Intel MKL-specific Environment Variables for OpenMP Threading Control

Intel MKL provides environment variables and support functions to control Intel MKL threading independently of OpenMP. The Intel MKL-specific threading controls take precedence over their OpenMP equivalents. Use the Intel MKL-specific threading controls to distribute OpenMP threads between Intel MKL and the rest of your program.

Note

Some Intel MKL routines may use fewer OpenMP threads than suggested by the threading controls if either the underlying algorithms do not support the suggested number of OpenMP threads or the routines perform better with fewer OpenMP threads because of lower OpenMP overhead and/or better data locality. Set the MKL_DYNAMIC environment variable to FALSE or call mkl_set_dynamic(0) to use the suggested number of OpenMP threads whenever the algorithms permit and regardless of OpenMP overhead and data locality.

Subsection "Number of User Threads" in the "Fourier Transform Functions" section of the Intel MKL Developer Reference shows how the Intel MKL threading controls help to set the number of threads for the FFT computation.

The table below lists the Intel MKL environment variables for threading control, their equivalent functions, and OMP counterparts:

Environment Variable

Support Function

Comment

Equivalent OpenMP* Environment Variable

MKL_NUM_THREADS

mkl_set_num_threads

mkl_set_num_threads_local

Suggests the number of OpenMP threads to use.

OMP_NUM_THREADS

MKL_DOMAIN_NUM_
THREADS

mkl_domain_set_num_threads

Suggests the number of OpenMP threads for a particular function domain.

 

MKL_DYNAMIC

mkl_set_dynamic

Enables Intel MKL to dynamically change the number of OpenMP threads.

OMP_DYNAMIC

Note

Call mkl_set_num_threads() to force Intel MKL to use a given number of OpenMP threads and prevent it from reacting to the environment variables MKL_NUM_THREADS, MKL_DOMAIN_NUM_THREADS, and OMP_NUM_THREADS.

The example below shows how to force Intel MKL to use one thread:

// ******* C language *******

#include <mkl.h>
...
mkl_set_num_threads ( 1 );
            

            
// ******* Fortran language *******
...
call mkl_set_num_threads( 1 )
            

See the Intel MKL Developer Reference for the detailed description of the threading control functions, their parameters, calling syntax, and more code examples.

For more complete information about compiler optimizations, see our Optimization Notice.
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