OpenMP set num threads is not working

Question

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  lt iostream gt   include  lt omp h gt   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   amp argc   amp argv       MPI Comm rank  MPI COMM WORLD   amp myrank        MPI Comm size MPI COMM WORLD  amp groupsize        omp set num threads 4        sum   0       pragma omp for  reduction   sum      for  int i   0  i  lt  n  i            sum   i  n 10        cout lt  lt  sum   lt  lt sum lt  lt endl      cout lt  lt  threads   lt  lt omp get num threads   lt  lt endl       MPI Finalize        return 0      The program outputs   sum   4 5e 007 threads 1   How to control the number of threads

User · Answer

Try setting your num threads inside your omp parallel code  it worked for me  This will give output as 4   pragma omp parallel      omp set num threads 4      int id   omp get num threads        pragma omp for    for  i   0 n  foo A       printf  Number of threads   d   id

User · Answer

I was facing the same problem   Solution is given below   Right click on Source Program   Properties   Configuration Properties   C C     Language   Now change Open MP support flag to Yes       You will get the desired result

User · Answer

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

User · Answer

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 lt  lt  sum   lt  lt sum lt  lt endl  cout lt  lt  threads   lt  lt omp get num threads   lt  lt endl    Should be changed to something like    pragma omp parallel       cout lt  lt  sum   lt  lt sum lt  lt endl      cout lt  lt  threads   lt  lt omp get num threads   lt  lt endl      The code I use follows Hristo s advice of disabling dynamic teams  too

User · Answer

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

[c++] OpenMP set_num_threads() is not working

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