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  • doc/proposals/concurrency/Makefile

    rbb9d8e8 rb7778c1  
    1616text/basics \
    1717text/concurrency \
     18text/internals \
    1819text/parallelism \
    1920text/together \

  • doc/proposals/concurrency/style/cfa-format.tex

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    133133  belowskip=3pt,
    134134  keepspaces=true,
     135  tabsize=4,
    135136  % frame=lines,
    136137  literate=,
    … …  
    150151  keywordstyle=\bfseries\color{blue},
    151152  keywordstyle=[2]\bfseries\color{Plum},
    152   commentstyle=\itshape\color{OliveGreen},                  % green and italic comments
     153  commentstyle=\sf\itshape\color{OliveGreen},             % green and italic comments
    153154  identifierstyle=\color{identifierCol},
    154155  stringstyle=\sf\color{Mahogany},                                % use sanserif font
    … …  
    158159  belowskip=3pt,
    159160  keepspaces=true,
     161  tabsize=4,
    160162  % frame=lines,
    161163  literate=,

  • doc/proposals/concurrency/text/basics.tex

    rbb9d8e8 rb7778c1  
    11% ======================================================================
    22% ======================================================================
    3 \chapter{Basics}\label{basics}
     3\chapter{Concurrency Basics}\label{basics}
    44% ======================================================================
    55% ======================================================================
    6 Before any detailed discussion of the concurrency and parallelism in \CFA, it is important to describe the basics of concurrency and how they are expressed in \CFA user code.
     6Before any detailed discussion of the concurrency and parallelism in \CFA, it is important to describe the basics of concurrency and how they are expressed in \CFA user-code.
    77
    88\section{Basics of concurrency}
    9 At its core, concurrency is based on having call-stacks and potentially multiple threads of execution for these stacks. Concurrency without parallelism only requires having multiple call stacks (or contexts) for a single thread of execution, and switching between these call stacks on a regular basis. A minimal concurrency product can be achieved by creating coroutines, which instead of context switching between each other, always ask an oracle where to context switch next. While coroutines do not technically require a stack, stackfull coroutines are the closest abstraction to a practical "naked"" call stack. When writing concurrency in terms of coroutines, the oracle effectively becomes a scheduler and the whole system now follows a cooperative threading-model \cit. The oracle/scheduler can either be a stackless or stackfull entity and correspondingly require one or two context switches to run a different coroutine. In any case, a subset of concurrency related challenges start to appear. For the complete set of concurrency challenges to occur, the only feature missing is preemption. Indeed, concurrency challenges appear with non-determinism. Guaranteeing mutual-exclusion or synchronisation are simply ways of limiting the lack of determinism in a system. A scheduler introduces order of execution uncertainty, while preemption introduces incertainty about where context-switches occur. Now it is important to understand that uncertainty is not necessarily undesireable; uncertainty can often be used by systems to significantly increase performance and is often the basis of giving a user the illusion that tasks are running in parallel. Optimal performance in concurrent applications is often obtained by having as much non-determinism as correctness allows\cit.
     9At its core, concurrency is based on having multiple call-stacks and scheduling among threads of execution executing on these stacks. Concurrency without parallelism only requires having multiple call stacks (or contexts) for a single thread of execution.
     10
     11Indeed, while execution with a single thread and multiple stacks where the thread is self-scheduling deterministically across the stacks is called coroutining, execution with a single and multiple stacks but where the thread is scheduled by an oracle (non-deterministic from the thread perspective) across the stacks is called concurrency.
     12
     13Therefore, a minimal concurrency system can be achieved by creating coroutines, which instead of context switching among each other, always ask an oracle where to context switch next. While coroutines can execute on the caller's stack-frame, stackfull coroutines allow full generality and are sufficient as the basis for concurrency. The aforementioned oracle is a scheduler and the whole system now follows a cooperative threading-model \cit. The oracle/scheduler can either be a stackless or stackfull entity and correspondingly require one or two context switches to run a different coroutine. In any case, a subset of concurrency related challenges start to appear. For the complete set of concurrency challenges to occur, the only feature missing is preemption. Indeed, concurrency challenges appear with non-determinism. Using mutual-exclusion or synchronisation are ways of limiting the lack of determinism in a system. A scheduler introduces order of execution uncertainty, while preemption introduces uncertainty about where context-switches occur. Now it is important to understand that uncertainty is not undesireable; uncertainty can often be used by systems to significantly increase performance and is often the basis of giving a user the illusion that tasks are running in parallel. Optimal performance in concurrent applications is often obtained by having as much non-determinism as correctness allows\cit.
    1014
    1115\section{\protect\CFA 's Thread Building Blocks}
    12 One of the important features that is missing in C is threading. On modern architectures, a lack of threading is becoming less and less forgivable\cite{Sutter05, Sutter05b}, and therefore modern programming languages must have the proper tools to allow users to write performant concurrent and/or parallel programs. As an extension of C, \CFA needs to express these concepts in a way that is as natural as possible to programmers used to imperative languages. And being a system-level language means programmers expect to choose precisely which features they need and which cost they are willing to pay.
     16One of the important features that is missing in C is threading. On modern architectures, a lack of threading is unacceptable\cite{Sutter05, Sutter05b}, and therefore modern programming languages must have the proper tools to allow users to write performant concurrent and/or parallel programs. As an extension of C, \CFA needs to express these concepts in a way that is as natural as possible to programmers familiar with imperative languages. And being a system-level language means programmers expect to choose precisely which features they need and which cost they are willing to pay.
