Open mp library functions and environment variables

Open mp library functions and environment variables

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  Presented by , SuveekshaJain MTech CSE
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  OpenMP Library Functions *Inaddition to programs, OpenMP provides a set of functions And environment variables. *OpenMP requires conditional compilation in programs #include<omp.h> #ifdef_OPENMP Omp_set_num_threads (4); #endif
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  The Most Heavilyused OpenMP Library Functions • int omp_get_num_threads (void); Returns the number of threads currently in use. • int omp_set_num_threads (int NumThreads); This function sets the number of threads that will be used when entering a parallel section. It overrides the OMP_NUM_THREADS environment. • int omp_get_thread_num(void); Returns the current thread number between 0 and total number of threads -1. • int omp_get_num_procs(void); Returns the number of available cores. HyperThreading Technology cores will count as two cores. • int omp_get_max_threads(void); Returns the maximum value that can be returned by calls to omp_get_num_threads.
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  *Some more OpenMplibrary functions, omp_init_lock Initializes a simple lock. omp_destroy_lock Removes a simple lock. omp_set_lock Waits until a simple lock is available. omp_unset_lock Releases a simple lock. omp_test_lock Tests a simple lock.
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  Float x[8000]; Omp_set_num_threads[4]; #pragma ompparallel private(k) { int num_thds = omp_get_num_threads(); Int ElementsperThread = 8000/ num_thds; Int Tid = omp_get_thread_num(); Int LowBound = Tid*ElementsPerThread; Int UpperBound = LowBound+ElementsPerThread; for( k = LowBound; k<UpperBound ; k++ ) DataProcess(x[k]); } Loop that uses OpenMP function
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  OpenMP Environment Variables *OMP_SCHEDULEsets the run time schedule type and chunk size Ex: setenv OMP_SCHEDULE "guided,4" setenv OMP_SCHEDULE "dynamic" *OMP_NUM_THREADS sets the number of threads to use during execution Ex: setenv OMP_NUM_THREADS 16 *OMP_DYNAMIC enables or disables dynamic adjustment of the number of threads Ex: setenv OMP_DYNAMIC TRUE