pragma omp parallel for vs. pragma omp parallel

Question

In C++ with openMP, is there any difference between

#pragma omp parallel for
for(int i=0; i<N; i++) {
   ...
}

and

#pragma omp parallel
for(int i=0; i<N; i++) {
   ...
}

?

Thanks!

Solution

#pragma omp parallel
for(int i=0; i<N; i++) {
   ...
}

This code creates a parallel region, and each individual thread executes what is in your loop. In other words, you do the complete loop N times, instead of N threads splitting up the loop and completing all iterations just once.

You can do:

#pragma omp parallel
{
    #pragma omp for
    for( int i=0; i < N; ++i )
    {
    }

    #pragma omp for
    for( int i=0; i < N; ++i )
    {
    }
}

This will create one parallel region (aka one fork/join, which is expensive and therefore you don't want to do it for every loop) and run multiple loops in parallel within that region. Just make sure if you already have a parallel region you use #pragma omp for as opposed to #pragma omp parrallel for as the latter will mean that each of your N threads spawns N more threads to do the loop.

Comments

[Admin]

I see; that's what I needed. So the first option (#pragma omp parallel) would execute Nthreads * N iterations in total, and any statement of the form vec[i] = result; inside the loop would mess everything up.

[RyanP]

@FranciscoJ.R.Ruiz that's correct. If this answer's your question, feel free to mark it as the answer :)

[Admin]

Sure! Thanks a lot.