pragma omp parallel for vs. pragma omp parallel

10
votes

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!

1
c++openmp
The second case has each thread to the same job (the entire for loop). The first case shares the work between threads so each thread does N/t iterations where t is the number of threads. – Z boson

1 Answers

7
votes
#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.