    1317
    1418\section{Coroutines: A stepping stone}\label{coroutine}
    15 While the main focus of this proposal is concurrency and parallelism, as mentionned above it is important to adress coroutines, which are actually a significant underlying aspect of a concurrency system. Indeed, while having nothing to do with parallelism and arguably little to do with concurrency, coroutines need to deal with context-switchs and other context-management operations. Therefore, this proposal includes coroutines both as an intermediate step for the implementation of threads, and a first class feature of \CFA. Furthermore, many design challenges of threads are at least partially present in designing coroutines, which makes the design effort that much more relevant. The core API of coroutines revolve around two features: independent call stacks and \code{suspend}/\code{resume}.
    16 
    17 Here is an example of a solution to the fibonnaci problem using \CFA coroutines:
    18 \begin{cfacode}
    19         coroutine Fibonacci {
    20               int fn; // used for communication
    21         };
    22 
    23         void ?{}(Fibonacci & this) { // constructor
    24               this.fn = 0;
    25         }
    26 
    27         // main automacically called on first resume
    28         void main(Fibonacci & this) {
    29                 int fn1, fn2;           // retained between resumes
    30                 this.fn = 0;
    31                 fn1 = this.fn;
    32                 suspend(this);          // return to last resume
    33 
    34                 this.fn = 1;
     19While the main focus of this proposal is concurrency and parallelism, it is important to address coroutines, which are actually a significant building block of a concurrency system. Coroutines need to deal with context-switchs and other context-management operations. Therefore, this proposal includes coroutines both as an intermediate step for the implementation of threads, and a first class feature of \CFA. Furthermore, many design challenges of threads are at least partially present in designing coroutines, which makes the design effort that much more relevant. The core \acrshort{api} of coroutines revolve around two features: independent call stacks and \code{suspend}/\code{resume}.
     20
     21A good example of a problem made easier with coroutines is genereting the fibonacci sequence. This problem comes with the challenge of decoupling how a sequence is generated and how it is used. Figure \ref{fig:fibonacci-c} shows conventional approaches to writing generators in C. All three of these approach suffer from strong coupling. The left and center approaches require that the generator have knowledge of how the sequence will be used, while the rightmost approach requires to user to hold internal state between calls on behalf of th sequence generator and makes it much harder to handle corner cases like the Fibonacci seed.
     22\begin{figure}
     23\label{fig:fibonacci-c}
     24\caption{Different implementations of a fibonacci sequence generator in C.}
     25\begin{center}
     26\begin{tabular}{c @{\hskip 0.025in}|@{\hskip 0.025in} c @{\hskip 0.025in}|@{\hskip 0.025in} c}
     27\begin{ccode}[tabsize=2]
     28//Using callbacks
     29void fibonacci_func(
     30        int n,
     31        void (*callback)(int)
     32) {
     33        int first = 0;
     34        int second = 1;
     35        int next, i;
     36        for(i = 0; i < n; i++)
     37        {
     38                if(i <= 1)
     39                        next = i;
     40                else {
     41                        next = f1 + f2;
     42                        f1 = f2;
     43                        f2 = next;
     44                }
     45                callback(next);
     46        }
     47}
     48\end{ccode}&\begin{ccode}[tabsize=2]
     49//Using output array
     50void fibonacci_array(
     51        int n,
     52        int * array
     53) {
     54        int f1 = 0; int f2 = 1;
     55        int next, i;
     56        for(i = 0; i < n; i++)
     57        {
     58                if(i <= 1)
     59                        next = i;
     60                else {
     61                        next = f1 + f2;
     62                        f1 = f2;
     63                        f2 = next;
     64                }
     65                *array = next;
     66                array++;
     67        }
     68}
     69\end{ccode}&\begin{ccode}[tabsize=2]
     70//Using external state
     71typedef struct {
     72        int f1, f2;
     73} iterator_t;
     74
     75int fibonacci_state(
     76        iterator_t * it
     77) {
     78        int f;
     79        f = it->f1 + it->f2;
     80        it->f2 = it->f1;
     81        it->f1 = f;
     82        return f;
     83}
     84
     85
     86
     87
     88
     89
     90\end{ccode}
     91\end{tabular}
     92\end{center}
     93\end{figure}
     94
     95
     96Figure \ref{fig:fibonacci-cfa} is an example of a solution to the fibonnaci problem using \CFA coroutines, using the coroutine stack to hold sufficient state for the generation. This solution has the advantage of having very strong decoupling between how the sequence is generated and how it is used. Indeed, this version is a easy to use as the \code{fibonacci_state} solution, while the imlpementation is very similar to the \code{fibonacci_func} example.
     97
     98\begin{figure}
     99\label{fig:fibonacci-cfa}
     100\caption{Implementation of fibonacci using coroutines}
     101\begin{cfacode}
     102coroutine Fibonacci {
     103        int fn; //used for communication
     104};
     105
     106void ?{}(Fibonacci & this) { //constructor
     107        this.fn = 0;
     108}
     109
     110//main automacically called on first resume
     111void main(Fibonacci & this) {
     112        int fn1, fn2;           //retained between resumes
     113        this.fn = 0;
     114        fn1 = this.fn;
     115        suspend(this);          //return to last resume
     116
     117        this.fn = 1;
     118        fn2 = fn1;
     119        fn1 = this.fn;
     120        suspend(this);          //return to last resume
     121
     122        for ( ;; ) {
     123                this.fn = fn1 + fn2;
    35124                fn2 = fn1;
    36125                fn1 = this.fn;
    37                 suspend(this);          // return to last resume
    38 
    39                 for ( ;; ) {
    40                         this.fn = fn1 + fn2;
    41                         fn2 = fn1;
    42                         fn1 = this.fn;
    43                         suspend(this);  // return to last resume
    44                 }
    45         }
    46 
    47         int next(Fibonacci & this) {
    48                 resume(this); // transfer to last suspend
    49                 return this.fn;
    50         }
    51 
