OpenMP specify thread number of a for loop iteration

Question

I'm using the following command to parallelize a single loop over the available threads of the program:

#pragma omp parallel for num_threads(threads)
for(long i = 0; i < threads; i++)
{
    array[i] = calculateStuff(i,...);
}

For technical reasons, I would like to guarantee that thread number 0 executes i=0, and thread number 1 executes i=1. In other words, I want always i=omp_get_thread_num(). Is this possible?

Hristo Iliev · Accepted Answer · 2015-04-29 12:10:01Z

5

Using a loop is a waste of resources in that particular case. Simply use omp_get_thread_num():

#include <omp.h>

#pragma omp parallel num_threads(threads)
{
    int i = omp_get_thread_num();
    array[i] = calculateStuff(i,...);
}

answered Apr 29, 2015 at 12:10

Hristo Iliev

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2 Comments

The Quantum Physicist Over a year ago

Thanks for the hint. I'm gonna do this, but I can't choose this as correct answer as this doesn't answer the question.

Hristo Iliev Over a year ago

It's not a an answer but more a suggestion to improve your code. Sometimes I wish that comments on Stack Overflow could be a bit more elaborate when it comes to formatting.

High Performance Mark · Accepted Answer · 2015-04-29 12:17:05Z

4

Try

#pragma omp parallel for num_threads(threads) schedule(static,1)

Now that @Hristo Iliev has turned up I'm 100% confident that the OpenMP standard requires the static schedule to assign iteration 0 to thread 0, 1 to 1, etc.

Try it and let us know how you get on.

Then again

#pragma omp parallel for num_threads(threads) schedule(dynamic,1)

ought to work too, since you only have as many iterations as you have threads. It's possible though that if thread 0 finishes its work before thread n starts then thread 0 will grab iteration n.

edited Apr 29, 2015 at 12:17

answered Apr 29, 2015 at 9:47

High Performance Mark

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2 Comments

The Quantum Physicist Over a year ago

I think you're right. Apparently openmp does that by default. I had a mistake in my program that made me not see this. Thank you.

Hristo Iliev Over a year ago

§2.7.1 Loop Construct: "When schedule(static, chunk_size) is specified, iterations are divided into chunks of size chunk_size, and the chunks are assigned to the threads in the team in a round-robin fashion in the order of the thread number." I.e. the standard does require it.

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