Intel(R) - Computational and Systems Biology at MIT

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6 Intel® Math Kernel Library User’s GuideNote that a number of other LAPACK routines, which are based on threaded LAPACK orBLAS routines, make effective use of parallelism: *gesv, *posv, *gels, *gesvd,*syev, *heev, etc.• All Level 3 BLAS, Sparse BLAS matrix-vector and matrix-matrix multiply routines forthe compressed sparse row and diagonal formats• VML• All FFTs (except 1D transformations when DFTI_NUMBER_OF_TRANSFORMS=1 and sizesare not power of two).NOTE. For power-of-two data in 1D FFTs, Intel MKL provides parallelismfor all the three supported architectures. For Intel® 64 architecture, theparallelism is provided for double complex out-of-place FFTs only.The library uses OpenMP* threading software, which responds to the environmentalvariable OMP_NUM_THREADS that sets the number of threads to use. Notice that there aredifferent means to set the number of threads. In Intel MKL releases earlier than 10.0, youcould use the environment variable OMP_NUM_THREADS (see Setting the Number of ThreadsUsing OpenMP* Environment Variable for details) or the equivalent OpenMP run-timefunction calls (detailed in section Changing the Number of Threads at Run Time). Startingwith version 10.0, Intel MKL also offers variables that are independent of OpenMP, such asMKL_NUM_THREADS, and equivalent Intel MKL functions for threading management (seeUsing Additional Threading Control for details). The Intel MKL variables are alwaysinspected first, then the OpenMP variables are examined, and if neither is used, theOpenMP software chooses the default number of threads. This is a change with respect toIntel MKL versions 9.x or earlier, which used a default value of one, as the Intel® CompilerOpenMP software uses the default number of threads equal to the number of processors inyour system.NOTE. In Intel MKL 10.0, the OpenMP* software determines the defaultnumber of threads. The default number of threads is equal to the numberof logical processors in your system for Intel OpenMP* libraries.To achieve higher performance, you are recommended to set the number of threads to thenumber of real processors or physical cores. Do this by any available means, which aresummarized in section Techniques to Set the Number of Threads.6-2
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
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INFORMATION IN THIS DOCUMENT IS PRO
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Page 13 and 14: Getting Started 2This chapter helps
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A Intel® Math Kernel Library User
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IndexAAbsoft compiler, linking with
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Intel MKL choice, particular cases,
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Intel(R) - Computational and Systems Biology at MIT

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