    52         void main() { // regular program main
    53                 Fibonacci f1, f2;
    54                 for ( int i = 1; i <= 10; i += 1 ) {
    55                         sout | next( f1 ) | next( f2 ) | endl;
    56                 }
    57         }
    58 \end{cfacode}
     126                suspend(this);  //return to last resume
     127        }
     128}
     129
     130int next(Fibonacci & this) {
     131        resume(this); //transfer to last suspend
     132        return this.fn;
     133}
     134
     135void main() { //regular program main
     136        Fibonacci f1, f2;
     137        for ( int i = 1; i <= 10; i += 1 ) {
     138                sout | next( f1 ) | next( f2 ) | endl;
     139        }
     140}
     141\end{cfacode}
     142\end{figure}
    59143
    60144\subsection{Construction}
    61 One important design challenge for coroutines and threads (shown in section \ref{threads}) is that the runtime system needs to run code after the user-constructor runs. In the case of coroutines, this challenge is simpler since there is no non-determinism from preemption or scheduling. However, the underlying challenge remains the same for coroutines and threads.
    62 
    63 The runtime system needs to create the coroutine's stack and more importantly prepare it for the first resumption. The timing of the creation is non-trivial since users both expect to have fully constructed objects once execution enters the coroutine main and to be able to resume the coroutine from the constructor. Like for regular objects, constructors can still leak coroutines before they are ready. There are several solutions to this problem but the chosen options effectively forces the design of the coroutine.
     145One important design challenge for coroutines and threads (shown in section \ref{threads}) is that the runtime system needs to run code after the user-constructor runs to connect the object into the system. In the case of coroutines, this challenge is simpler since there is no non-determinism from preemption or scheduling. However, the underlying challenge remains the same for coroutines and threads.
     146
     147The runtime system needs to create the coroutine's stack and more importantly prepare it for the first resumption. The timing of the creation is non-trivial since users both expect to have fully constructed objects once execution enters the coroutine main and to be able to resume the coroutine from the constructor. As regular objects, constructors can leak coroutines before they are ready. There are several solutions to this problem but the chosen options effectively forces the design of the coroutine.
    64148
    65149Furthermore, \CFA faces an extra challenge as polymorphic routines create invisible thunks when casted to non-polymorphic routines and these thunks have function scope. For example, the following code, while looking benign, can run into undefined behaviour because of thunks:
    … …  
    78162}
    79163\end{cfacode}
     164
    80165The generated C code\footnote{Code trimmed down for brevity} creates a local thunk to hold type information:
    81166
    … …  
    95180}
    96181\end{ccode}
    97 The problem in this example is a race condition between the start of the execution of \code{noop} on the other thread and the stack frame of \code{bar} being destroyed. This extra challenge limits which solutions are viable because storing the function pointer for too long only increases the chances that the race will end in undefined behavior; i.e. the stack based thunk being destroyed before it was used. This challenge is an extension of challenges that come with second-class routines. Indeed, GCC nested routines also have the limitation that the routines cannot be passed outside of the scope of the functions these were declared in. The case of coroutines and threads is simply an extension of this problem to multiple call-stacks.
     182The problem in this example is a storage management issue, the function pointer \code{_thunk0} is only valid until the end of the block. This extra challenge limits which solutions are viable because storing the function pointer for too long causes undefined behavior; i.e. the stack based thunk being destroyed before it was used. This challenge is an extension of challenges that come with second-class routines. Indeed, GCC nested routines also have the limitation that the routines cannot be passed outside of the scope of the functions these were declared in. The case of coroutines and threads is simply an extension of this problem to multiple call-stacks.
    98183
    99184\subsection{Alternative: Composition}
    100 One solution to this challenge would be to use composition/containement,
    101 
    102 \begin{cfacode}
    103         struct Fibonacci {
    104               int fn; // used for communication
    105               coroutine c; //composition
    106         };
    107 
    108         void ?{}(Fibonacci & this) {
    109               this.fn = 0;
    110                 (this.c){};
    111         }
    112 \end{cfacode}
    113 There are two downsides to this approach. The first, which is relatively minor, is that the base class needs to be made aware of the main routine pointer, regardless of whether a parameter or a virtual pointer is used, this means the coroutine data must be made larger to store a value that is actually a compile time constant (address of the main routine). The second problem, which is both subtle and significant, is that now users can get the initialisation order of there coroutines wrong. Indeed, every field of a \CFA struct is constructed but in declaration order, unless users explicitly write otherwise. This semantics means that users who forget to initialize a the coroutine may resume the coroutine with an uninitilized object. For coroutines, this is unlikely to be a problem, for threads however, this is a significant problem.
     185One solution to this challenge is to use composition/containement, where uses add insert a coroutine field which contains the necessary information to manage the coroutine.
     186
     187\begin{cfacode}
     188struct Fibonacci {
     189        int fn; //used for communication
     190        coroutine c; //composition
     191};
     192
     193void ?{}(Fibonacci & this) {
     194        this.fn = 0;
     195        (this.c){}; //Call constructor to initialize coroutine
     196}
     197\end{cfacode}
