The MKL_NUM_STRIPES environment variable controls the Intel MKL threading algorithm for ?gemm functions. When MKL_NUM_STRIPES is set to a positive integer value nstripes, Intel MKL tries to use a number of partitions equal to nstripes along the leading dimension of the output matrix.

The following table explains how the value nstripes of MKL_NUM_STRIPES defines the partitioning algorithm used by Intel MKL for ?gemm output matrix; max_threads_for_mkl denotes the maximum number of OpenMP threads for Intel MKL:

Value of


Partitioning Algorithm

1 < nstripes < (max_threads_for_mkl/2)

2D partitioning with the number of partitions equal to nstripes:

  • Horizontal, for column-major ordering.
  • Vertical, for row-major ordering.

nstripes = 1

1D partitioning algorithm along the opposite direction of the leading dimension.

nstripes ≥ (max_threads_for_mkl /2)

1D partitioning algorithm along the leading dimension.

nstripes < 0

The default Intel MKL threading algorithm.

The following figure shows the partitioning of an output matrix for nstripes = 4 and a total number of 8 OpenMP threads for column-major and row-major orderings:

You can use support functions mkl_set_num_stripes and mkl_get_num_stripes to set and query the number of stripes, respectively.

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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

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