3.17. omp_set_dynamic and omp_set_num_threads Routines

Some programs rely on a fixed, prespecified number of threads to execute correctly. Because the default setting for the dynamic adjustment of the number of threads is implementation defined, such programs can choose to turn off the dynamic threads capability and set the number of threads explicitly to ensure portability. The following example shows how to do this using omp_set_dynamic, and omp_set_num_threads.

In this example, the program executes correctly only if it is executed by 16 threads. If the implementation is not capable of supporting 16 threads, the behavior of this example is implementation defined. Note that the number of threads executing a parallel region remains constant during the region, regardless of the dynamic threads setting. The dynamic threads mechanism determines the number of threads to use at the start of the parallel region and keeps it constant for the duration of the region.

//%compiler: clang
//%cflags: -fopenmp

/*
* name: set_dynamic_nthrs.1
* type: C
*/
#include <omp.h>
#include <stdlib.h>

void do_by_16(float *x, int iam, int ipoints) {}

void dynthreads(float *x, int npoints)
{
  int iam, ipoints;

  omp_set_dynamic(0);
  omp_set_num_threads(16);

  #pragma omp parallel shared(x, npoints) private(iam, ipoints)
  {
    if (omp_get_num_threads() != 16)
      abort();

    iam = omp_get_thread_num();
    ipoints = npoints/16;
    do_by_16(x, iam, ipoints);
  }
}

!!%compiler: gfortran
!!%cflags: -fopenmp

! name: set_dynamic_nthrs.1
! type: F-fixed
      SUBROUTINE DO_BY_16(X, IAM, IPOINTS)
        REAL X(*)
        INTEGER IAM, IPOINTS
      END SUBROUTINE DO_BY_16

      SUBROUTINE DYNTHREADS(X, NPOINTS)

        INCLUDE "omp_lib.h"      ! or USE OMP_LIB

        INTEGER NPOINTS
        REAL X(NPOINTS)

        INTEGER IAM, IPOINTS

        CALL OMP_SET_DYNAMIC(.FALSE.)
        CALL OMP_SET_NUM_THREADS(16)

!$OMP   PARALLEL SHARED(X,NPOINTS) PRIVATE(IAM, IPOINTS)

          IF (OMP_GET_NUM_THREADS() .NE. 16) THEN
            STOP
          ENDIF

          IAM = OMP_GET_THREAD_NUM()
          IPOINTS = NPOINTS/16
          CALL DO_BY_16(X,IAM,IPOINTS)

!$OMP   END PARALLEL

      END SUBROUTINE DYNTHREADS