Interaction with the Message-passing Interface

To improve performance of cluster applications, it is critical for Intel MKL to use the optimal number of threads, as well as the correct thread affinity. Usually, the optimal number is the number of available cores per node divided by the number of MPI processes per node. You can set the number of threads using one of the available methods, described in Techniques to Set the Number of Threads.

If the number of threads is not set, Intel MKL checks whether it runs under MPI provided by the Intel® MPI Library. If this is true, the following environment variables define Intel MKL threading behavior:

  • I_MPI_THREAD_LEVEL
  • MKL_MPI_PPN
  • I_MPI_NUMBER_OF_MPI_PROCESSES_PER_NODE
  • I_MPI_PIN_MAPPING
  • OMPI_COMM_WORLD_LOCAL_SIZE
  • MPI_LOCALNRANKS

The threading behavior depends on the value of I_MPI_THREAD_LEVEL as follows:

  • 0 or undefined.

    Intel MKL considers that thread support level of Intel MPI Library is MPI_THREAD_SINGLE and defaults to sequential execution.

  • 1, 2, or 3.

    This value determines Intel MKL conclusion of the thread support level:

    • 1 - MPI_THREAD_FUNNELED
    • 2 - MPI_THREAD_SERIALIZED
    • 3 - MPI_THREAD_MULTIPLE

    In all these cases, Intel MKL determines the number of MPI processes per node using the other environment variables listed and defaults to the number of threads equal to the number of available cores per node divided by the number of MPI processes per node.

Important

Instead of relying on the discussed implicit settings, explicitly set the number of threads for Intel MKL.

Optimization Notice

Intel's compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice.

Notice revision #20110804

See Also

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