OpenMP set_num_threads() is not working

Besides calling omp_get_num_threads() outside of the parallel region in your case, calling omp_set_num_threads() still doesn't guarantee that the OpenMP runtime will use exactly the specified number of threads. omp_set_num_threads() is used to override the value of the environment variable OMP_NUM_THREADS and they both control the upper limit of the size of the thread team that OpenMP would spawn for all parallel regions (in the case of OMP_NUM_THREADS) or for any consequent parallel region (after a call to omp_set_num_threads()). There is something called dynamic teams that could still pick smaller number of threads if the run-time system deems it more appropriate. You can disable dynamic teams by calling omp_set_dynamic(0) or by setting the environment variable OMP_DYNAMIC to false.

To enforce a given number of threads you should disable dynamic teams and specify the desired number of threads with either omp_set_num_threads():

omp_set_dynamic(0);     // Explicitly disable dynamic teamsomp_set_num_threads(4); // Use 4 threads for all consecutive parallel regions#pragma omp parallel ...{    ... 4 threads used here ...}

or with the num_threads OpenMP clause:

omp_set_dynamic(0);     // Explicitly disable dynamic teams// Spawn 4 threads for this parallel region only#pragma omp parallel ... num_threads(4){    ... 4 threads used here ...}

The omp_get_num_threads() function returns the number of threads that are currently in the team executing the parallel region from which it is called. You are calling it outside of the parallel region, which is why it returns 1.

According to the GCC manual for omp_get_num_threads:

In a sequential section of the program omp_get_num_threads returns 1

So this:

cout<<"sum="<<sum<<endl;cout<<"threads="<<omp_get_num_threads()<<endl;

Should be changed to something like:

#pragma omp parallel{    cout<<"sum="<<sum<<endl;    cout<<"threads="<<omp_get_num_threads()<<endl;}

The code I use follows Hristo's advice of disabling dynamic teams, too.