openmp code not running in parallel

问题

omp_set_num_threads( 8 );
#pragma omp parallel for
for( int tx = 0; tx < numThread; tx++ )
{
    cout<<"\nThread :"<<omp_get_num_threads()<<"\n";

}

My understanding is that the above code is supposed to print 8. But the output I am getting is

Thread :1

Thread :1

Thread :1

Thread :1

Thread :1

Thread :1

Thread :1

Thread :1

Please let me know what is going wrong here. I am beginner to openmp and so I am quite sure i must have made some stupid mistake.

Thanks in advance

回答1:

I'm not sure what's happening here.

It can be the case that you compiled with a stub OpenMP library, which provides all the OpenMP library API, but only acting as if sequential mode (see this link for example for the corresponding Intel compiler switch).

Another possibility is that the OMP_THREAD_LIMIT environment variable is set to 1 in your environment. See for example this code:

#include <iostream>
#include <omp.h>

int main() {
    omp_set_num_threads(8);
    #pragma omp parallel
    #pragma omp single
    std::cout << "Number of threads in the current parallel region is " << omp_get_num_threads() << std::endl;

    return 0;
}

When compiled with OpenMP support and run, it gives me:

$ g++ -fopenmp nbthreads.cc -o nbthreads
$ ./nbthreads 
Number of threads in the current parallel region is 8
$ OMP_THREAD_LIMIT=1 ./nbthreads 
Number of threads in the current parallel region is 1

Aside from these two possibilities, I've no idea.

Edit: thanks to Z boson's comment, I'm pretty sure I've got the key of the mystery.

With the very same code as the hereabove one, here is what I've got:

$ g++ -o nbthreads nbthreads.cc -lgomp
$ ./nbthreads 
Number of threads in the current parallel region is 1

So you simply mistakenly used -lgomp instead of -fopenmp while compiling / linking your code. This gives you an OpenMP code equivalent to having only 1 thread, even if you explicitly ask for more.

来源：https://stackoverflow.com/questions/33160934/openmp-code-not-running-in-parallel

标签

openmp