OpenMP set_num_threads() is not working

c++, multithreading, numbers, openmp, set

Solution

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 teams
omp_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 ...
}

Problem

I am writing a parallel program using OpenMP in C++. I want to control the number of threads in the program using `omp_set_num_threads()`, but it does not work. ``` #include <iostream> #include <omp.h> #include "mpi.h" using namespace std; int myrank; int groupsize; double sum; double t1,t2; int n = 10000000; int main(int argc, char *argv[]) { MPI_Init( &argc, &argv); MPI_Comm_rank( MPI_COMM_WORLD, &myrank ); MPI_Comm_size(MPI_COMM_WORLD,&groupsize); omp_set_num_threads(4); sum = 0; #pragma omp for reduction(+:sum) for (int i = 0; i < n; i++) sum+= i/(n/10); cout<<"sum="<<sum<<endl; cout<<"threads="<<omp_get_num_threads()<<endl; MPI_Finalize(); return 0; } ``` The program outputs: ``` sum = 4.5e+007 threads=1 ``` How to control the number of threads?

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