Developer Guide

  • 2021.1
  • 12/04/2020
  • Public Content

Improving Performance for Small Size Problems

The overhead of calling an
Intel® oneAPI Math Kernel Library
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® oneAPI Math Kernel Library
direct call
feature works in conjunction with the compiler to preprocess the calling parameters to supported
Intel® oneAPI Math Kernel Library
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® oneAPI Math Kernel Library
    is required by supplying the compiler option as explained below:
    Intel® oneAPI Math Kernel Library
    Compiler Option
  • For Fortran applications:
    • Enable preprocessor by using the
      option for Intel® Fortran Compiler.
    • Include the
      Intel® oneAPI Math Kernel Library
      Fortran include file
Intel® oneAPI Math Kernel Library
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
, 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


Performance varies by use, configuration and other factors. Learn more at