Ignore:
Timestamp:
Nov 8, 2017, 5:43:33 PM (8 years ago)
Author:
Aaron Moss <a3moss@…>
Branches:
ADT, aaron-thesis, arm-eh, ast-experimental, cleanup-dtors, deferred_resn, demangler, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, new-env, no_list, persistent-indexer, pthread-emulation, qualifiedEnum, resolv-new, with_gc
Children:
954908d
Parents:
78315272 (diff), e35f30a (diff)
Note: this is a merge changeset, the changes displayed below correspond to the merge itself.
Use the (diff) links above to see all the changes relative to each parent.
Message:

Merge branch 'master' of plg.uwaterloo.ca:software/cfa/cfa-cc

File:
1 edited

Legend:

Unmodified
Added
Removed

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

    r78315272 r3f7e12cb  
    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
     11Execution 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.
     14
     15A scheduler introduces order of execution uncertainty, while preemption introduces uncertainty about where context-switches occur. Mutual-exclusion and synchronisation are ways of limiting non-determinism in a concurrent system. Now it is important to understand that uncertainty is desireable; uncertainty can be used by runtime 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.
    1016
    1117\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.
     18One 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 programs to take advantage of parallelism. 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.
    1319
    1420\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;
     21While 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-switches 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}.
     22
     23\begin{figure}
     24\begin{center}
     25\begin{tabular}{c @{\hskip 0.025in}|@{\hskip 0.025in} c @{\hskip 0.025in}|@{\hskip 0.025in} c}
     26\begin{ccode}[tabsize=2]
     27//Using callbacks
     28void fibonacci_func(
     29        int n,
     30        void (*callback)(int)
     31) {
     32        int first = 0;
     33        int second = 1;
     34        int next, i;
     35        for(i = 0; i < n; i++)
     36        {
     37                if(i <= 1)
     38                        next = i;
     39                else {
     40                        next = f1 + f2;
     41                        f1 = f2;
     42                        f2 = next;
     43                }
     44                callback(next);
     45        }
     46}
     47
     48int main() {
     49        void print_fib(int n) {
     50                printf("%d\n", n);
     51        }
     52
     53        fibonacci_func(
     54                10, print_fib
     55        );
     56
     57
     58
     59}
     60\end{ccode}&\begin{ccode}[tabsize=2]
     61//Using output array
     62void fibonacci_array(
     63        int n,
     64        int * array
     65) {
     66        int f1 = 0; int f2 = 1;
     67        int next, i;
     68        for(i = 0; i < n; i++)
     69        {
     70                if(i <= 1)
     71                        next = i;
     72                else {
     73                        next = f1 + f2;
     74                        f1 = f2;
     75                        f2 = next;
     76                }
     77                array[i] = next;
     78        }
     79}
     80
     81
     82int main() {
     83        int a[10];
     84
     85        fibonacci_func(
     86                10, a
     87        );
     88
     89        for(int i=0;i<10;i++){
     90                printf("%d\n", a[i]);
     91        }
     92
     93}
     94\end{ccode}&\begin{ccode}[tabsize=2]
     95//Using external state
     96typedef struct {
     97        int f1, f2;
     98} Iterator_t;
     99
     100int fibonacci_state(
     101        Iterator_t * it
     102) {
     103        int f;
     104        f = it->f1 + it->f2;
     105        it->f2 = it->f1;
     106        it->f1 = max(f,1);
     107        return f;
     108}
     109
     110
     111
     112
     113
     114
     115
     116int main() {
     117        Iterator_t it={0,0};
     118
     119        for(int i=0;i<10;i++){
     120                printf("%d\n",
     121                        fibonacci_state(
     122                                &it
     123                        );
     124                );
     125        }
     126
     127}
     128\end{ccode}
     129\end{tabular}
     130\end{center}
     131\caption{Different implementations of a fibonacci sequence generator in C.}
     132\label{lst:fibonacci-c}
     133\end{figure}
     134
     135A good example of a problem made easier with coroutines is generators, like the fibonacci sequence. This problem comes with the challenge of decoupling how a sequence is generated and how it is used. Figure \ref{lst: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 is used, while the rightmost approach requires holding internal state between calls on behalf of the generator and makes it much harder to handle corner cases like the Fibonacci seed.
     136
     137Figure \ref{lst:fibonacci-cfa} is an example of a solution to the fibonnaci problem using \CFA coroutines, where the coroutine stack holds 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 as easy to use as the \code{fibonacci_state} solution, while the imlpementation is very similar to the \code{fibonacci_func} example.
     138
     139\begin{figure}
     140\begin{cfacode}
     141coroutine Fibonacci {
     142        int fn; //used for communication
     143};
     144
     145void ?{}(Fibonacci & this) { //constructor
     146        this.fn = 0;
     147}
     148
     149//main automacically called on first resume
     150void main(Fibonacci & this) with (this) {
     151        int fn1, fn2;           //retained between resumes
     152        fn  = 0;
     153        fn1 = fn;
     154        suspend(this);          //return to last resume
     155
     156        fn  = 1;
     157        fn2 = fn1;
     158        fn1 = fn;
     159        suspend(this);          //return to last resume
     160
     161        for ( ;; ) {
     162                fn  = fn1 + fn2;
    35163                fn2 = fn1;
    36                 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
     164                fn1 = fn;
     165                suspend(this);  //return to last resume
     166        }
     167}
     168
     169int next(Fibonacci & this) {
     170        resume(this); //transfer to last suspend
     171        return this.fn;
     172}
     173
     174void main() { //regular program main
     175        Fibonacci f1, f2;
     176        for ( int i = 1; i <= 10; i += 1 ) {
     177                sout | next( f1 ) | next( f2 ) | endl;
     178        }
     179}
     180\end{cfacode}
     181\caption{Implementation of fibonacci using coroutines}
     182\label{lst:fibonacci-cfa}
     183\end{figure}
     184
     185Figure \ref{lst:fmt-line} shows the \code{Format} coroutine which rearranges text in order to group characters into blocks of fixed size. The example takes advantage of resuming coroutines in the constructor to simplify the code and highlights the idea that interesting control flow can occur in the constructor.
     186
     187\begin{figure}
     188\begin{cfacode}[tabsize=3]
     189//format characters into blocks of 4 and groups of 5 blocks per line
     190coroutine Format {
     191        char ch;                                                                        //used for communication
     192        int g, b;                                                               //global because used in destructor
     193};
     194
     195void  ?{}(Format & fmt) {
     196        resume( fmt );                                                  //prime (start) coroutine
     197}
     198
     199void ^?{}(Format & fmt) with fmt {
     200        if ( fmt.g != 0 || fmt.b != 0 )
     201        sout | endl;
     202}
     203
     204void main(Format & fmt) with fmt {
     205        for ( ;; ) {                                                    //for as many characters
     206                for(g = 0; g < 5; g++) {                //groups of 5 blocks
     207                        for(b = 0; b < 4; fb++) {       //blocks of 4 characters
     208                                suspend();
     209                                sout | ch;                                      //print character
     210                        }
     211                        sout | "  ";                                    //print block separator
    44212                }
    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}
     213                sout | endl;                                            //print group separator
     214        }
     215}
     216
     217void prt(Format & fmt, char ch) {
     218        fmt.ch = ch;
     219        resume(fmt);
     220}
     221
     222int main() {
     223        Format fmt;
     224        char ch;
     225        Eof: for ( ;; ) {                                               //read until end of file
     226                sin | ch;                                                       //read one character
     227                if(eof(sin)) break Eof;                 //eof ?
     228                prt(fmt, ch);                                           //push character for formatting
     229        }
     230}
     231\end{cfacode}
     232\caption{Formatting text into lines of 5 blocks of 4 characters.}
     233\label{lst:fmt-line}
     234\end{figure}
    59235
    60236\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.
     237One 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 fully constructed 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.
     238
     239The 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.
    64240
    65241Furthermore, \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:
    … …  
    71247
    72248forall(otype T)
    73 void noop(T *) {}
     249void noop(T*) {}
    74250
    75251void bar() {
    76252        int a;
    77         async(noop, &a);
    78 }
    79 \end{cfacode}
     253        async(noop, &a); //start thread running noop with argument a
     254}
     255\end{cfacode}
     256
    80257The generated C code\footnote{Code trimmed down for brevity} creates a local thunk to hold type information:
    81258
    … …  
    95272}
    96273\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.
     274The problem in this example is a storage management issue, the function pointer \code{_thunk0} is only valid until the end of the block, which limits the viable solutions because storing the function pointer for too long causes undefined behavior; i.e., the stack-based thunk being destroyed before it can be used. This challenge is an extension of challenges that come with second-class routines. Indeed, GCC nested routines also have the limitation that nested routine cannot be passed outside of the declaration scope. The case of coroutines and threads is simply an extension of this problem to multiple call-stacks.
    98275
    99276\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.
     277One solution to this challenge is to use composition/containement, where coroutine fields are added to manage the coroutine.
     278
     279\begin{cfacode}
     280struct Fibonacci {
     281        int fn; //used for communication
     282        coroutine c; //composition
     283};
     284
     285void FibMain(void *) {
     286        //...
     287}
     288
     289void ?{}(Fibonacci & this) {
     290        this.fn = 0;
     291        //Call constructor to initialize coroutine
     292        (this.c){myMain};
     293}
     294\end{cfacode}
     295The downside of this approach is that users need to correctly construct the coroutine handle before using it. Like any other objects, doing so the users carefully choose construction order to prevent usage of unconstructed objects. However, in the case of coroutines, users must also pass to the coroutine information about the coroutine main, like in the previous example. This opens the door for user errors and requires extra runtime storage to pass at runtime information that can be known statically.
    114296
    115297\subsection{Alternative: Reserved keyword}
    … …  
    117299
    118300\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.