     198There are two downsides to this approach. The first, which is relatively minor, made aware of the main routine pointer. This information must either be store in the coroutine runtime data or in its static type structure. When using composition, all coroutine handles have the same static type structure which means the pointer to the main needs to be part of the runtime data. This requirement means the coroutine data must be made larger to store a value that is actually a compile time constant (address of the main routine). The second problem, which is both subtle and significant, is that now users can get the initialisation order of coroutines wrong. Indeed, every field of a \CFA struct is constructed but in declaration order, unless users explicitly write otherwise. This semantics means that users who forget to initialize the coroutine handle may resume the coroutine with an uninitilized object. For coroutines, this is unlikely to be a problem, for threads however, this is a significant problem. Figure \ref{fig:fmt-line} shows the \code{Format} coroutine which rearranges text in order to group characters into blocks of fixed size. This is a good example where the control flow is made much simpler from being able to resume the coroutine from the constructor and highlights the idea that interesting control flow can occor in the constructor.
     199\begin{figure}
     200\label{fig:fmt-line}
     201\caption{Formatting text into lines of 5 blocks of 4 characters.}
     202\begin{cfacode}[tabsize=3]
     203//format characters into blocks of 4 and groups of 5 blocks per line
     204coroutine Format {
     205        char ch;                                                                        //used for communication
     206        int g, b;                                                               //global because used in destructor
     207};
     208
     209void  ?{}(Format & fmt) {
     210        resume( fmt );                                                  //prime (start) coroutine
     211}
     212
     213void ^?{}(Format & fmt) with fmt {
     214        if ( fmt.g != 0 || fmt.b != 0 )
     215        sout | endl;
     216}
     217
     218void main(Format & fmt) with fmt {
     219        for ( ;; ) {                                                    //for as many characters
     220                for(g = 0; g < 5; g++) {                //groups of 5 blocks
     221                        for(b = 0; b < 4; fb++) {       //blocks of 4 characters
     222                                suspend();
     223                                sout | ch;                                      //print character
     224                        }
     225                        sout | "  ";                                    //print block separator
     226                }
     227                sout | endl;                                            //print group separator
     228        }
     229}
     230
     231void prt(Format & fmt, char ch) {
     232        fmt.ch = ch;
     233        resume(fmt);
     234}
     235
     236int main() {
     237        Format fmt;
     238        char ch;
     239        Eof: for ( ;; ) {                                               //read until end of file
     240                sin | ch;                                                       //read one character
     241                if(eof(sin)) break Eof;                 //eof ?
     242                prt(fmt, ch);                                           //push character for formatting
     243        }
     244}
     245\end{cfacode}
     246\end{figure}
     247
    114248
    115249\subsection{Alternative: Reserved keyword}
    … …  
    117251
    118252\begin{cfacode}
    119         coroutine Fibonacci {
    120               int fn; // used for communication
    121         };
    122 \end{cfacode}
    123 This mean the compiler can solve problems by injecting code where needed. The downside of this approach is that it makes coroutine a special case in the language. Users who would want to extend coroutines or build their own for various reasons can only do so in ways offered by the language. Furthermore, implementing coroutines without language supports also displays the power of \CFA.
    124 While this is ultimately the option used for idiomatic \CFA code, coroutines and threads can both be constructed by users without using the language support. The reserved keywords are only present to improve ease of use for the common cases.
     253coroutine Fibonacci {
     254        int fn; //used for communication
     255};
     256\end{cfacode}
     257This mean the compiler can solve problems by injecting code where needed. The downside of this approach is that it makes coroutine a special case in the language. Users who would want to extend coroutines or build their own for various reasons can only do so in ways offered by the language. Furthermore, implementing coroutines without language supports also displays the power of the programming language used. While this is ultimately the option used for idiomatic \CFA code, coroutines and threads can both be constructed by users without using the language support. The reserved keywords are only present to improve ease of use for the common cases.
    125258
    126259\subsection{Alternative: Lamda Objects}
    … …  
    159292      coroutine_desc * get_coroutine(T & this);
    160293};
    161 \end{cfacode}
    162 This ensures an object is not a coroutine until \code{resume} (or \code{prime}) is called on the object. Correspondingly, any object that is passed to \code{resume} is a coroutine since it must satisfy the \code{is_coroutine} trait to compile. The advantage of this approach is that users can easily create different types of coroutines, for example, changing the memory foot print of a coroutine is trivial when implementing the \code{get_coroutine} routine. The \CFA keyword \code{coroutine} only has the effect of implementing the getter and forward declarations required for users to only have to implement the main routine.
     294
     295forall( dtype T | is_coroutine(T) ) void suspend(T &);
     296forall( dtype T | is_coroutine(T) ) void resume (T &);
     297\end{cfacode}
     298This ensures an object is not a coroutine until \code{resume} is called on the object. Correspondingly, any object that is passed to \code{resume} is a coroutine since it must satisfy the \code{is_coroutine} trait to compile. The advantage of this approach is that users can easily create different types of coroutines, for example, changing the memory layout of a coroutine is trivial when implementing the \code{get_coroutine} routine. The \CFA keyword \code{coroutine} only has the effect of implementing the getter and forward declarations required for users to only have to implement the main routine.
    163299
    164300\begin{center}
    … …  
    186322\end{center}
    187323
    188 The combination of these two approaches allows users new to concurrency to have a easy and concise method while more advanced users can expose themselves to otherwise hidden pitfalls at the benefit of tighter control on memory layout and initialization.
     324The combination of these two approaches allows users new to coroutinning and concurrency to have an easy and concise specification, while more advanced users have tighter control on memory layout and initialization.
    189325
    190326\section{Thread Interface}\label{threads}
    … …  
    205341\end{cfacode}
