Examples of OpenMP* Usage

The following examples show how to use the OpenMP* feature.

A Simple Difference Operator

This example shows a simple parallel loop where the amount of work in each iteration is different. Dynamic scheduling is used to get good load balancing. The for has a nowait because there is an implicit barrier at the end of the parallel region.

void for_1 (float a[], float b[], int n)

{

  int i, j;

  #pragma omp parallel shared(a,b,n) private(i,j)

  {

      #pragma omp for schedule(dynamic,1) nowait

          for(i = 1; i < n; i++)

          {

             for(j = 0; j <= i; j++)

             b[j + n*i] = (a[j + n*i] + a[j + n*(i-1)])/2.0;

          }

   }

}

Two Difference Operators

The following example uses two parallel loops fused to reduce fork/join overhead. The first for has a nowait because all the data used in the second loop is different than all the data used in the first loop.

void for_2 (float a[], float b[], float c[], \

float d[], int n, int m)

{

  int i, j;

  #pragma omp parallel shared(a,b,c,d,n,m) private(i,j)

  {

    #pragma omp for schedule(dynamic,1) nowait

    for(i = 1; i < n; i++)

    {

      for(j = 0; j <= i; j++)

      b[j + n*i] = ( a[j + n*i] + a[j + n*(i-1)] )/2.0;

    }

 

    #pragma omp for schedule(dynamic,1) nowait

    for(i = 1; i < m; i++)

    {

      for(j = 0; j <= i; j++)

      d[j + m*i] = ( c[j + m*i] + c[j + m*(i-1)] )/2.0;

    }

  }

}