Developer Guide

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Interaction with the Message-passing Interface

To improve performance of cluster applications, it is critical for
Intel® oneAPI Math Kernel Library
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 inTechniques to Set the Number of Threads.
If the number of threads is not set,
Intel® oneAPI Math Kernel Library
checks whether it runs under MPI provided by the Intel® MPI Library. If this is true, the following environment variables define
Intel® oneAPI Math Kernel Library
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® oneAPI Math Kernel Library
    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® oneAPI Math Kernel Library
    conclusion of the thread support level:
    • 1 -
      MPI_THREAD_FUNNELED
    • 2 -
      MPI_THREAD_SERIALIZED
    • 3 -
      MPI_THREAD_MULTIPLE
    In all these cases,
    Intel® oneAPI Math Kernel Library
    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.
Instead of relying on the discussed implicit settings, explicitly set the number of threads for
Intel® oneAPI Math Kernel Library
.
Product and Performance Information
Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.
Notice revision #20201201

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.