assessment the authors' preferred definition is not the most common + one. (Although the mention of Hoare's apparent use of this definition is + one data point, countervailing ones are found in many contemporaneous or + later papers, e.g. Habermann's 1972 "Synchronization of Communicating + Processes" (CACM 15(3)), Reed & Kanodia's 1979 "Synchronization with + eventcounts and sequencers" (CACM (22(2)) and so on.) + +Fixed, "We contend these two properties are independent, ...". + +With respect to the two papers, Habermann fundamentally agrees with our +definitions, where the term mutual exclusion is the same but Habermann uses the +term "communication" for our synchronization. However, the term "communication" +is rarely used to mean synchronization. The fact that Habermann collectively +calls these two mechanisms synchronization is the confusion. Reed & Kanodia +state: + + By mutual exclusion we mean any mechanism that forces the time ordering of + execution of pieces of code, called critical regions, in a system of + concurrent processes to be a total ordering. + +But there is no timing order for a critical region (which I assume means +critical section); threads can arrive at any time in any order, where the +mutual exclusion for the critical section ensures one thread executes at a +time. Interestingly, Reed & Kanodia's critical region is Habermann's +communication not critical section. These papers only buttress our contention +about the confusion of these terms in the literature. + + + section 2 (an expanded version of what was previously section 5.9) lacks + examples and is generally obscure and allusory ("the most advanced feature" + -- name it! "in triplets" -- there is only one triplet!; + +Fixed. + + These independent properties can be used to compose different language + features, forming a compositional hierarchy, where the combination of all + three is the most advanced feature, called a thread/task/process. While it is + possible for a language to only provide threads for composing + programs~\cite{Hermes90}, this unnecessarily complicates and makes inefficient + solutions to certain classes of problems. + + + what are "execution locations"? "initialize" and "de-initialize" + +Fixed through an example at the end of the sentence. + + \item[\newterm{execution state}:] is the state information needed by a + control-flow feature to initialize, manage compute data and execution + location(s), and de-initialize, \eg calling a function initializes a stack + frame including contained objects with constructors, manages local data in + blocks and return locations during calls, and de-initializes the frame by + running any object destructors and management operations. + + + what? "borrowed from the invoker" is a concept in need of explaining or at + least a fully explained example -- in what sense does a plain function + borrow" its stack frame? + +A function has no storage except for static storage; it gets its storage from +the program stack. For a function to run it must borrow storage from somewhere. +When the function returns, the borrowed storage is returned. Do you have a more +appropriate word? + + "computation only" as opposed to what? + +This is a term in concurrency for operations that compute without blocking, +i.e., the operation starts with everything it needs to compute its result and +runs to completion, blocking only when it is done and returns its result. + + + in 2.2, in what way is a "request" fundamental to "synchronization"? + +I assume you are referring to the last bullet. + + Synchronization must be able to control the service order of requests + including prioritizing selection from different kinds of outstanding requests, + and postponing a request for an unspecified time while continuing to accept + new requests. + +Habermann states on page 173 + + Looking at deposit we see that sender and receiver should be synchronized with + respect to buffer overflow: deposit of another message must be delayed if + there is no empty message frame, and such delay should be removed when the + first empty frame becomes available. This is programmed as + synchronization(frame) : deposit is preceded by wait(frame), accept is + followed by signal( frame), and the constant C[frame] is set equal to + "bufsize." + +Here synchronization is controlling the service order of requests: when the +buffer is full, requests for insert (sender) are postponed until a request to +remove (receiver) occurs. Without the ability to control service order among +requests, the producer/consumer problem with a bounded buffer cannot be +solved. Hence, this capability is fundamental. + + + and the "implicitly" versus "explicitly" point needs stating as elsewhere, + with a concrete example e.g. Java built-in mutexes versus + java.util.concurrent). + +Fixed. + + MES must