    206342
    207 Obviously, for this thread implementation to be usefull it must run some user code. Several other threading interfaces use a function-pointer representation as the interface of threads (for example \Csharp~\cite{Csharp} and Scala~\cite{Scala}). However, this proposal considers that statically tying a \code{main} routine to a thread superseeds this approach. Since the \code{main} routine is already a special routine in \CFA (where the program begins), it is possible naturally extend the semantics using overloading to declare mains for different threads (the normal main being the main of the initial thread). As such the \code{main} routine of a thread can be defined as
     343Obviously, for this thread implementation to be usefull it must run some user code. Several other threading interfaces use a function-pointer representation as the interface of threads (for example \Csharp~\cite{Csharp} and Scala~\cite{Scala}). However, this proposal considers that statically tying a \code{main} routine to a thread superseeds this approach. Since the \code{main} routine is already a special routine in \CFA (where the program begins), it is a natural extension of the semantics using overloading to declare mains for different threads (the normal main being the main of the initial thread). As such the \code{main} routine of a thread can be defined as
    208344\begin{cfacode}
    209345        thread foo {};
    … …  
    214350\end{cfacode}
    215351
    216 In this example, threads of type \code{foo} start execution in the \code{void main(foo*)} routine which prints \code{"Hello World!"}. While this proposoal encourages this approach to enforce strongly-typed programming, users may prefer to use the routine based thread semantics for the sake of simplicity. With these semantics it is trivial to write a thread type that takes a function pointer as parameter and executes it on its stack asynchronously
    217 \begin{cfacode}
    218         typedef void (*voidFunc)(void);
     352In this example, threads of type \code{foo} start execution in the \code{void main(foo &)} routine, which prints \code{"Hello World!"}. While this thesis encourages this approach to enforce strongly-typed programming, users may prefer to use the routine-based thread semantics for the sake of simplicity. With these semantics it is trivial to write a thread type that takes a function pointer as a parameter and executes it on its stack asynchronously
     353\begin{cfacode}
     354        typedef void (*voidFunc)(int);
    219355
    220356        thread FuncRunner {
    221357                voidFunc func;
     358                int arg;
    222359        };
    223360
    224         //ctor
    225         void ?{}(FuncRunner & this, voidFunc inFunc) {
     361        void ?{}(FuncRunner & this, voidFunc inFunc, int arg) {
    226362                this.func = inFunc;
    227363        }
    228364
    229         //main
    230365        void main(FuncRunner & this) {
    231                 this.func();
    232         }
    233 \end{cfacode}
    234 
    235 An advantage of the overloading approach to main is to clearly highlight where and what memory is required to pass parameters and return values to/from a thread.
    236 
    237 Of course for threads to be useful, it must be possible to start and stop threads and wait for them to complete execution. While using an \acrshort{api} such as \code{fork} and \code{join} is relatively common in the literature, such an interface is unnecessary. Indeed, the simplest approach is to use \acrshort{raii} principles and have threads \code{fork} once the constructor has completed and \code{join} before the destructor runs.
     366                this.func( this.arg );
     367        }
     368\end{cfacode}
     369
     370An consequence of the strongly typed approach to main is that memory layout of parameters and return values to/from a thread are now explicitly specified in the \acrshort{api}.
     371
     372Of course for threads to be useful, it must be possible to start and stop threads and wait for them to complete execution. While using an \acrshort{api} such as \code{fork} and \code{join} is relatively common in the literature, such an interface is unnecessary. Indeed, the simplest approach is to use \acrshort{raii} principles and have threads \code{fork} after the constructor has completed and \code{join} before the destructor runs.
    238373\begin{cfacode}
    239374thread World;
    … …  
    254389\end{cfacode}
    255390
    256 This semantic has several advantages over explicit semantics typesafety is guaranteed, a thread is always started and stopped exaclty once and users cannot make any progamming errors. Another advantage of this semantic is that it naturally scale to multiple threads meaning basic synchronisation is very simple
     391This semantic has several advantages over explicit semantics: a thread is always started and stopped exaclty once and users cannot make any progamming errors and it naturally scales to multiple threads meaning basic synchronisation is very simple
    257392
    258393\begin{cfacode}
    … …  
    276411\end{cfacode}
    277412
    278 However, one of the apparent drawbacks of this system is that threads now always form a lattice, that is they are always destroyed in opposite order of construction because of block structure. However, storage allocation is not limited to blocks; dynamic allocation can create threads that outlive the scope in which the thread is created much like dynamically allocating memory lets objects outlive the scope in which they are created
     413However, one of the drawbacks of this approach is that threads now always form a lattice, that is they are always destroyed in opposite order of construction because of block structure. This restriction is relaxed by using dynamic allocation, so threads can outlive the scope in which they are created, much like dynamically allocating memory lets objects outlive the scope in which they are created
    279414
    280415\begin{cfacode}
    … …  
    283418};
    284419
    285 //main
    286420void main(MyThread & this) {
    287421        //...
    … …  
    291425        MyThread * long_lived;
    292426        {
     427                //Start a thread at the beginning of the scope
    293428                MyThread short_lived;
    294                 //Start a thread at the beginning of the scope
    295 
    296                 DoStuff();
    297429
    298430                //create another thread that will outlive the thread in this scope
    299431                long_lived = new MyThread;
    300432
     433                DoStuff();
     434
    301435                //Wait for the thread short_lived to finish
    302436        }
    303437        DoMoreStuff();
    304438
    305         //Now wait for the short_lived to finish
     439        //Now wait for the long_lived to finish
    306440        delete long_lived;
    307441}

