University:
Boston University
Course:
CAS CS 505 | Introduction to Natural Language Processing
Academic year:

2020
Views:

295

Pages:

20
Author:

kendappaohnz

Sections construct • Syntax in C/C++ programs • Syntax in Fortran programs #pragma omp sections [clause[[,] clause]. . . ] !$omp sections [clause[[,] clause]. . . ] { [!$omp section ] [#pragma omp section ] …… code block 1 ...... …… code block 1 ...... [!$omp section [#pragma omp section …… code block 2 ...... ] …… code block 2 ...... ] ... ... !$omp end sections } • The work in each section must be independent. • Each section is distributed to one thread. • Example of parallel sections #pragma omp parallel sections { #pragma omp section (void) funcA(); #pragma omp section (void) funcB(); } /*-- End of parallel region --*/ • Although the sections construct can be generally used to get threads to perform different tasks independently, its most common use is probably to execute function or subroutine calls in parallel. • There is a load-balancing problem, if the works in different sections are not equal. Single construct • Syntax in C/C++ programs • Syntax in Fortran programs #pragma omp single [clause[[,] clause]. . . !$omp single [clause[[,] clause]. . . ] …… code block ...... …… code block ...... !$omp end single • The code block following the single construct is executed by one thread only. • The executing thread could be any thread (not necessary the master one). • The other threads wait at a barrier until the executing thread has completed. • Example of the single construct #pragma omp parallel shared(a,b) private(i) { #pragma omp single { a = 10; } /* A barrier is automatically inserted here */ #pragma omp for for (i=0; i 1), the size of each chunk is determined in the same way, with the restriction that the chunks do not contain fewer than k iterations (with a possible exception for the last chunk to be assigned, which may have fewer than k iterations). When no chunk_size is specified, it defaults to 1. runtime The schedule and (optional) chunk size are set through the OMP_SCHEDULE environment variable. • Example of schedule clause: The workload in the inner loop depends on the value of the outer loop iteration variable i. Therefore, the workload is not balanced, and the static schedule is probably not the best choice. Dynamic or guided schedules are required. #pragma omp parallel for default(none) schedule(runtime) private(i,j) shared(n) for (i=0; i

Using OpenMP Lecture Part 2

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