Set number of threads using omp_set_num_threads() to 2, but omp_get_num_threads() returns 1

Question

I have the following C/C++ code using OpenMP:

    int nProcessors=omp_get_max_threads();
    if(argv[4]!=NULL){
        printf(\"argv[4]: %s\\n\",argv[4]);
              


        

Answer 1

It has been already pointed out that omp_get_num_threads() returns 1 in sequential sections of the code. Accordingly, even if setting, by omp_set_num_threads(), an overall number of threads larger than 1, any call to omp_get_num_threads() will return 1, unless we are in a parallel section. The example below tries to clarify this point

#include <stdio.h>

#include <omp.h>

int main() {

    const int maxNumThreads = omp_get_max_threads();

    printf("Maximum number of threads for this machine: %i\n", maxNumThreads);

    printf("Not yet started a parallel Section: the number of threads is %i\n", omp_get_num_threads());

    printf("Setting the maximum number of threads...\n");
    omp_set_num_threads(maxNumThreads);

    printf("Once again, not yet started a parallel Section: the number of threads is still %i\n", omp_get_num_threads());

    printf("Starting a parallel Section...\n");

#pragma omp parallel for 
    for (int i = 0; i < maxNumThreads; i++) {
        int tid = omp_get_thread_num();
        printf("This is thread %i announcing that the number of launched threads is %i\n", tid, omp_get_num_threads());
    }

}

Answer 2

you are using the wrong function. use omp_get_max_threads to check for the maximum number of allowed threads.

Answer 3

The omp_get_num_threads() call returns 1 in the serial section of the code. See Link

So you need to have parallel code to get the correct value, here how your code should look like:

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

int main (int argc, const char * argv[])
{
    int nProcessors = omp_get_max_threads();

    std::cout<<nProcessors<<std::endl;

    omp_set_num_threads(nProcessors);

    std::cout<<omp_get_num_threads()<<std::endl;

#pragma omp parallel for 
    for(int i = 0; i < 5; i++){
        int tid = omp_get_thread_num();
        std::cout<<tid<<"\t tid"<<std::endl;
        int nThreads = omp_get_num_threads();
        std::cout<<nThreads<<"\t nThreads"<<std::endl;
    }

    exit(0);
}

This code produces:

2

1
0    tid
2    nThreads
0    tid
2    nThreads
0    tid
2    nThreads
1    tid
2    nThreads
1    tid
2    nThreads

It seems that you have either open mp not enabled or your loop is not in the form that can be parallized by openmp