  • doc/proposals/concurrency/text/cforall.tex

    rbb9d8e8 rb7778c1  
    55% ======================================================================
    66
    7 As mentionned in the introduction, the document presents the design for the concurrency features in \CFA. Since it is a new language here is a quick review of the language specifically tailored to the features needed to support concurrency.
     7This thesis presents the design for a set of concurrency features in \CFA. Since it is a new dialect of C, the following is a quick introduction to the language, specifically tailored to the features needed to support concurrency.
    88
    9 \CFA is a extension of ISO C and therefore supports much of the same paradigms as C. It is a non-object oriented system level language, meaning it has very most of the major abstractions have either no runtime cost or can be opt-out easily. Like C, the basics of \CFA revolve around structures and routines, which are thin abstractions over assembly. The vast majority of the code produced by a \CFA compiler respects memory-layouts and calling-conventions laid out by C. However, while \CFA is not an object-oriented language according to a strict definition. It does have some notion of objects, most importantly construction and destruction of objects. Most of the following pieces of code can be found as is on the \CFA website : \cite{www-cfa}
     9\CFA is a extension of ISO-C and therefore supports all of the same paradigms as C. It is a non-object oriented system language, meaning most of the major abstractions have either no runtime overhead or can be opt-out easily. Like C, the basics of \CFA revolve around structures and routines, which are thin abstractions over machine code. The vast majority of the code produced by the \CFA translator respects memory-layouts and calling-conventions laid out by C. Interestingly, while \CFA is not an object-oriented language, lacking the concept of a received (e.g.: this), it does have some notion of objects\footnote{C defines the term objects as : [Where to I get the C11 reference manual?]}, most importantly construction and destruction of objects. Most of the following pieces of code can be found on the \CFA website \cite{www-cfa}
    1010
    1111\section{References}
    1212
    13 Like \CC, \CFA introduces references as an alternative to pointers. In regards to concurrency, the semantics difference between pointers and references aren't particularly relevant but since this document uses mostly references here is a quick overview of the semantics :
     13Like \CC, \CFA introduces references as an alternative to pointers. In regards to concurrency, the semantics difference between pointers and references are not particularly relevant but since this document uses mostly references here is a quick overview of the semantics :
    1414\begin{cfacode}
    1515int x, *p1 = &x, **p2 = &p1, ***p3 = &p2,
    1616&r1 = x,    &&r2 = r1,   &&&r3 = r2;
    17 ***p3 = 3;                              // change x
    18 r3 = 3;                                 // change x, ***r3
    19 **p3 = ...;                             // change p1
    20 &r3 = ...;                              // change r1, (&*)**r3
    21 *p3 = ...;                              // change p2
    22 &&r3 = ...;                             // change r2, (&(&*)*)*r3
    23 &&&r3 = p3;                             // change r3 to p3, (&(&(&*)*)*)r3
    24 int y, z, & ar[3] = { x, y, z };        // initialize array of references
    25 &ar[1] = &z;                            // change reference array element
    26 typeof( ar[1] ) p;                      // is int, i.e., the type of referenced object
    27 typeof( &ar[1] ) q;                     // is int &, i.e., the type of reference
    28 sizeof( ar[1] ) == sizeof( int );       // is true, i.e., the size of referenced object
    29 sizeof( &ar[1] ) == sizeof( int *);     // is true, i.e., the size of a reference
     17***p3 = 3;                                                      //change x
     18r3    = 3;                                                      //change x, ***r3
     19**p3  = ...;                                            //change p1
     20*p3   = ...;                                            //change p2
     21int y, z, & ar[3] = {x, y, z};          //initialize array of references
     22typeof( ar[1]) p;                                       //is int, i.e., the type of referenced object
     23typeof(&ar[1]) q;                                       //is int &, i.e., the type of reference
     24sizeof( ar[1]) == sizeof(int);          //is true, i.e., the size of referenced object
     25sizeof(&ar[1]) == sizeof(int *);        //is true, i.e., the size of a reference
    3026\end{cfacode}
    3127The important thing to take away from this code snippet is that references offer a handle to an object much like pointers but which is automatically derefferenced when convinient.
    … …  
    3329\section{Overloading}
    3430
    35 Another important feature \CFA has in common with \CC is function overloading :
     31Another important feature of \CFA is function overloading as in Java and \CC, where routine with the same name are selected based on the numbers and type of the arguments. As well, \CFA uses the return type as part of the selection criteria, as in Ada\cite{Ada}. For routines with multiple parameters and returns, the selection is complex.
    3632\begin{cfacode}
    37 // selection based on type and number of parameters
    38 void f( void );                         // (1)
    39 void f( char );                         // (2)
    40 void f( int, double );                  // (3)
    41 f();                                    // select (1)
    42 f( 'a' );                               // select (2)
    43 f( 3, 5.2 );                            // select (3)
     33//selection based on type and number of parameters
     34void f(void);                   //(1)
     35void f(char);                   //(2)
     36void f(int, double);    //(3)
     37f();                                    //select (1)
     38f('a');                                 //select (2)
     39f(3, 5.2);                              //select (3)
    4440
    45 // selection based on  type and number of returns
    46 char f( int );                          // (1)
    47 double f( int );                        // (2)
    48 [ int, double ] f( int );               // (3)
    49 char c = f( 3 );                        // select (1)
    50 double d = f( 4 );                      // select (2)
    51 [ int, double ] t = f( 5 );             // select (3)
     41//selection based on  type and number of returns
     42char   f(int);                  //(1)
     43double f(int);                  //(2)
     44char   c = f(3);                //select (1)
     45double d = f(4);                //select (2)
    5246\end{cfacode}
    53 This feature is particularly important for concurrency since the runtime system relies on creating different types do represent concurrency objects. Therefore, overloading is necessary to prevent the need for long prefixes and other naming conventions that prevent clashes. As seen in chapter \ref{basics}, the main is an example of routine that benefits from overloading when concurrency in introduced.
     47This feature is particularly important for concurrency since the runtime system relies on creating different types to represent concurrency objects. Therefore, overloading is necessary to prevent the need for long prefixes and other naming conventions that prevent name clashes. As seen in chapter \ref{basics}, routines main is an example that benefits from overloading.
    5448
    5549\section{Operators}
    5650Overloading also extends to operators. The syntax for denoting operator-overloading is to name a routine with the symbol of the operator and question marks where the arguments of the operation would be, like so :
    5751\begin{cfacode}
    58 int ++?( int op );                      // unary prefix increment
    59 int ?++( int op );                      // unary postfix increment
    60 int ?+?( int op1, int op2 );            // binary plus
    61 int ?<=?( int op1, int op2 );           // binary less than
    62 int ?=?( int & op1, int op2 );          // binary assignment
