Ignore:
Timestamp:
Oct 17, 2017, 12:06:15 PM (4 years ago)
Author:
Thierry Delisle <tdelisle@…>
Branches:
aaron-thesis, arm-eh, cleanup-dtors, deferred_resn, demangler, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, new-env, no_list, persistent-indexer, resolv-new, with_gc
Children:
633c711
Parents:
0aaac0e
Message:

Updated thesis up to implementation

File:
1 edited

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

    r0aaac0e rfb31cb8  
    2828A monitor is a set of routines that ensure mutual exclusion when accessing shared state. This concept is generally associated with Object-Oriented Languages like Java~\cite{Java} or \uC~\cite{uC++book} but does not strictly require OO semantics. The only requirements is the ability to declare a handle to a shared object and a set of routines that act on it :
    2929\begin{cfacode}
    30         typedef /*some monitor type*/ monitor;
    31         int f(monitor & m);
    32 
    33         int main() {
    34                 monitor m;  //Handle m
    35                 f(m);       //Routine using handle
    36         }
     30typedef /*some monitor type*/ monitor;
     31int f(monitor & m);
     32
     33int main() {
     34        monitor m;  //Handle m
     35        f(m);       //Routine using handle
     36}
    3737\end{cfacode}
    3838
     
    4747
    4848\begin{cfacode}
    49         monitor counter_t { /*...see section $\ref{data}$...*/ };
    50 
    51         void ?{}(counter_t & nomutex this); //constructor
    52         size_t ++?(counter_t & mutex this); //increment
    53 
    54         //need for mutex is platform dependent
    55         void ?{}(size_t * this, counter_t & mutex cnt); //conversion
     49monitor counter_t { /*...see section $\ref{data}$...*/ };
     50
     51void ?{}(counter_t & nomutex this); //constructor
     52size_t ++?(counter_t & mutex this); //increment
     53
     54//need for mutex is platform dependent
     55void ?{}(size_t * this, counter_t & mutex cnt); //conversion
    5656\end{cfacode}
    5757This counter is used as follows:
     
    125125The capacity to acquire multiple locks before entering a critical section is called \emph{\gls{bulk-acq}}. In practice, writing multi-locking routines that do not lead to deadlocks is tricky. Having language support for such a feature is therefore a significant asset for \CFA. In the case presented above, \CFA guarantees that the order of aquisition is consistent across calls to routines using the same monitors as arguments. However, since \CFA monitors use \gls{multi-acq} locks, users can effectively force the acquiring order. For example, notice which routines use \code{mutex}/\code{nomutex} and how this affects aquiring order:
    126126\begin{cfacode}
    127         void foo(A & mutex a, B & mutex b) { //acquire a & b
    128                 ...
    129         }
    130 
    131         void bar(A & mutex a, B & /*nomutex*/ b) { //acquire a
    132                 ... foo(a, b); ... //acquire b
    133         }
    134 
    135         void baz(A & /*nomutex*/ a, B & mutex b) { //acquire b
    136                 ... foo(a, b); ... //acquire a
    137         }
     127void foo(A & mutex a, B & mutex b) { //acquire a & b
     128        ...
     129}
     130
     131void bar(A & mutex a, B & /*nomutex*/ b) { //acquire a
     132        ... foo(a, b); ... //acquire b
     133}
     134
     135void baz(A & /*nomutex*/ a, B & mutex b) { //acquire b
     136        ... foo(a, b); ... //acquire a
     137}
    138138\end{cfacode}
    139139The \gls{multi-acq} monitor lock allows a monitor lock to be acquired by both \code{bar} or \code{baz} and acquired again in \code{foo}. In the calls to \code{bar} and \code{baz} the monitors are acquired in opposite order.
     
