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OpenMP Environment Variables

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Provides links to environment variables used in the OpenMP API.

The Visual C++ implementation of the OpenMP standard includes the following environment variables. These environment variables are read at program startup and modifications to their values are ignored at runtime (for example, using _putenv, _wputenv).

Environment variable Description
OMP_SCHEDULE Modifies the behavior of the schedule clause when schedule(runtime) is specified in a for or parallel for directive.
OMP_NUM_THREADS Sets the maximum number of threads in the parallel region, unless overridden by omp_set_num_threads or num_threads.
OMP_DYNAMIC Specifies whether the OpenMP run time can adjust the number of threads in a parallel region.
OMP_NESTED Specifies whether nested parallelism is enabled, unless nested parallelism is enabled or disabled with omp_set_nested.

OMP_DYNAMIC

Specifies whether the OpenMP run time can adjust the number of threads in a parallel region.

set OMP_DYNAMIC[=TRUE | =FALSE]

Remarks

The OMP_DYNAMIC environment variable can be overridden by the omp_set_dynamic function.

The default value in the Visual C++ implementation of the OpenMP standard is OMP_DYNAMIC=FALSE.

For more information, see 4.3 OMP_DYNAMIC.

Example

The following command sets the OMP_DYNAMIC environment variable to TRUE:

set OMP_DYNAMIC=TRUE

The following command displays the current setting of the OMP_DYNAMIC environment variable:

set OMP_DYNAMIC

OMP_NESTED

Specifies whether nested parallelism is enabled, unless nested parallelism is enabled or disabled with omp_set_nested.

set OMP_NESTED[=TRUE | =FALSE]

Remarks

The OMP_NESTED environment variable can be overridden by the omp_set_nested function.

The default value in the Visual C++ implementation of the OpenMP standard is OMP_NESTED=FALSE.

For more information, see 4.4 OMP_NESTED.

Example

The following command sets the OMP_NESTED environment variable to TRUE:

set OMP_NESTED=TRUE

The following command displays the current setting of the OMP_NESTED environment variable:

set OMP_NESTED

OMP_NUM_THREADS

Sets the maximum number of threads in the parallel region, unless overridden by omp_set_num_threads or num_threads.

set OMP_NUM_THREADS[=num]

Parameters

num
The maximum number of threads you want in the parallel region, up to 64 in the Visual C++ implementation.

Remarks

The OMP_NUM_THREADS environment variable can be overridden by the omp_set_num_threads function or by num_threads.

The default value of num in the Visual C++ implementation of the OpenMP standard is the number of virtual processors, including hyperthreading CPUs.

For more information, see 4.2 OMP_NUM_THREADS.

Example

The following command sets the OMP_NUM_THREADS environment variable to 16:

set OMP_NUM_THREADS=16

The following command displays the current setting of the OMP_NUM_THREADS environment variable:

set OMP_NUM_THREADS

OMP_SCHEDULE

Modifies the behavior of the schedule clause when schedule(runtime) is specified in a for or parallel for directive.

set OMP_SCHEDULE[=type[,size]]

Parameters

size
(Optional) Specifies the size of iterations. size must be a positive integer. The default is 1, except when type is static. Not valid when type is runtime.

type
The kind of scheduling, either dynamic, guided, runtime, or static.

Remarks

The default value in the Visual C++ implementation of the OpenMP standard is OMP_SCHEDULE=static,0.

For more information, see 4.1 OMP_SCHEDULE.

Example

The following command sets the OMP_SCHEDULE environment variable:

set OMP_SCHEDULE="guided,2"

The following command displays the current setting of the OMP_SCHEDULE environment variable:

set OMP_SCHEDULE