Cluster Grid

OpenMP Environment Variables

OpenMP provides the following environment variables for controlling the execution of parallel code. All environment variable names are uppercase. The values assigned to them are not case sensitive.

OMP_SCHEDULE

Applies only to DO, PARALLEL DO (Fortran) and for, parallel for C/C++ directives which have their schedule clause set to RUNTIME. The value of this variable determines how iterations of the loop are scheduled on processors. For example:

OMP_NUM_THREADS

Sets the maximum number of threads to use during execution. For example:

OMP_DYNAMIC

Enables or disables dynamic adjustment of the number of threads available for execution of parallel regions. Valid values are TRUE or FALSE. For example:

OMP_NESTED

Enables or disables nested parallelism. Valid values are TRUE or FALSE. For example:

Implementation notes:

Your implementation may or may not support nested parallelism and/or dynamic threads. If nested parallelism is supported, it is often only nominal, meaning that a nested parallel region may only have one thread. Consult your implementation's documentation for details - or experiment and find out for yourself.

OMP_STACKSIZE

New feature available with OpenMP 3.0. Controls the size of the stack for created (non-Master) threads. Examples:

OMP_WAIT_POLICY

New feature available with OpenMP 3.0. Provides a hint to an OpenMP implementation about the desired behaviour of waiting threads. A compliant OpenMP implementation may or may not abide by the setting of the environment variable. Valid values are ACTIVE and PASSIVE. ACTIVE specifies that waiting threads should mostly be active, i.e. consume processor cycles, while waiting. PASSIVE specifies that waiting threads should mostly be passive, i.e. not consume processor cycles, while waiting. The details of the ACTIVE and PASSIVE behaviours are implementation defined. Examples:

OMP_MAX_ACTIVE_LEVELS

New feature available with OpenMP 3.0. Controls the maximum number of nested active parallel regions. The value of this environment variable must be a non-negative integer. The behaviour of the program is implementation-defined if the requested value of OMP MAX ACTIVE LEVELS is greater than the maximum number of nested active parallel levels an implementation can support or if the value is not a non-negative integer. Example:

OMP_THREAD_LIMIT

New feature available with OpenMP 3.0. Sets the number of OpenMP threads to use for the whole OpenMP program. The value of this environment variable must be a positive integer. The behaviour of the program is implementation-defined if the requested value of OMP THREAD LIMIT is greater than the number of threads an implementation can support or if the value is not a positive integer. Example: