Developer Guide

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Improving Performance for Small Size Problems

The overhead of calling an
Intel® MKL
function for small problem sizes can be significant when the functionhas a large number of parameters or internally checks parameter errors. To reduce the performance overhead for these small size problems, the
Intel® MKL
direct call
feature works in conjunction with the compiler to preprocess the calling parameters to supported
Intel® MKL
functions and directly call or inline special optimized small-matrix kernels that bypass error checking.For a list of functions supporting direct call, see Limitations of the Direct Call.
To activate the feature, do the following:
  • Compile your C or Fortran code with the preprocessor macro depending on whether a threaded or sequential mode of
    Intel® MKL
    is required by supplying the compiler option as explained below:
    Intel® MKL
    Mode
    Macro
    Compiler Option
    Threaded
    MKL_DIRECT_CALL
    -
    DMKL_DIRECT_CALL
    Sequential
    MKL_DIRECT_CALL_SEQ
    -
    DMKL_DIRECT_CALL_SEQ
  • For Fortran applications:
    • Enable preprocessor by using the
      -
      fpp
      option for Intel® Fortran Compiler and
      -Mpreprocess
      option for PGI* compilers.
    • Include the
      Intel® MKL
      Fortran include file
      mkl_direct_call.fi
      .
Intel® MKL
skips error checking and intermediate function calls if the problem size is small enough (for example: a call to a function that supports direct call, such as
dgemm
, with matrix ranks smaller than 50).
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

Product and Performance Information

1

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