CODEWITHSUNDEEP
c++multithreadingparallel-processingopenmpscheduling
Zoltán Jánosi
Zoltán Jánosi

Reputation: 47

OpenMP optimize scheduling of for loop

I need some help with OpenMP. Is it possible that if a thread ended in a for loop it helps then to another thread, dividing it? I have a loop in a loop where are breaks; and the threads doesn't end at the same time, so there are threads which has much work, and other threads which are done. (so there are unused cores). I run my program on a corei7, and it seems that OpenMP divide the loop to 8 threads. But the utilization starts to drop after some time when one thread did the job.

#pragma omp parallel for
for(i = 0; i < Vector.size(); i++) {
    for(j = 0; j < othervector.size(); j++) {
        {some code}
        if(sth is true) break;
    }
}

Thank you.

Upvotes: 1

Views: 1311

Answers (1)

el_tenedor
el_tenedor

Reputation: 664

The default division/SCHEDULE of the loop iterations in a for loop is implementation dependent. In your case, when using the omp parallel for the default shedule may be STATIC, which means that depending on the size of your vector each thread gets assigned a fixed chunk of data. Since apparently the work load can't be balanced by statically dividing it, you should check out the DYNAMIC, GUIDED and RUNTIME clause and see if this helps you to reestablish a high utilization of your (virtual) cores. Depending on the chunk size this will of course cause additional overhead, but it may become negligible comparing it with the time your cores spend in idle when scheduling statically.

To answer the original question: I don't think that you can tell a thread to continue the work of another one. When the work gets assigned each thread has to deal with it on its own. Here is what I would try out.

#define CHUNKSIZE 100

#pragma omp parallel for schedule(dynamic,chunk) nowait
for(i = 0; i < Vector.size(); i++) {
    for(j = 0; j < othervector.size(); j++) {
        {some code}
        if(sth is true) break;
    }
}

Actually Hristo Iliev wrote a very nice answer to a similar question some time ago.

Upvotes: 2

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