be available implicitly in language constructs, \eg Java built-in + monitors, as well as explicitly for specialized requirements, \eg + @java.util.concurrent@, because requiring programmers to build MES using + low-level locks often leads to incorrect programs. + + + section 6: 6.2 omits the most important facts in preference for otherwise + inscrutable detail: "identify the kind of parameter" (first say *that there + are* kinds of parameter, and what "kinds" means!); "mutex parameters are + documentation" is misleading (they are also semantically significant!) and + fails to say *what* they mean; the most important thing is surely that + 'mutex' is a language feature for performing lock/unlock operations at + function entry/exit. So say it! + +These sections have been rewritten address the comments. + + + The meanings of examples f3 and f4 remain unclear. + +Rewrote the paragraph. + + + Meanwhile in 6.3, "urgent" is not introduced (we are supposed to infer its + meaning from Figure 12, + +Defined Hoare's urgent list at the start of the paragraph. + + + but that Figure is incomprehensible to me), and we are told of "external + scheduling"'s long history in Ada but not clearly what it actually means; + +We do not know how to address your comment because the description is clear to +us and other non-reviewers who have read the paper. I had forgotten an +important citation to prior work on this topic, which is now referenced: + + Buhr Peter A., Fortier Michel, Coffin Michael H.. Monitor + Classification. ACM Computing Surveys. 1995; 27(1):63-107. + +This citation is a 45 page paper expanding on the topic of internal scheduling. +I also added a citation to Hoare's monitor paper where signal_block is defined: + + When a process signals a condition on which another process is waiting, the + signalling process must wait until the resumed process permits it to + proceed. + +External scheduling from Ada was subsequently added to uC++ and Cforall. +Furthermore, Figure 20 shows a direct comparison of CFA procedure-call +external-scheduling with Go channel external-scheduling. So external scheduling +exists in languages beyond Ada. + + + 6.4's description of "waitfor" tells us it is different from an if-else + chain but tries to use two *different* inputs to tell us that the behavior + is different; tell us an instance where *the same* values of C1 and C2 give + different behavior (I even wrote out a truth table and still don't see the + semantic difference) + +Again, it is unclear what is the problem. For the if-statement, if C1 is true, +it only waits for a call to mem1, even if C2 is true and there is an +outstanding call to mem2. For the waitfor, if both C1 and C2 are true and there +is a call to mem2 but not mem1, it accepts the call to mem2, which cannot +happen with the if-statement. So for all true when clauses, any outstanding +call is immediately accepted. If there are no outstanding calls, the waitfor +blocks until the next call to any of the true when clauses occurs. I added the +following sentence to further clarify. + + Hence, the @waitfor@ has parallel semantics, accepting any true @when@ clause. + +Note, the same parallel semantics exists for the Go select statement with +respect to waiting for a set of channels to receive data. While Go select does +not have a when clause, it would be trivial to add it, making the select more +expressive. + + + The authors frequently use bracketed phrases, and sometimes slashes "/", in + ways that are confusing and/or detrimental to readability. Page 13 line + 2's "forward (backward)" is one particularly egregious example. In general + I would recommend the the authors try to limit their use of parentheses and + slashes as a means of forcing a clearer wording to emerge. + +Many of the slashes and parentheticals have been removed. Some are retained to +express joined concepts: call/return, suspend/resume, resume/resume, I/O. + + + Also, the use of "eg." is often cursory and does not explain the examples + given, which are frequently a one- or two-word phrase of unclear referent. + +A few of these are fixed. + + + Considering the revision more broadly, none of the more extensive or + creative rewrites I suggested in my previous review have been attempted, + nor any equivalent efforts to improve its readability. + +If you reread the previous response, we addressed all of your suggestions except + + An expositional idea occurs: start the paper with a strawman + naive/limited realisation of coroutines -- say, Simon Tatham's popular + "Coroutines in C" web page -- and identify point by point what the + limitations are and how C\/ overcomes them. Currently the presentation + is often flat (lacking motivating contrasts) and backwards (stating + solutions before problems). The foregoing approach might fix both of + these. + + We