openMP is not creating threads in visual studio

openmp
Nishanth picture Nishanth · Dec 23, 2010 · Viewed 10k times · Source

My openMP version did not give any speed boost. I have a dual core machine and the CPU usage is always 50%. So I tried the sample program given in Wiki. Looks like the openMP compiler (Visual Studio 2008) is not creating more than one thread.

This is the program:

 #include <omp.h>
 #include <stdio.h>
 #include <stdlib.h>

 int main (int argc, char *argv[]) {
   int th_id, nthreads;
   #pragma omp parallel private(th_id)
   {
     th_id = omp_get_thread_num();
     printf("Hello World from thread %d\n", th_id);
     #pragma omp barrier
     if ( th_id == 0 ) {
       nthreads = omp_get_num_threads();
       printf("There are %d threads\n",nthreads);
     }
   }
   return EXIT_SUCCESS;
 }

This is the output that I get:

Hello World from thread 0
There are 1 threads
Press any key to continue . . .

Answer

Jonathan Dursi picture Jonathan Dursi · Dec 23, 2010

There's nothing wrong with the program - so presumably there's some issue with how it's being compiled or run. Is this VS2008 Pro? A quick google around suggests OpenMP is not enabled in Standard. Is OpenMP enabled in Properties -> C/C++ -> Language -> OpenMP? (Eg, are you compiling with /openmp)? Is the environment variable OMP_NUM_THREADS being set to 1 somewhere when you run this?

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