Intel(R) Math Kernel Library for Linux* OS User's Guide

6 Intel® Math Kernel Library User’s GuideThe example below illustrates the use of the Intel MKL function mkl_set_num_threads()to set one thread.Example 6-2Setting the Number of Threads to One// ******* C language *******#include #include …mkl_set_num_threads ( 1 );// ******* Fortran language *******…call mkl_set_num_threads( 1 )The section further explains the Intel MKL environment variables for threading control. Seethe Intel MKL Reference Manual for the detailed description of the threading controlfunctions, their parameters, calling syntax, and more code examples.MKL_DYNAMICThe MKL_DYNAMIC environment variable enables Intel MKL to dynamically change thenumber of threads.The default value of MKL_DYNAMIC is TRUE, regardless of OMP_DYNAMIC, whose defaultvalue may be FALSE.When MKL_DYNAMIC is TRUE, Intel MKL tries to use what it considers the best number ofthreads, up to the maximum number you specify.For example, MKL_DYNAMIC set to TRUE enables optimal choice of the number of threads inthe following cases:• If the requested number of threads exceeds the number of physical cores (perhapsbecause of hyper-threading), and MKL_DYNAMIC is not changed from its default valueof TRUE, Intel MKL will scale down the number of threads to the number of physicalcores.• If you are able to detect the presence of MPI, but cannot determine if it has beencalled in a thread-safe mode (it is impossible to detect this with MPICH 1.2.x, forinstance), and MKL_DYNAMIC has not been changed from its default value of TRUE,Intel MKL will run one thread.When MKL_DYNAMIC is FALSE, Intel MKL tries not to deviate from the number of threadsthe user requested. However, setting MKL_DYNAMIC=FALSE does not ensure that Intel MKLwill use the number of threads that you request. The library may have no choice on this6-10