    63 int ?+=?( int & op1, int op2 );         // binary plus-assignment
     52int ++? (int op);                       //unary prefix increment
     53int ?++ (int op);                       //unary postfix increment
     54int ?+? (int op1, int op2);             //binary plus
     55int ?<=?(int op1, int op2);             //binary less than
     56int ?=? (int & op1, int op2);           //binary assignment
     57int ?+=?(int & op1, int op2);           //binary plus-assignment
    6458
    65 struct S { int i, j; };
    66 S ?+?( S op1, S op2 ) {                 // add two structures
    67         return (S){ op1.i + op2.i, op1.j + op2.j };
     59struct S {int i, j;};
     60S ?+?(S op1, S op2) {                           //add two structures
     61        return (S){op1.i + op2.i, op1.j + op2.j};
    6862}
    69 S s1 = { 1, 2 }, s2 = { 2, 3 }, s3;
    70 s3 = s1 + s2;                           // compute sum: s3 == { 2, 5 }
     63S s1 = {1, 2}, s2 = {2, 3}, s3;
     64s3 = s1 + s2;                                           //compute sum: s3 == {2, 5}
    7165\end{cfacode}
    72 
    73 Since concurrency does not use operator overloading, this feature is more important as an introduction for the syntax of constructors.
     66While concurrency does not use operator overloading directly, this feature is more important as an introduction for the syntax of constructors.
    7467
    7568\section{Constructors/Destructors}
    76 Object life time is often a challenge in concurrency. \CFA uses the approach of giving concurrent meaning to object life time as a mean of synchronization and/or mutual exclusion. Since \CFA relies heavily on the life time of objects, Constructors \& Destructors are a the core of the features required for concurrency and parallelism. \CFA uses the following syntax for constructors and destructors :
     69Object life-time is often a challenge in concurrency. \CFA uses the approach of giving concurrent meaning to object life-time as a mean of synchronization and/or mutual exclusion. Since \CFA relies heavily on the life time of objects, constructors and destructors are a core feature required for concurrency and parallelism. \CFA uses the following syntax for constructors and destructors :
    7770\begin{cfacode}
    7871struct S {
    … …  
    8073        int * ia;
    8174};
    82 void ?{}( S & s, int asize ) with s {   // constructor operator
    83         size = asize;                   // initialize fields
    84         ia = calloc( size, sizeof( S ) );
     75void ?{}(S & s, int asize) {    //constructor operator
     76        s.size = asize;                         //initialize fields
     77        s.ia = calloc(size, sizeof(S));
    8578}
    86 void ^?{}( S & s ) with s {             // destructor operator
    87         free( ia );                     // de-initialization fields
     79void ^?{}(S & s) {                              //destructor operator
     80        free(ia);                                       //de-initialization fields
    8881}
    8982int main() {
    90         S x = { 10 }, y = { 100 };      // implict calls: ?{}( x, 10 ), ?{}( y, 100 )
    91         ...                             // use x and y
    92         ^x{};  ^y{};                    // explicit calls to de-initialize
    93         x{ 20 };  y{ 200 };             // explicit calls to reinitialize
    94         ...                             // reuse x and y
    95 }                                       // implict calls: ^?{}( y ), ^?{}( x )
     83        S x = {10}, y = {100};          //implict calls: ?{}(x, 10), ?{}(y, 100)
     84        ...                                                     //use x and y
     85        ^x{};  ^y{};                            //explicit calls to de-initialize
     86        x{20};  y{200};                         //explicit calls to reinitialize
     87        ...                                                     //reuse x and y
     88}                                                               //implict calls: ^?{}(y), ^?{}(x)
    9689\end{cfacode}
    97 The language guarantees that every object and all their fields are constructed. Like \CC construction is automatically done on declaration and destruction done when the declared variables reach the end of its scope.
     90The language guarantees that every object and all their fields are constructed. Like \CC, construction of an object is automatically done on allocation and destruction of the object is done on deallocation. Allocation and deallocation can occur on the stack or on the heap.
     91\begin{cfacode}
     92{
     93        struct S s = {10};      //allocation, call constructor
     94        ...
     95}                                               //deallocation, call destructor
     96struct S * s = new();   //allocation, call constructor
     97...
     98delete(s);                              //deallocation, call destructor
     99\end{cfacode}
     100Note that like \CC, \CFA introduces \code{new} and \code{delete}, which behave like \code{malloc} and \code{free} in addition to constructing and destructing objects, after calling \code{malloc} and before calling \code{free} respectively.
    98101
    99 For more information see \cite{cforall-ug,rob-thesis,www-cfa}.
     102\section{Parametric Polymorphism}
     103Routines in \CFA can also be reused for multiple types. This is done using the \code{forall} clause which gives \CFA it's name. \code{forall} clauses allow seperatly compiled routines to support generic usage over multiple types. For example, the following sum function will work for any type which support construction from 0 and addition :
     104\begin{cfacode}
     105//constraint type, 0 and +
     106forall(otype T | { void ?{}(T *, zero_t); T ?+?(T, T); })
     107T sum(T a[ ], size_t size) {
     108        T total = 0;                            //construct T from 0
     109        for(size_t i = 0; i < size; i++)
     110                total = total + a[i];   //select appropriate +
     111        return total;
     112}
     113
     114S sa[5];
     115int i = sum(sa, 5);                             //use S's 0 construction and +
     116\end{cfacode}
     117
     118Since writing constraints on types can become cumbersome for more constrained functions, \CFA also has the concept of traits. Traits are named collection of constraints which can be used both instead and in addition to regular constraints:
     119\begin{cfacode}
     120trait sumable( otype T ) {
     121        void ?{}(T *, zero_t);          //constructor from 0 literal
     122        T ?+?(T, T);                            //assortment of additions
     123        T ?+=?(T *, T);
     124        T ++?(T *);
     125        T ?++(T *);
     126};
     127forall( otype T | sumable(T) )  //use trait
     128T sum(T a[], size_t size);
     129\end{cfacode}
     130
     131\section{with Clause/Statement}
     132Since \CFA lacks the concept of a receiver, certain functions end-up needing to repeat variable names often, to solve this \CFA offers the \code{with} statement which opens an aggregate scope making its fields directly accessible (like Pascal).
     133\begin{cfacode}
     134struct S { int i, j; };
     135int mem(S & this) with this             //with clause
     136        i = 1;                                          //this->i
     137        j = 2;                                          //this->j
     138}
     139int foo() {
     140        struct S1 { ... } s1;
     141        struct S2 { ... } s2;
     142        with s1                                         //with statement
     143        {
     144                //access fields of s1
     145                //without qualification
     146                with s2                                 //nesting
     147                {
     148                        //access fields of s1 and s2
     149                        //without qualification
     150                }
     151        }
     152        with s1, s2                             //scopes open in parallel
     153        {
     154                //access fields of s1 and s2
     155                //without qualification
     156        }
     157}
     158\end{cfacode}
     159
     160For more information on \CFA see \cite{cforall-ug,rob-thesis,www-cfa}.