     301coroutine Fibonacci {
     302        int fn; //used for communication
     303};
     304\end{cfacode}
     305The \code{coroutine} keyword means the compiler can find and inject code where needed. The downside of this approach is that it makes coroutine a special case in the language. Users wantint 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 still be constructed by users without using the language support. The reserved keywords are only present to improve ease of use for the common cases.
    125306
    126307\subsection{Alternative: Lamda Objects}
    … …  
    135316Often, the canonical threading paradigm in languages is based on function pointers, pthread being one of the most well known examples. The main problem of this approach is that the thread usage is limited to a generic handle that must otherwise be wrapped in a custom type. Since the custom type is simple to write in \CFA and solves several issues, added support for routine/lambda based coroutines adds very little.
    136317
    137 A variation of this would be to use an simple function pointer in the same way pthread does for threads :
     318A variation of this would be to use a simple function pointer in the same way pthread does for threads :
    138319\begin{cfacode}
    139320void foo( coroutine_t cid, void * arg ) {
    … …  
    148329}
    149330\end{cfacode}
    150 This semantic is more common for thread interfaces than coroutines but would work equally well. As discussed in section \ref{threads}, this approach is superseeded by static approaches in terms of expressivity.
     331This semantics is more common for thread interfaces than coroutines works equally well. As discussed in section \ref{threads}, this approach is superseeded by static approaches in terms of expressivity.
    151332
    152333\subsection{Alternative: Trait-based coroutines}
    … …  
    159340      coroutine_desc * get_coroutine(T & this);
    160341};
    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.
     342
     343forall( dtype T | is_coroutine(T) ) void suspend(T &);
     344forall( dtype T | is_coroutine(T) ) void resume (T &);
     345\end{cfacode}
     346This 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.
    163347
    164348\begin{center}
    … …  
    186370\end{center}
    187371
    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.
     372The 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.
    189373
    190374\section{Thread Interface}\label{threads}
    … …  
    192376
    193377\begin{cfacode}
    194         thread foo {};
     378thread foo {};
    195379\end{cfacode}
    196380
    … …  
    205389\end{cfacode}
    206390
    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
    208 \begin{cfacode}
    209         thread foo {};
    210 
    211         void main(foo & this) {
    212                 sout | "Hello World!" | endl;
    213         }
    214 \end{cfacode}
    215 
    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);
    219 
    220         thread FuncRunner {
    221                 voidFunc func;
    222         };
    223 
    224         //ctor
    225         void ?{}(FuncRunner & this, voidFunc inFunc) {
    226                 this.func = inFunc;
    227         }
    228 
    229         //main
    230         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.
     391Obviously, 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
     392\begin{cfacode}
     393thread foo {};
     394
     395void main(foo & this) {
     396        sout | "Hello World!" | endl;
     397}
     398\end{cfacode}
     399
     400In 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 the static semantics it is trivial to write a thread type that takes a function pointer as a parameter and executes it on its stack asynchronously.
     401\begin{cfacode}
     402typedef void (*voidFunc)(int);
     403
     404thread FuncRunner {
     405        voidFunc func;
     406        int arg;
     407};
     408
     409void ?{}(FuncRunner & this, voidFunc inFunc, int arg) {
     410        this.func = inFunc;
     411        this.arg  = arg;
     412}
     413
     414void main(FuncRunner & this) {
     415        //thread starts here and runs the function
     416        this.func( this.arg );
     417}
     418\end{cfacode}
     419
     420A 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}.
     421
     422Of 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.
    238423\begin{cfacode}
    239424thread World;
    … …  
    254439\end{cfacode}
    255440
    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
     441This semantic has several advantages over explicit semantics: a thread is always started and stopped exaclty once, users cannot make any progamming errors, and it naturally scales to multiple threads meaning basic synchronisation is very simple.
    257442
    258443\begin{cfacode}
    … …  
    276461\end{cfacode}
    277462
    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
     463However, one of the drawbacks of this approach is that threads now always form a lattice, that is they are always destroyed in the 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.
    279464
    280465\begin{cfacode}
    … …  
    283468};
    284469
    285 //main
    286470void main(MyThread & this) {
    287471        //...
    … …  
    291475        MyThread * long_lived;
    292476        {
     477                //Start a thread at the beginning of the scope
    293478                MyThread short_lived;
    294                 //Start a thread at the beginning of the scope
    295 
    296                 DoStuff();
    297479
    298480                //create another thread that will outlive the thread in this scope
    299481                long_lived = new MyThread;
    300482
     483                DoStuff();
     484
    301485                //Wait for the thread short_lived to finish
    302486        }
    303487        DoMoreStuff();
    304488
    305         //Now wait for the short_lived to finish
     489        //Now wait for the long_lived to finish
    306490        delete long_lived;
    307491}
Note: See TracChangeset for help on using the changeset viewer.