OpenMP: how to make for loop distribution depend only on array size?

Question

Consider the following chunk of code:

int array[30000]

#pragma omp parallel
{
   for( int a = 0; a < 1000; a++ )
   {
       #pragma omp for nowait
       for( int i = 0; i < 30000; i++ )
       {
           /*calculations with array[i] and also other array entries happen here*/
       }
   }
}

Race conditions are not a concern in my application but I would like to enforce that each thread in the parallel regions takes care of exactly the same chunk of array at each run through the inner for loop.

It is my understanding that schedule(static) distributes the for-loop items based on the number of threads and the array length. However, it is not clear whether the distribution changes for different loops or different repetitions of the same loop (even when number of threads and length are the same).

What does the standard say about this? Is schedule(static) sufficient to enforce this?

Daniel Langr · Accepted Answer · 2019-09-06 11:20:35Z

4

I believe this quote from OpenMP Specification provides such a guarantee:

A compliant implementation of the static schedule must ensure that the same assignment of logical iteration numbers to threads will be used in two worksharing-loop regions if the following conditions are satisfied: 1) both worksharing-loop regions have the same number of loop iterations, 2) both worksharing-loop regions have the same value of chunk_size specified, or both worksharing-loop regions have no chunk_size specified, 3) both worksharing-loop regions bind to the same parallel region, and 4) neither loop is associated with a SIMD construct.

answered Sep 6, 2019 at 11:20

Daniel Langr

24.1k3 gold badges55 silver badges110 bronze badges

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

Collectives™ on Stack Overflow

OpenMP: how to make for loop distribution depend only on array size?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related