Parallel and Distributed Programming. OpenMP

Transcription

1 Parallel and Distributed Programming OpenMP

2 OpenMP Portability of software SPMD model Detailed versions (bindings) for different programming languages Components: directives for compiler library functions environmental variables

3 Directives syntax format (for linking with the languages C and C++): # pragma omp directives clauses the most important directives are work sharing constructs, occurring in the parallel area and used to separate commands executed by an individual processors the most important clauses define the variable treatment by the threads in the parallel region each directive has its own set of allowable clauses

4 Directivessyntax parallel #pragma omp parallel list of clauses { /* parallel region */ } list of clauses (empty or any combination of the following): if( condition ) num_threads ( constatnt ) data sharing attributes ( private, firstprivate, shared, reduction ) scheduling etc.

5 Numberof Threads the number of threads can be determined explicitly by: the use of the num_threads clause in parallel directive, eg: #pragma omp parallel num_threads(10) calling of omp_set_num_threads procedure, eg: omp_set_num_threads(10); environment variable OMP_NUM_THREADS, eg: $ set OMP_NUM_THREADS = 10 in other cases, the number of threads is determined by the OpenMP implementation in a system there is the ability to dynamically determine the number of threads

6 Library functions functions associated with the execution environment: header file: omp.h syntax of the set function: void function (int); syntax of the remaining functions: int function (void) omp_set_num_threads - to determine the number of threads omp_get_num_threads - to get the number of threads omp_get_num_procs - to get the number of processors omp_get_thread_num - to get the rank of a particular thread (master 0) omp_in_parallel - to check of parallel execution omp_get_max_threads - to get the maximum number of threads omp_set_dynamic, omp_get_dynamic to customize the number of threads omp_set_nested, omp_get_nested to provide nesting

7 Library functions functions for locks handle: type of lock: omp_lock_t (function argument is always omp_lock_t*) omp_init_lock - initiating omp_destroy_lock - destruction omp_set_lock - closing omp_test_lock - attempt to shutdown without blocking omp_unset_lock - opening versions for nested locks functions for time measurment: omp_get_wtime omp_get_wtick - resolution (accuracy) of the clock

8 Preprocessor macro and enviroment variables #ifdef _OPENMP printf( Compiler recognizes OpenMP directives\n ); #endif Enviromental variables: OMP_SCHEDULE name for parallel loops with schedule(runtime) clause OMP_NUM_THREADS number OMP_DYNAMIC TRUE/FALSE OMP_NESTED TRUE/FALSE

9 Example #include<omp.h> int main() { } int nthr; #ifdef #endif _OPENMP printf(" Compiler recognizes OpenMP directives\n"); printf("maximum number of threads:\n"); scanf("%d",&nthr); omp_set_num_threads(lwat); printf("actual number of threads: %d, my ID %d\n", omp_get_num_threads(), omp_get_thread_num()); #pragma omp parallel { } printf( Actual number of threads: %d, my ID %d\n", omp_get_num_threads(), omp_get_thread_num());

10 Data sharing Data sharing clauses: shared - shared variable private - local variable firstprivate - local variable with the copied initial value lastprivate - local variable with the final value equal to the value after sequential execution threadprivate directive

11 Data sharing Variable is shared (accessible to all threads) if: it exist before entrance to the parallel region and it not occurs in the directives and clauses that are making it private is defined inside of parallel region as a static variable Variable is private (local to the thread) if: it was declared in threadprivate directive was placed in a private or a similar clause (firstprivate, lastprivate, reduction) is defined inside of the parallel region as an automatic variable is the parallel loop control variable