    159159This example shows a trivial solution to the bank account transfer problem\cit. Without \gls{multi-acq} and \gls{bulk-acq}, the solution to this problem is much more involved and requires carefull engineering.
    160160
     161\subsubsection{\code{mutex} statement} \label{mutex-stmt}
     162
     163The call semantics discussed aboved have one software engineering issue, only a named routine can acquire the mutual-exclusion of a set of monitor. \CFA offers the \code{mutex} statement to workaround the need for unnecessary names, avoiding a major software engineering problem\cit. Listing \ref{lst:mutex-stmt} shows an example of the \code{mutex} statement, which introduces a new scope in which the mutual-exclusion of a set of monitor is acquired. Beyond naming, the \code{mutex} statement has no semantic difference from a routine call with \code{mutex} parameters.
     164
     165\begin{figure}
     166\begin{center}
     167\begin{tabular}{|c|c|}
     168function call & \code{mutex} statement \\
     169\hline
     170\begin{cfacode}[tabsize=3]
     171monitor M {};
     172void foo( M & mutex m ) {
     173        //critical section
     174}
     175
     176void bar( M & m ) {
     177        foo( m );
     178}
     179\end{cfacode}&\begin{cfacode}[tabsize=3]
     180monitor M {};
     181void bar( M & m ) {
     182        mutex(m) {
     183                //critical section
     184        }
     185}
     186
     187
     188\end{cfacode}
     189\end{tabular}
     190\end{center}
     191\caption{Regular call semantics vs. \code{mutex} statement}
     192\label{lst:mutex-stmt}
     193\end{figure}
     194
    161195% ======================================================================
    162196% ======================================================================
     
    195229
    196230\begin{cfacode}
    197         monitor A {
    198                 condition e;
    199         }
    200 
    201         void foo(A & mutex a) {
    202                 ...
    203                 //Wait for cooperation from bar()
    204                 wait(a.e);
    205                 ...
    206         }
    207 
    208         void bar(A & mutex a) {
    209                 //Provide cooperation for foo()
    210                 ...
    211                 //Unblock foo
    212                 signal(a.e);
    213         }
     231monitor A {
     232        condition e;
     233}
     234
     235void foo(A & mutex a) {
     236        ...
     237        //Wait for cooperation from bar()
     238        wait(a.e);
     239        ...
     240}
     241
     242void bar(A & mutex a) {
     243        //Provide cooperation for foo()
     244        ...
     245        //Unblock foo
     246        signal(a.e);
     247}
    214248\end{cfacode}
    215249
     
    223257% ======================================================================
    224258% ======================================================================
    225 It is easier to understand the problem of multi-monitor scheduling using a series of pseudo-code. Note that for simplicity in the following snippets of pseudo-code, waiting and signalling is done using an implicit condition variable, like Java built-in monitors.
     259It is easier to understand the problem of multi-monitor scheduling using a series of pseudo-code. Note that for simplicity in the following snippets of pseudo-code, waiting and signalling is done using an implicit condition variable, like Java built-in monitors. Indeed, \code{wait} statements always use a single condition as paremeter and waits on the monitors associated with the condition.
    226260
    227261\begin{multicols}{2}
     
    305339\end{multicols}
    306340
    307 The next example is where \gls{bulk-acq} adds a significant layer of complexity to the internal signalling semantics.
    308 
     341Listing \ref{lst:int-bulk-pseudo} shows an example where \gls{bulk-acq} adds a significant layer of complexity to the internal signalling semantics. Listing \ref{lst:int-bulk-cfa} shows the corresponding \CFA code which implements the pseudo-code in listing \ref{lst:int-bulk-pseudo}. Note that listing \ref{lst:int-bulk-cfa} uses non-\code{mutex} parameter to introduce monitor \code{b} into context. However, for the purpose of translating the given pseudo-code into \CFA-code any method of introducing new monitors into context, other than a \code{mutex} parameter, is acceptable, e.g. global variables, pointer parameters or using locals with the \code{mutex}-statement.
     342
     343\begin{figure}[!b]
    309344\begin{multicols}{2}
    310345Waiting thread
     
