c++ - OpenMP omp_get_num_threads()V.S. omp_get_max_threads()

Question

我不了解omp_get_num_threads()和omp_get_max_threads()之间的区别。我按以下方式复制演示代码。

    omp_set_nested(1);
    omp_set_max_active_levels(10);
    omp_set_dynamic(0);
    omp_set_num_threads(2);
    #pragma omp parallel 
    {
        omp_set_num_threads(3);

        #pragma omp parallel
        {
            omp_set_num_threads(4);
            #pragma omp single
            {
                std::cout << omp_get_max_active_levels() << " " << omp_get_num_threads() << " " 
                << omp_get_max_threads() << std::endl;
            }
        }

        #pragma omp barrier
        #pragma omp single 
        {
            std::cout << omp_get_max_active_levels() << " " << omp_get_num_threads() << " " 
                << omp_get_max_threads() << std::endl;
        }
    }

然后我得到以下输出。

10 3 4
10 3 4
10 3 4
10 3 3

我已经检查了官方文档，但对此仍然感到困惑。

Answer 1

从文档:
omp_get_num_threads

The omp_get_num_threads routine returns the number of threads in the team executing the parallel region to which the routine region binds. If called from the sequential part of a program, this routine returns 1.

omp_get_max_threads

The value returned by omp_get_max_threads is the value of the first element of the nthreads-var ICV of the current task. This value is also an upper bound on the number of threads that could be used to form a new team if a parallel region without a num_threadsclause were encountered after execution returns from this routine.

下图说明了线程流。您的输出可能不正确，我无法使用clang + libomp或gcc + libGOMP复制它。



如果未同时指定线程数，则 omp_get_max_threads始终返回新的 parallel构造可以创建的线程数。当在内部并行区域的 omp_set_num_threads上设置4时，可以创建的新的不同线程的最大数量为4，但是在该区域中正在使用3。对于外部平行区域，最大值为3，并且正在使用2。

在串行代码中，在任何编译指示中，线程数均为1，但如果未通过 omp_set_num_threads或 OMP_NUM_THREADS环境变量进行更改，则最大值是系统的默认值(通常是内核数)