Development Reference Guides

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OpenMP* Examples

The following examples show how to use several OpenMP* features.

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 improve load balancing.
The
for
pragma
has a
nowait
clause because there is an implicit barrier at the end of the parallel region. Therefore it is not necessary to also have a barrier at the end of the
for
region.
void for1(float a[], float b[], int n) { int i, j; #pragma omp parallel shared(a,b,n) { #pragma omp for schedule(dynamic,1) private (i,j) 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:
for
Loop Version

This example uses two parallel loops fused to reduce fork/join overhead. The first
for
pragma
has a
nowait
clause because all the data used in the second loop is different than all the data used in the first loop.
void for2(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; } }

Two Difference Operators:
sections
Version

This example demonstrates the use of the
sections
pragma
. The logic is identical to the preceding
for
pragma
example, but uses a
sections
pragma
instead of a
for
pragma
. Here the speedup is limited to two because there are only two units of work whereas in the example above there are
(n-1) + (m-1)
units of work.
void sections1(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 sections nowait { #pragma omp section 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 section 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; } } }

Update a Shared Scalar

This example demonstrates how to use a
single
construct to update an element of the shared array
a
. The optional
nowait
clause after the first loop is omitted because it is necessary to wait at the end of the loop before proceeding into the
single
construct to avoid a race condition.
void sp_1a(float a[], float b[], int n) { int i; #pragma omp parallel shared(a,b,n) private(i) { #pragma omp for for (i = 0; i < n; i++) a[i] = 1.0 / a[i]; #pragma omp single a[0] = MIN( a[0], 1.0 ); #pragma omp for nowait for (i = 0; i < n; i++) b[i] = b[i] / a[i]; } }

Product and Performance Information

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Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.