    336371\end{pseudo}
    337372\end{multicols}
    338 \begin{center}
    339 Listing 1
    340 \end{center}
     373\caption{Internal scheduling with \gls{bulk-acq}}
     374\label{lst:int-bulk-pseudo}
     375\end{figure}
     376
     377\begin{figure}[!b]
     378\begin{multicols}{2}
     379Waiting thread
     380\begin{cfacode}
     381monitor A;
     382monitor B;
     383extern condition c;
     384void foo(A & mutex a, B & b) {
     385        //Code Section 1
     386        mutex(a, b) {
     387                //Code Section 2
     388                wait(c);
     389                //Code Section 3
     390        }
     391        //Code Section 4
     392}
     393\end{cfacode}
     394
     395\columnbreak
     396
     397Signalling thread
     398\begin{cfacode}
     399monitor A;
     400monitor B;
     401extern condition c;
     402void foo(A & mutex a, B & b) {
     403        //Code Section 5
     404        mutex(a, b) {
     405                //Code Section 6
     406                signal(c);
     407                //Code Section 7
     408        }
     409        //Code Section 8
     410}
     411\end{cfacode}
     412\end{multicols}
     413\caption{Equivalent \CFA code for listing \ref{lst:int-bulk-pseudo}}
     414\label{lst:int-bulk-cfa}
     415\end{figure}
    341416
    342417It is particularly important to pay attention to code sections 4 and 8, which are where the existing semantics of internal scheduling need to be extended for multiple monitors. The root of the problem is that \gls{bulk-acq} is used in a context where one of the monitors is already acquired and is why it is important to define the behaviour of the previous pseudo-code. When the signaller thread reaches the location where it should "release A \& B" (line 16), it must actually transfer ownership of monitor B to the waiting thread. This ownership trasnfer is required in order to prevent barging. Since the signalling thread still needs monitor A, simply waking up the waiting thread is not an option because it would violate mutual exclusion. There are three options.
     
    388463
    389464Thread 3
    390 \begin{pseudo}[numbers=left, firstnumber=10]
     465\begin{pseudo}[numbers=left, firstnumber=9]
    391466acquire A
    392467        acquire A & B
     
    480555\end{center}
    481556\label{fig:dependency}
    482 \caption{Dependency graph of the statments in listing \ref{lst:dependency}}
     557\caption{Dependency graph of the statements in listing \ref{lst:dependency}}
    483558\end{figure}
    484559
    485 Listing \ref{lst:dependency} is the three thread example rewritten for dependency graphs as well as the corresponding dependency graph. Figure \ref{fig:dependency} shows the corresponding dependency graph that results, where every node is a statment of one of the three threads, and the arrows the dependency of that statment. The extra challenge is that this dependency graph is effectively post-mortem, but the run time system needs to be able to build and solve these graphs as the dependency unfolds. Resolving dependency graph being a complex and expensive endeavour, this solution is not the preffered one.
     560Listing \ref{lst:dependency} is the three thread example rewritten for dependency graphs as well as the corresponding dependency graph. Figure \ref{fig:dependency} shows the corresponding dependency graph that results, where every node is a statement of one of the three threads, and the arrows the dependency of that statement. The extra challenge is that this dependency graph is effectively post-mortem, but the run time system needs to be able to build and solve these graphs as the dependency unfolds. Resolving dependency graph being a complex and expensive endeavour, this solution is not the preffered one.
    486561
    487562\subsubsection{Partial signalling} \label{partial-sig}
     
    675750
    676751\begin{cfacode}
    677         monitor A {};
    678 
    679         void f(A & mutex a);
    680         void g(A & mutex a) {
    681                 waitfor(f); //Obvious which f() to wait for
    682         }
    683 
    684         void f(A & mutex a, int); // New different F added in scope
    685         void h(A & mutex a) {
    686                 waitfor(f); //Less obvious which f() to wait for
    687         }
     752monitor A {};
     753
     754void f(A & mutex a);
     755void g(A & mutex a) {
     756        waitfor(f); //Obvious which f() to wait for
     757}
     758
     759void f(A & mutex a, int); //New different F added in scope
     760void h(A & mutex a) {
     761        waitfor(f); //Less obvious which f() to wait for
     762}
    688763\end{cfacode}
    689764
     
