Intel(R) - Computational and Systems Biology at MIT

Managing Performance and Memory 6Techniques to Set the Number of ThreadsYou can employ different techniques to specify the number of threads to use in Intel MKL.• Set OpenMP or Intel MKL environment variable:— OMP_NUM_THREADS— MKL_NUM_THREADS— MKL_DOMAIN_NUM_THREADS• Call OpenMP or Intel MKL function:— omp_set_num_threads()— mkl_set_num_threads()— mkl_domain_set_num_threads().When choosing the appropriate technique, take into account the following rules:• If you employ the OpenMP techniques (OMP_NUM_THREADS andomp_set_num_threads()) only, which was the case with earlier Intel MKL versions,the library will still respond to them.• The Intel MKL threading controls take precedence over the OpenMP techniques.• A subroutine call takes precedence over any environment variables. The exception isthe OpenMP subroutine omp_set_num_threads(), which does not have precedenceover Intel MKL environment variables, such as MKL_NUM_THREADS.• The environment variables cannot be used to change run-time behavior in the courseof the run, as they are read only once.Avoiding Conflicts in the Execution EnvironmentThere are situations in which conflicts can exist in the execution environment that makethe use of threads in Intel MKL problematic. They are listed here with recommendations fordealing with these. First, a brief discussion of why the problem exists is appropriate.If the user threads the program using OpenMP directives and compiles the program withIntel® compilers, Intel MKL and the program will both use the same threading library. IntelMKL tries to determine if it is in a parallel region in the program, and if it is, it does notspread its operations over multiple threads unless the user specifically requests Intel MKLto do so via the MKL_DYNAMIC functionality (see Using Additional Threading Control fordetails). However, Intel MKL can be aware that it is in a parallel region only if the threadedprogram and Intel MKL are using the same threading library. If the user’s program isthreaded by some other means, Intel MKL may operate in multithreaded mode and theperformance may suffer due to overuse of the resources.6-3