  • doc/proposals/concurrency/text/concurrency.tex

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    700700\end{tabular}
    701701\end{center}
    702 This method is more constrained and explicit, which may help users tone down the undeterministic nature of concurrency. Indeed, as the following examples demonstrates, external scheduling allows users to wait for events from other threads without the concern of unrelated events occuring. External scheduling can generally be done either in terms of control flow (e.g., \uC) or in terms of data (e.g. Go). Of course, both of these paradigms have their own strenghts and weaknesses but for this project control-flow semantics were chosen to stay consistent with the rest of the languages semantics. Two challenges specific to \CFA arise when trying to add external scheduling with loose object definitions and multi-monitor routines. The previous example shows a simple use \code{_Accept} versus \code{wait}/\code{signal} and its advantages. Note that while other languages often use \code{accept} as the core external scheduling keyword, \CFA uses \code{waitfor} to prevent name collisions with existing socket APIs.
     702This method is more constrained and explicit, which may help users tone down the undeterministic nature of concurrency. Indeed, as the following examples demonstrates, external scheduling allows users to wait for events from other threads without the concern of unrelated events occuring. External scheduling can generally be done either in terms of control flow (e.g., \uC) or in terms of data (e.g. Go). Of course, both of these paradigms have their own strenghts and weaknesses but for this project control-flow semantics were chosen to stay consistent with the rest of the languages semantics. Two challenges specific to \CFA arise when trying to add external scheduling with loose object definitions and multi-monitor routines. The previous example shows a simple use \code{_Accept} versus \code{wait}/\code{signal} and its advantages. Note that while other languages often use \code{accept} as the core external scheduling keyword, \CFA uses \code{waitfor} to prevent name collisions with existing socket \acrshort{api}s.
    703703
    704704In the case of internal scheduling, the call to \code{wait} only guarantees that \code{V} is the last routine to access the monitor. This entails that the routine \code{V} may have acquired mutual exclusion several times while routine \code{P} was waiting. On the other hand, external scheduling guarantees that while routine \code{P} was waiting, no routine other than \code{V} could acquire the monitor.

  • doc/proposals/concurrency/text/intro.tex

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    33% ======================================================================
    44
    5 This proposal provides a minimal concurrency API that is simple, efficient and can be reused to build higher-level features. The simplest possible concurrency system is a thread and a lock but this low-level approach is hard to master. An easier approach for users is to support higher-level constructs as the basis of the concurrency, in \CFA. Indeed, for highly productive concurrent programming, high-level approaches are much more popular~\cite{HPP:Study}. Examples are task based, message passing and implicit threading. Therefore a high-level approach is adopted in \CFA
     5This thesis provides a minimal concurrency \acrshort{api} that is simple, efficient and can be reused to build higher-level features. The simplest possible concurrency system is a thread and a lock but this low-level approach is hard to master. An easier approach for users is to support higher-level constructs as the basis of concurrency. Indeed, for highly productive concurrent programming, high-level approaches are much more popular~\cite{HPP:Study}. Examples are task based, message passing and implicit threading. The high-level approach and its minimal \acrshort{api} are tested in a dialect of C, call \CFA. [Is there value to say that this thesis is also an early definition of the \CFA language and library in regards to concurrency?]
    66
    7 There are actually two problems that need to be solved in the design of concurrency for a programming language: which concurrency and which parallelism tools are available to the programmers. While these two concepts are often combined, they are in fact distinct, requiring different tools~\cite{Buhr05a}. Concurrency tools need to handle mutual exclusion and synchronization, while parallelism tools are about performance, cost and resource utilization.
     7There are actually two problems that need to be solved in the design of concurrency for a programming language: which concurrency and which parallelism tools are available to the programmer. While these two concepts are often combined, they are in fact distinct, requiring different tools~\cite{Buhr05a}. Concurrency tools need to handle mutual exclusion and synchronization, while parallelism tools are about performance, cost and resource utilization.

  • doc/proposals/concurrency/thesis.tex

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    103103\input{parallelism}
    104104
     105\input{internals}
     106
    105107\input{together}
    106108

  • doc/proposals/concurrency/version

    rbb9d8e8 rb7778c1  
    1 0.10.2
     10.10.95
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