    732807External scheduling, like internal scheduling, becomes significantly more complex when introducing multi-monitor syntax. Even in the simplest possible case, some new semantics need to be established:
    733808\begin{cfacode}
    734         monitor M {};
    735 
    736         void f(M & mutex a);
    737 
    738         void g(M & mutex a, M & mutex b) {
    739                 waitfor(f); //ambiguous, keep a pass b or other way around?
    740         }
     809monitor M {};
     810
     811void f(M & mutex a);
     812
     813void g(M & mutex a, M & mutex b) {
     814        waitfor(f); //ambiguous, keep a pass b or other way around?
     815}
    741816\end{cfacode}
    742817
     
    744819
    745820\begin{cfacode}
    746         monitor M {};
    747 
    748         void f(M & mutex a);
    749 
    750         void g(M & mutex a, M & mutex b) {
    751                 waitfor( f, b );
    752         }
    753 \end{cfacode}
    754 
    755 This syntax is unambiguous. Both locks are acquired and kept. When routine \code{f} is called, the lock for monitor \code{b} is temporarily transferred from \code{g} to \code{f} (while \code{g} still holds lock \code{a}). This behavior can be extended to multi-monitor waitfor statment as follows.
    756 
    757 \begin{cfacode}
    758         monitor M {};
    759 
    760         void f(M & mutex a, M & mutex b);
    761 
    762         void g(M & mutex a, M & mutex b) {
    763                 waitfor( f, a, b);
    764         }
     821monitor M {};
     822
     823void f(M & mutex a);
     824
     825void g(M & mutex a, M & mutex b) {
     826        waitfor( f, b );
     827}
     828\end{cfacode}
     829
     830This syntax is unambiguous. Both locks are acquired and kept. When routine \code{f} is called, the lock for monitor \code{b} is temporarily transferred from \code{g} to \code{f} (while \code{g} still holds lock \code{a}). This behavior can be extended to multi-monitor waitfor statement as follows.
     831
     832\begin{cfacode}
     833monitor M {};
     834
     835void f(M & mutex a, M & mutex b);
     836
     837void g(M & mutex a, M & mutex b) {
     838        waitfor( f, a, b);
     839}
    765840\end{cfacode}
    766841
     
    770845
    771846\begin{cfacode}
    772         mutex struct A {};
    773 
    774         mutex struct B {};
    775 
    776         void g(A & mutex a, B & mutex b) {
    777                 waitfor(f, a, b);
    778         }
    779 
    780         A a1, a2;
    781         B b;
    782 
    783         void foo() {
    784                 g(a1, b); //block on accept
    785         }
    786 
    787         void bar() {
    788                 f(a2, b); //fufill cooperation
    789         }
     847mutex struct A {};
     848
     849mutex struct B {};
     850
     851void g(A & mutex a, B & mutex b) {
     852        waitfor(f, a, b);
     853}
     854
     855A a1, a2;
     856B b;
     857
     858void foo() {
     859        g(a1, b); //block on accept
     860}
     861
     862void bar() {
     863        f(a2, b); //fufill cooperation
     864}
    790865\end{cfacode}
    791866
     
