c++ OpenMP multiple threads in each loop

Question

I have a function myFunc() which utilizes OMP loops internally, but does not consume all the cores I have available. At a higher level, I call this function several times within a loop. Is there a simple way to use OMP on this outer loop, but not restrict each loop pass to a single thread?

e.g:

// I have 24 cores
// Would like to allow each for loop pass to have 12 cores
for (int outIter=0; outIter<2; outIter++) {
    x[outIter] = myFunc(...)  // This function has OMP inside
}

Kaveh Vahedipour · Accepted Answer · 2017-09-26 18:00:15Z

1

You should first find out, why myFunc cannot exhaust all 24 cores. Often you are limited by memory bandwidth. Having said that, the above loop is obviously not parallel. You would have to try something like this:

#pragma omp parallel num_threads(2)
{
   myFunc(...);
}

where myFunc's input would be a function of omp_get_thread_num(). But again, do not be to hopeful of being able to achieve more parallelism. If your problem is limited by memory bandwidth there is little hope for speedup. What happens roughly in you myFunc?

answered Sep 26, 2017 at 18:00

Kaveh Vahedipour

3,4971 gold badge18 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Mastiff Over a year ago

Thanks, yeah, I know it's not parallel as is. If I put the pragma statement right above the for loop, my understanding is that this provides only a single thread to each loop iteration (normal use case being loop_iterations >> number_of_cores). Any additional OMP calls inside the loop would be negated because there is only one thread available. Inside the function, the for loops are over small numbers of iterations which is why I'm trying to do this.

Kaveh Vahedipour Over a year ago

No go on with your omp calls inside. This just breaks down the outer loop in two. It does not limit the inside operation to 2 cores. You might have to look, if you gain anything from specifying KMP_AFFINITY additionally.

Collectives™ on Stack Overflow

c++ OpenMP multiple threads in each loop

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related