Techniques to Set the Number of Threads

Use the following techniques to specify the number of OpenMP threads to use in Intel® oneAPI Math Kernel Library:

Set one of the OpenMP or Intel® oneAPI Math Kernel Library environment variables:
- OMP_NUM_THREADS
- MKL_NUM_THREADS
- MKL_DOMAIN_NUM_THREADS
Call one of the OpenMP or Intel® oneAPI Math Kernel Library functions:
- omp_set_num_threads()
- mkl_set_num_threads()
- mkl_domain_set_num_threads()
- mkl_set_num_threads_local()

Note

A call to the mkl_set_num_threads or mkl_domain_set_num_threadsfunction changes the number of OpenMP threads available to all in-progress calls (in concurrent threads) and future calls to Intel® oneAPI Math Kernel Library and may result in slow Intel® oneAPI Math Kernel Library performance and/or race conditions reported by run-time tools, such as Intel® Inspector.

To avoid such situations, use the mkl_set_num_threads_local function (see the "Support Functions" section in the Intel® oneAPI Math Kernel Library Developer Reference for the function description).

When choosing the appropriate technique, take into account the following rules:

The Intel® oneAPI Math Kernel Library threading controls take precedence over the OpenMP controls because they are inspected first.
A function call takes precedence over any environment settings. The exception, which is a consequence of the previous rule, is that a call to the OpenMP subroutine omp_set_num_threads()does not have precedence over the settings of Intel® oneAPI Math Kernel Library environment variables such asMKL_NUM_THREADS. See Using Additional Threading Control for more details.
You cannot change run-time behavior in the course of the run using the environment variables because they are read only once at the first call to Intel® oneAPI Math Kernel Library.

If you use the Intel TBB threading technology, read the documentation for the tbb::task_scheduler_init class at https://www.threadingbuildingblocks.org/documentation to find out how to specify the number of threads.