    794869% ======================================================================
    795870% ======================================================================
    796 \subsection{Waitfor semantics}
    797 % ======================================================================
    798 % ======================================================================
     871\subsection{\code{waitfor} semantics}
     872% ======================================================================
     873% ======================================================================
     874
     875Syntactically, the \code{waitfor} statement takes a function identifier and a set of monitors. While the set of monitors can be any list of expression, the function name is more restricted. This is because the compiler validates at compile time the validity of the waitfor statement. It checks that the set of monitor passed in matches the requirements for a function call. Listing \ref{lst:waitfor} shows various usage of the waitfor statement and which are acceptable. The choice of the function type is made ignoring any non-\code{mutex} parameter. One limitation of the current implementation is that it does not handle overloading.
     876\begin{figure}
     877\begin{cfacode}
     878monitor A{};
     879monitor B{};
     880
     881void f1( A & mutex );
     882void f2( A & mutex, B & mutex );
     883void f3( A & mutex, int );
     884void f4( A & mutex, int );
     885void f4( A & mutex, double );
     886
     887void foo( A & mutex a1, A & mutex a2, B & mutex b1, B & b2 ) {
     888        A * ap = & a1;
     889        void (*fp)( A & mutex ) = f1;
     890
     891        waitfor(f1, a1);     //Correct : 1 monitor case
     892        waitfor(f2, a1, b1); //Correct : 2 monitor case
     893        waitfor(f3, a1);     //Correct : non-mutex arguments are ignored
     894        waitfor(f1, *ap);    //Correct : expression as argument
     895
     896        waitfor(f1, a1, b1); //Incorrect : Too many mutex arguments
     897        waitfor(f2, a1);     //Incorrect : Too few mutex arguments
     898        waitfor(f2, a1, a2); //Incorrect : Mutex arguments don't match
     899        waitfor(f1, 1);      //Incorrect : 1 not a mutex argument
     900        waitfor(f4, a1);     //Incorrect : f9 not a function
     901        waitfor(*fp, a1 );   //Incorrect : fp not a identifier
     902        waitfor(f4, a1);     //Incorrect : f4 ambiguous
     903
     904        waitfor(f2, a1, b2); //Undefined Behaviour : b2 may not acquired
     905}
     906\end{cfacode}
     907\caption{Various correct and incorrect uses of the waitfor statement}
     908\label{lst:waitfor}
     909\end{figure}
     910
     911Finally, for added flexibility, \CFA supports constructing complex waitfor mask using the \code{or}, \code{timeout} and \code{else}. Indeed, multiple \code{waitfor} can be chained together using \code{or}; this chain will form a single statement which will baton-pass to any one function that fits one of the function+monitor set which was passed in. To eanble users to tell which was the accepted function, \code{waitfor}s are followed by a statement (including the null statement \code{;}) or a compound statement. When multiple \code{waitfor} are chained together, only the statement corresponding to the accepted function is executed. A \code{waitfor} chain can also be followed by a \code{timeout}, to signify an upper bound on the wait, or an \code{else}, to signify that the call should be non-blocking, that is only check of a matching function already arrived and return immediately otherwise. Any and all of these clauses can be preceded by a \code{when} condition to dynamically construct the mask based on some current state. Listing \ref{lst:waitfor2}, demonstrates several complex masks and some incorrect ones.
     912
     913\begin{figure}
     914\begin{cfacode}
     915monitor A{};
     916
     917void f1( A & mutex );
     918void f2( A & mutex );
     919
     920void foo( A & mutex a, bool b, int t ) {
     921        //Correct : blocking case
     922        waitfor(f1, a);
     923
     924        //Correct : block with statement
     925        waitfor(f1, a) {
     926                sout | "f1" | endl;
     927        }
     928
     929        //Correct : block waiting for f1 or f2
     930        waitfor(f1, a) {
     931                sout | "f1" | endl;
     932        } or waitfor(f2, a) {
     933                sout | "f2" | endl;
     934        }
     935
     936        //Correct : non-blocking case
     937        waitfor(f1, a); or else;
     938
     939        //Correct : non-blocking case
     940        waitfor(f1, a) {
     941                sout | "blocked" | endl;
     942        } or else {
     943                sout | "didn't block" | endl;
     944        }
     945
     946        //Correct : block at most 10 seconds
     947        waitfor(f1, a) {
     948                sout | "blocked" | endl;
     949        } or timeout( 10`s) {
     950                sout | "didn't block" | endl;
     951        }
     952
     953        //Correct : block only if b == true
     954        //if b == false, don't even make the call
     955        when(b) waitfor(f1, a);
     956
     957        //Correct : block only if b == true
     958        //if b == false, make non-blocking call
     959        waitfor(f1, a); or when(!b) else;
     960
     961        //Correct : block only of t > 1
     962        waitfor(f1, a); or when(t > 1) timeout(t); or else;
     963
     964        //Incorrect : timeout clause is dead code
     965        waitfor(f1, a); or timeout(t); or else;
     966
     967        //Incorrect : order must be
     968        //waitfor [or waitfor... [or timeout] [or else]]
     969        timeout(t); or waitfor(f1, a); or else;
     970}
     971\end{cfacode}
     972\caption{Various correct and incorrect uses of the or, else, and timeout clause around a waitfor statement}
     973\label{lst:waitfor2}
     974\end{figure}
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