prefer the current structure of our paper and believe the paper does + explain basic coding limitations and how they are overcome in using + high-level control-floe mechanisms. + +and we have addressed readability issues in this version. + + + The hoisting of the former section 5.9 is a good idea, but the newly added + material accompanying it (around Table 1) suffers fresh deficiencies in + clarity. Overall the paper is longer than before, even though (as my + previous review stated), I believe a shorter paper is required in order to + serve the likely purpose of publication. (Indeed, the authors' letter + implies that a key goal of publication is to build community and gain + external users.) + +This comment is the referee's opinion, which we do not agree with it. Our +previous 35 page SP&E paper on Cforall: + + Moss Aaron, Schluntz Robert, Buhr Peter A.. Cforall: Adding Modern + Programming Language Features to C. Softw. Pract. Exper. 2018; + 48(12):2111-2146. + +has a similar structure and style to this paper, and it received an award from +John Wiley & Sons: + + Software: Practice & Experience for articles published between January 2017 + and December 2018, "most downloads in the 12 months following online + publication showing the work generated immediate impact and visibility, + contributing significantly to advancement in the field". + +So we have demonstrated an ability to build community and gain external users. + + + Given this trajectory, I no longer see a path to an acceptable revision of + the present submission. Instead I suggest the authors consider splitting + the paper in two: one half about coroutines and stack management, the other + about mutexes, monitors and the runtime. (A briefer presentation of the + runtime may be helpful in the first paper also, and a brief recap of the + generator and coroutine support is obviously needed in the second too.) + +Again we disagree with the referee's suggestion to vastly restructure the +paper. What advantage is there is presenting exactly the same material across +two papers, which will likely end up longer than a single paper? The current +paper has a clear theme that fundamental execution properties generate a set of +basic language mechanisms, and we then proceed to show how these mechanisms can +be designed into the programing language Cforall. + + + I do not buy the authors' defense of the limited practical experience or + "non-micro" benchmarking presented. Yes, gaining external users is hard and + I am sympathetic on that point. But building something at least *somewhat* + substantial with your own system should be within reach, and without it the + "practice and experience" aspects of the work have not been explored. + Clearly C\/ is the product of a lot of work over an extended period, so it + is a surprise that no such experience is readily available for inclusion. + +Understood. There are no agreed-upon concurrency benchmarks, which is why +micro-benchmarks are often used. Currently, the entire Cforall runtime is +written in Cforall (10,000+ lines of code (LOC)). This runtime is designed to +be thread safe, automatically detects the use of concurrency features at link +time, and bootstraps into a threaded runtime almost immediately at program +startup so threads can be declared as global variables and may run to +completion before the program main starts. The concurrent core of the runtime +is 3,500 LOC and bootstraps from low-level atomic primitives into Cforall locks +and high-level features. In other words, the concurrent core uses itself as +quickly as possible to start using high-level concurrency. There are 12,000+ +LOC in the Cforall tests-suite used to verify language features, which are run +nightly. Of theses, there are 2000+ LOC running standard concurrent tests, such +as aggressively testing each language feature, and classical examples such as +bounded buffer, dating service, matrix summation, quickSort, binary insertion +sort, etc. More experience will be available soon, based on ongoing work in +the "future works" section. Specifically, non-blocking I/O is working with the +new Linux io_uring interface and a new high-performance ready-queue is under +construction to take into account this change. With non-blocking I/O, it will +be possible to write applications like high-performance web servers, as is now +done in Rust and Go. Also, completed is Java-style executors for work-based +concurrent programming and futures. Under construction is a high-performance +actor system. + + + It does not seem right to state that a stack is essential to Von Neumann + architectures -- since the earliest Von Neumann machines (and indeed early + Fortran) did not use one. + +Reference Manual Fortran II for the IBM 704 Data Processing System, 1958 IBM, page 2 +https://archive.computerhistory.org/resources/text/Fortran/102653989.05.01.acc.pdf + + Since a subprogram may call for other subprograms to any desired depth, a + particular CALL statement may be defined by a pyramid of multi-level + subprograms. + +I think we may be differing on the meaning of stack. You may be imagining a +modern stack that grows and shrinks dynamically. Whereas early Fortran +preallocated a stack frame for each function, like Python allocates a frame for +a generator. Within each preallocated Fortran function is a frame for local +variables and a pointer to store the return value for a call. The Fortran +call/return mechanism uses these frames to build a traditional call stack +linked by the return pointer. The only restriction is that a function stack +frame can only be used once, implying no direct or indirect recursion. Hence, +without a stack mechanism, there can be no call/return to "any desired depth", +where the maximum desired depth is limited by the number of functions. So +call/return requires some form of a stack, virtually all programming language +have call/return, past and present, and these languages run on Von Neumann +machines that do not distinguish between program and memory space, have mutable +state, and the concept of a pointer to data or code. + + + To elaborate on something another reviewer commented on: it is a surprise + to find a "Future work" section *after* the "Conclusion" section. A + "Conclusions and future work" section often works well. + +Done. + + + +Reviewing: 3 + + but it remains really difficult to have a good sense of which idea I should + use and when. This applies in different ways to different features from the + language: + + * coroutines/generators/threads: here there is some discussion, but it can + be improved. + * interal/external scheduling: I didn't find any direct comparison between + these features, except by way of example. + +See changes below. + + + I would have preferred something more like a table or a few paragraphs + highlighting the key reasons one would pick one construct or the other. + +Section 2.1 is changed to address this suggestion. + + + The discussion of clusters and pre-emption in particular feels quite rushed. + +We believe a brief introduction to the Cforall runtime structure is important +because clustering within a user-level versus distributed system is unusual, +Furthermore, the explanation of preemption is important because several new +languages, like Go and Rust tokio, are not preemptive. Rust threads are +preemptive only because it uses kernel threads, which UNIX preempts. + + + * Recommend to shorten the comparison on coroutine/generator/threads in + Section 2 to a paragraph with a few examples, or possibly a table + explaining the trade-offs between the constructs + +Not done, see below. + + + * Recommend to clarify the relationship between internal/external + scheduling -- is one more general but more error-prone or low-level? + +Done, see below. + + + There is obviously a lot of overlap between these features, and in + particular between coroutines and generators. As noted in the previous + review, many languages have chosen to offer *only* generators, and to + build coroutines by stacks of generators invoking one another. + +Coroutines built from stacks of generators have problems, such as no symmetric +control-flow. Furthermore, stacks of generators have a problem with the +following programming pattern. logger is a library function called from a +function or a coroutine, where the doit for the coroutine suspends. With stacks +of generators, there has to be a function and generator version of logger to +support these two scenarios. If logger is a library function, it may be +impossible to create the generator logger because the logger function is +opaque. + + #include + #include + + forall( otype T | { void doit( T ); } ) + void logger( T & t ) { + doit( t ); + } + + coroutine C {}; + void main( C & c ) with( c ) { + void doit( C & c ) { suspend; } + logger( c ); + } + void mem( C & c ) { + resume( c ); + } + + int main() { + C c; + mem( c ); + mem( c ); + + struct S {}; + S s; + void doit( S & s ) {} + logger( s ); + } + +Additonal text has been added to the start of Section 3.2 address this comment. + + + In fact, the end of Section 2.1 (on page 5) contains a particular paragraph + that embodies this "top down" approach. It starts, "programmers can now + answer three basic questions", and thus gives some practical advice for + which construct you should use and when. I think giving some examples of + specific applications that this paragraph, combined with some examples of + cases where each construct was needed, would be a better approach. + + I don't think this comparison needs to be very long. It seems clear enough + that one would + + * prefer generators for simple computations that yield up many values, + * prefer coroutines for more complex processes that have significant + internal structure, + +We do not believe the number of values yielded is an important factor is +choosing between a generator or coroutine; either form can receive (input) or +return (output) millions of values. As well, simple versus complex computations +is also not an absolute criterion for selection as both forms can be simple or +complex. + +As stated in the paper, a coroutine brings generality to the implementation +because of the addition stack, whereas generators have restrictions on standard +software-engineering practices: variable placement, no helper functions without +creating an explicit generator stack, and no symmetric control-flow. It is +true that simple generators probably suffer less from restrictions. Also, +simplicity means the amount of work done is small so the cost of the +suspend/resume becomes a factor, where experiments show suspend/resume for a +generator is 10 times faster than for a coroutines. + + + * prefer threads for cases where parallel execution is desired or needed. + +Agreed, but this description does not mention mutual exclusion and +synchronization, which is essential in any meaningful concurrent program. + +Our point here is to illustrate that a casual "top down" explanation is +insufficient to explain the complexity of the underlying execution properties. +We presented some rule-of-thumbs at the end of Section 2 but programmers must +understand all the underlying mechanisms and their interactions to exploit the +execution properties to their fullest, and to understand when a programming +language does or does not provide a desired mechanism. + + + I did appreciate the comparison in Section 2.3 between async-await in + JS/Java and generators/coroutines. I agree with its premise that those + mechanisms are a poor replacement for generators (and, indeed, JS has a + distinct generator mechanism, for example, in part for this reason). I + believe I may have asked for this in a previous review, but having read it, + I wonder if it is really necessary, since those mechanisms are so different + in purpose. + +Given that you asked about this before, I believe other readers might also ask +the same question because async-await is very popular. So I think this section +does help to position the work in the paper among other work, and hence, it is +appropriate to keep it in the paper. + + + I find the motivation for supporting both internal and external scheduling + to be fairly implicit. After several reads through the section, I came to + the conclusion that internal scheduling is more expressive than external + scheduling, but sometimes less convenient or clear. Is this correct? If + not, it'd be useful to clarify where external scheduling is more + expressive. + + I would find it very interesting to try and capture some of the properties + that make internal vs external scheduling the better choice. + + For example, it seems to me that external scheduling works well if there + are only a few "key" operations, but that internal scheduling might be + better otherwise, simply because it would be useful to have the ability to + name a signal that can be referenced by many methods. + +To address this point, the last paragraph on page 22 (now page 23) has been +augmented to the following: + + Given external and internal scheduling, what guidelines can a programmer use + to select between them? In general, external scheduling is easier to + understand and code because only the next logical action (mutex function(s)) + is stated, and the monitor implicitly handles all the details. Therefore, + there are no condition variables, and hence, no wait and signal, which reduces + coding complexity and synchronization errors. If external scheduling is + simpler than internal, why not use it all the time? Unfortunately, external + scheduling cannot be used if: scheduling depends on parameter value(s) or + scheduling must block across an unknown series of calls on a condition + variable (\ie internal scheduling). For example, the dating service cannot be + written using external scheduling. First, scheduling requires knowledge of + calling parameters to make matching decisions and parameters of calling + threads are unavailable within the monitor. Specifically, a thread within the + monitor cannot examine the @ccode@ of threads waiting on the calling queue to + determine if there is a matching partner. (Similarly, if the bounded buffer + or readers/writer are restructured with a single interface function with a + parameter denoting producer/consumer or reader/write, they cannot be solved + with external scheduling.) Second, a scheduling decision may be delayed + across an unknown number of calls when there is no immediate match