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Parallelism Generation and Control

parallel Construct

When a thread encounters a parallel construct, a team of threads is created to execute the parallel region. The thread that encountered the parallel construct becomes the primary thread of the new team, with a thread number of zero for the duration of the new parallel region. All threads in the new team, including the primary thread, execute the region. At the end of the parallel region, there will be a implicit sync.

The number of threads to execute a parallel region can be specified by:

  • Conditional mode: if clause

    if(cond) parallelly else serially

  • Static mode

    • num_threads clause
    • omp_set_num_threads()
    • OMP_NUM_THREADS

  • default

  • omp_set_dynamic()

    开启时会将指定的线程数作为线程数上限。默认开启。

  • omp_set_nested()

    Enable or disable nested parallel operations. Disabled by default.

A thread may obtain its own thread number by a call to the omp_get_thread_num library routine.