Techniques to Set the Number of Threads

Use the following techniques to specify the number of 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()

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.