so the + thread in the monitor must block on a condition. Specifically, if a thread + determines there is no opposite calling thread with the same @ccode@, it must + wait an unknown period until a matching thread arrives. For complex + synchronization, both external and internal scheduling can be used to take + advantage of best of properties of each. + + + Consider the bounded buffer from Figure 13: if it had multiple methods for + removing elements, and not just `remove`, then the `waitfor(remove)` call + in `insert` might not be sufficient. + +Section 6.4 Extended waitfor shows the waitfor is very powerful and can handle +your request: + + waitfor( remove : buffer ); or waitfor( remove2 : buffer ); + +and its shorthand form (not shown in the paper) + + waitfor( remove, remove2 : t ); + +A call to one these remove functions satisfies the waitfor (exact selection +details are discussed in Section 6.4). + + + The same is true, I think, of the `signal_block` function, which I + have not encountered before; + +In Tony Hoare's seminal paper on Monitors "Monitors: An Operating System +Structuring Concept", it states on page 551: + + When a process signals a condition on which another process is waiting, the + signalling process must wait until the resumed process permits it to + proceed. We therefore introduce for each monitor a second semaphore "urgent" + (initialized to 0), on which signalling processes suspend themselves by the + operation P(urgent). + +Hence, the original definition of signal is in fact signal_block, i.e., the +signaller blocks. Later implementations of monitor switched to signaller +nonblocking because most signals occur before returns, which allows the +signaller to continue execution, exit the monitor, and run concurrently with +the signalled thread that restarts in the monitor. When the signaller is not +going to exit immediately, signal_block is appropriate. + + + it seems like its behavior can be modeled with multiple condition + variables, but that's clearly more complex. + +Yes. Buhr, Fortier and Coffin show in Monitor Classification, ACM Computing +Surveys, 27(1):63-107, that all extant monitors with different signalling +semantics can be transformed into each other. However, some transformations are +complex and runtime expensive. + + + One question I had about `signal_block`: what happens if one signals + but no other thread is waiting? Does it block until some other thread + waits? Or is that user error? + +On page 20, it states: + + Signalling is unconditional because signalling an empty condition queue does + nothing. + +To the best of our knowledge, all monitors have the same semantics for +signalling an empty condition queue, regardless of the kind of signal, i.e., +signal or signal_block. + + I believe that one difference between the Go program and the Cforall + equivalent is that the Goroutine has an associated queue, so that + multiple messages could be enqueued, whereas the Cforall equivalent is + effectively a "bounded buffer" of length 1. Is that correct? + +Actually, the buffer length is 0 for the Cforall call and the Go unbuffered +send so both are synchronous communication. + + I think this should be stated explicitly. (Presumably, one could modify the + Cforall program to include an explicit vector of queued messages if + desired, but you would also be reimplementing the channel abstraction.) + +Fixed, by adding the following sentences: + + The different between call and channel send occurs for buffered channels + making the send asynchronous. In \CFA, asynchronous call and multiple + buffers is provided using an administrator and worker + threads~\cite{Gentleman81} and/or futures (not discussed). + + + Also, in Figure 20, I believe that there is a missing `mutex` keyword. + +Fixed. + + + I was glad to see that the paper acknowledged that Cforall still had + low-level atomic operations, even if their use is discouraged in favor of + higher-level alternatives. + +There was never an attempt to not acknowledged that Cforall had low-level +atomic operations. The original version of the paper stated: + + 6.6 Low-level Locks + For completeness and efficiency, Cforall provides a standard set of low-level + locks: recursive mutex, condition, semaphore, barrier, etc., and atomic + instructions: fetchAssign, fetchAdd, testSet, compareSet, etc. + +and that section is still in the paper. In fact, we use these low-level +mechanisms to build all of the high-level concurrency constructs in Cforall. + + + However, I still feel that the conclusion overstates the value of the + contribution here when it says that "Cforall high-level race-free monitors + and threads provide the core mechanisms for mutual exclusion and + synchronization, without the need for volatile and atomics". I feel + confident that Java programmers, for example, would be advised to stick + with synchronized methods whenever possible, and it seems to me that they + offer similar advantages -- but they sometimes wind up using volatiles for + performance reasons. + +I think we are agreeing violently. 99.9% of Java/Cforall/Go/Rust concurrent +programs can achieve very good performance without volatile or atomics because +the runtime system has already used these low-level capabilities to build an +efficient set of high-level concurrency constructs. + +0.1% of the time programmers need to build their own locks and synchronization +mechanisms. This need also occurs for storage management. Both of these +mechanisms are allowed in Cforall but are fraught with danger and should be +discouraged. Specially, it takes a 7th dan Black Belt programmer to understand +fencing for a WSO memory model, such as on the ARM. It doesn't help that the +C++ atomics are baroque and incomprehensible. I'm sure Hans Boehm, Doug Lea, +Dave Dice and me would agree that 99% of hand-crafted locks created by +programmers are broken and/or non-portable. + + + I was also confused by the term "race-free" in that sentence. In + particular, I don't think that Cforall has any mechanisms for preventing + *data races*, and it clearly doesn't prevent "race conditions" (which would + bar all sorts of useful programs). I suppose that "race free" here might be + referring to the improvements such as removing barging behavior. + +We use the term "race free" to mean the same as Boehm/Adve's "data-race +freedom" in + + Boehm Hans-J., Adve Sarita V., You Don't Know Jack About Shared Variables or + Memory Models. Communications ACM. 2012; 55(2):48-54. + https://queue.acm.org/detail.cfm?id=2088916 + +which is cited in the paper. Furthermore, we never said that Cforall has +mechanisms for preventing *all* data races. We said Cforall high-level +race-free monitors and threads[, when used with mutex access function] (added +to the paper), implies no data races within these constructs, unless a +programmer directly publishes shared state. This approach is exactly what +Boehm/Adve advocate for the vast majority of concurrent programming. + + + It would perhaps be more interesting to see a comparison built using [tokio] or + [async-std], two of the more prominent user-space threading libraries that + build on Rust's async-await feature (which operates quite differently than + Javascript's async-await, in that it doesn't cause every aync function call to + schedule a distinct task). + +Done. + + + Several figures used the `with` keyword. I deduced that `with(foo)` permits + one to write `bar` instead of `foo.bar`. It seems worth introducing. + Apologies if this is stated in the paper, if so I missed it. + +Page 6, footnote F states: + + The Cforall "with" clause opens an aggregate scope making its fields directly + accessible, like Pascal "with", but using parallel semantics; multiple + aggregates may be opened. + + + On page 20, section 6.3, "external scheduling and vice versus" should be + "external scheduling and vice versa". + +Fixed. + + + On page 5, section 2.3, the paper states "we content" but it should be "we + contend". + +Fixed. + + + Page 1. I don't believe that it s fair to imply that Scala is "research + vehicle" as it is used by major players, Twitter being the most prominent + example. + +Fixed. Removed "research". + + + Page 15. Must Cforall threads start after construction (e.g. see your + example on page 15, line 21)? + +Yes. Our experience in Java, uC++ and Cforall is that 95% of the time +programmers want threads to start immediately. (Most Java programs have no code +between a thread declaration and the call to start the thread.) Therefore, +this semantic should be the default because (see page 13): + + Alternatives, such as explicitly starting threads as in Java, are repetitive + and forgetting to call start is a common source of errors. + +To handle the other 5% of the cases, there is a trivial Cforall pattern +providing Java-style start/join. The additional cost for this pattern is small +in comparison to the work performed between the start and join. + + thread T {}; + void start( T & mutex ) {} // any function name + void join( T & mutex ) {} // any function name + void main( T & t ) { + sout | "start"; + waitfor( start : t ); // wait to be started + sout | "restart"; // perform work + waitfor( join : t ); // wait to be joined + sout | "join"; + } + int main() { + T t[3]; // threads start and delay + sout | "need to start"; + for ( i; 3 ) start( t[i] ); + sout | "need to join"; + for ( i; 3 ) join( t[i] ); + sout | "threads stopped"; + } // threads deleted from stack + + $ a.out + need to start + start + start + start + need to join + restart + restart + restart + threads stopped + join + join + join + + + Page 18, line 17: is using + +Fixed.