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parallel-processingopenmp
shuhalo
shuhalo

Reputation: 6472

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

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?

Upvotes: 4

Views: 341

Answers (1)

Daniel Langr
Daniel Langr

Reputation: 23527

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.

Upvotes: 4

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