OpenMP parallel for not working as expected

Question

I have simple program:

int index;
unsigned int count = 0;

#pragma omp parallel for
for (index = 0; index < 10000; index++)
{
    count++;
}

Problem is that I think count should be equal to 10000. But instead it is usually somewhere around 5000. In about 10% it is equal to 10000.

What is going on here?

count++ protected by atomic is one well-trodden path to writing serial programs with the overhead of OpenMP. Don't follow that path. Instead, learn how to use OpenMP's reduction variables. You'll find enough to get you started by searching here on SO. — High Performance Mark
– High Performance Mark, Commented Sep 4, 2016 at 13:32

petrmikheev · Accepted Answer · 2016-09-04 15:17:42Z

5

As suggested in comments, you need reduction directive:

int index;
unsigned int count = 0;

#pragma omp parallel for reduction(+:count)
for (index = 0; index < 10000; index++)
{
    count++;
}

answered Sep 4, 2016 at 15:17

petrmikheev

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1 Comment

I wouldn't say need probably although this is a way to fix the issue.