Improving Performance for Small Size Problems

The overhead of calling an Intel MKL function for small problem sizes can be significant when the function has 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

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