![]() ![]() OpenMP functionalities are added to a program by use of pragmas. If this is not specified, OpenMP runtime will use the environment variable OMP_NUM_THREADS, and if it is not set choose a number of threads according implementation-defined rules (often: 'use all cores'). The number of work threads to be spawned is controlled by an internal control variable, which value can be defined explicitely at the entry point. Furthermore there may only be exactly one entry point at the top of the region and exactly one exit point at the bottom and Branching out is not allowed. As a result all parallel regions must be defined explicitly. ![]() This Fork-Join concept allows incremental parallelization: starting from serial code, the hot spots are parallelised one by one (typically still allowing serial compilation and execution). Upon leaving the parallel region these worker threads are typically put to sleep (instead of actual joining) by the OpenMP runtime until the program reaches its next parallel region. When entering a parallel region, worker threads are spawned and form a team with the master. In the OpenMP Shared-Memory Parallel Programming Model all processors/cores access a shared main memory and employ multiple threads.Ī program is launched with one single thread as a master thread.
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