Techniques to Set the Number of Threads

Use the following techniques to specify the number of OpenMP threads to use in Intel MKL:

  • Set one of the OpenMP or Intel MKL environment variables:
    • OMP_NUM_THREADS
    • MKL_NUM_THREADS
    • MKL_DOMAIN_NUM_THREADS
  • Call one of the OpenMP or Intel MKL 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_threads function changes the number of OpenMP threads available to all in-progress calls (in concurrent threads) and future calls to Intel MKL and may result in slow Intel MKL 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 MKL Developer Reference for the function description).

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

  • The Intel MKL 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 MKL environment variables such as MKL_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 MKL.

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.

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