start a thread waiting on a condition variable - Maple Help

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Threads[ConditionVariable][Signal] - start a thread waiting on a condition variable

Calling Sequence

Signal( condId )

Parameters

condId

-

(integer) the condition variable identifier

Description

• 

The Signal command wakes up one thread that is waiting on the condition variable identified by condId.  The Signal command does not guarantee which thread will be started.

• 

The Signal command does not require the calling thread to hold a lock on the mutex that was used when the waiting threads called Wait.  However, holding the lock while calling Signal allows for a greater level of control over the scheduling of the signaled thread.

• 

If no threads are waiting on the condition variable then nothing happens. Any threads calling Wait after the signal wait as normal.

• 

For more information on using condition variables, see the Condition Variable help page.

Examples

This is an example of a Producer/Consumer pattern.

The Producer creates jobs (here represented by integers in the global table tab).  The Consumers remove a job, process it, then insert the result back into the table.

We have two condition variables and one mutex.  One condition variable (cp) is used for the Producer thread.  The Producer thread will pause on the producer condition variable if enough jobs are available for processing. The other condition variable (cc) is used for the Consumer threads.  When no jobs are available, the Consumer threads will pause on that condition variable until the Producer creates more jobs.

The mutex is used to protect access to the global variable tab.

Producer := proc( m, cp, cc, max, mindiff )
    global tab, e;
    local i,j,n;

    Threads[Mutex][Lock]( m );
    j := 0;

    tab[ "maxjob" ] := mindiff;
    tab[ "curjob" ] := 1;

    for j from 1 to mindiff
    do
        tab[ j ] := 2*j;
    end do;

    Threads[ConditionVariable][Signal]( cp );

    n := false;
    while ( e )
    do
        j := tab[ "maxjob" ];
        if ( j - tab[ "curjob" ] > mindiff/2 ) then
            n := true;
            Threads[ConditionVariable][Wait]( cp, m );
        end if;

        for i from j to tab[ "curjob" ] + mindiff
        do
            tab[ i ] := 2*i;
        end do;

        tab[ "maxjob" ] := tab[ "curjob" ] + mindiff;

        if ( n ) then
            Threads[ConditionVariable][Broadcast]( cc );
            n := false;
        end if;

    end do;
    Threads[Mutex][Unlock]( m );

end proc:
Consumer := proc( m, cp, cc, max )
    global tab, e;
    local n, i, j, num;

    num := 0;
    Threads[Mutex][Lock]( m );
    while ( num < max )
    do
        while ( tab[ "curjob" ] = tab[ "maxjob" ] )
        do
            Threads[ConditionVariable][Signal]( cp );
            Threads[ConditionVariable][Wait]( cc, m );
        end do;

        n := tab[ "curjob" ];
        j := tab[ n ];
        tab[ "curjob" ] := n + 1;

        Threads[Mutex][Unlock]( m );

        j := add( i, i=1..j );
        num := num+1;

        Threads[Mutex][Lock]( m );
        tab[ n ] := j;
    end do;
    Threads[Mutex][Unlock]( m );
end proc:

tab:=table&colon;

m:=Threads&lsqb;Mutex&rsqb;Create

m:=1

(1)

cc:=Threads&lsqb;ConditionVariable&rsqb;Create

cc:=1

(2)

cp:=Threads&lsqb;ConditionVariable&rsqb;Create

cp:=2

(3)

e:=true&colon;

Start the Producer thread. We wait on cp until the Producer thread has started.

Threads&lsqb;Mutex&rsqb;Lockm

id1:=Threads&lsqb;Create&rsqb;Producerm&comma;cp&comma;cc&comma;31&comma;10

id1:=1

(4)

Threads&lsqb;ConditionVariable&rsqb;Waitcp&comma;m

Threads&lsqb;Mutex&rsqb;Unlockm

Start the Consumer threads.  They will each consume 100 jobs and there are 5 threads so we should process 500 jobs.

id2:=seqThreads&lsqb;Create&rsqb;Consumerm&comma;cp&comma;cc&comma;100&comma;i&equals;1..5

id2:=2&comma;3&comma;4&comma;5&comma;6

(5)

Wait for the Consumer threads to finish.

Threads&lsqb;Wait&rsqb;opid2

Threads&lsqb;Mutex&rsqb;Lockm

Shutdown the Producer thread.

e:=false&colon;

Threads&lsqb;ConditionVariable&rsqb;Signalcp

Threads&lsqb;Mutex&rsqb;Unlockm

Threads&lsqb;Wait&rsqb;id1

Check the number of processed jobs.

printtabcurjob

501

(6)

Check the results of one job.

printtab233

108811

(7)

printaddi&comma;i&equals;1..2233

108811

(8)

See Also

Threads, Threads[ConditionVariable][Broadcast], Threads[ConditionVariable][Create], Threads[ConditionVariable][Destroy], Threads[ConditionVariable][Wait], Threads[Mutex]


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