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Oracle Solaris Studio 12.2：OpenMP API 用户指南

4.2.1 OMP_NESTED

可通过设置 OMP_NESTED 环境变量或调用 omp_set_nested() 来启用或禁用嵌套并行操作。

以下示例中的嵌套并行构造具有三个级别。

示例 4–1 嵌套并行操作示例


#include <omp.h>
#include <stdio.h>
void report_num_threads(int level)
{
    #pragma omp single
    {
        printf("Level %d: number of threads in the team - %d\n",
                  level, omp_get_num_threads());
    }
 }
int main()
{
    omp_set_dynamic(0);
    #pragma omp parallel num_threads(2)
    {
        report_num_threads(1);
        #pragma omp parallel num_threads(2)
        {
            report_num_threads(2);
            #pragma omp parallel num_threads(2)
            {
                report_num_threads(3);
            }
        }
    }
    return(0);
}

启用嵌套并行操作时，编译和运行此程序会产生以下（经过排序的）输出：


% setenv OMP_NESTED TRUE
% a.out
Level 1: number of threads in the team - 2
Level 2: number of threads in the team - 2
Level 2: number of threads in the team - 2
Level 3: number of threads in the team - 2
Level 3: number of threads in the team - 2
Level 3: number of threads in the team - 2
Level 3: number of threads in the team - 2

比较禁用嵌套并行操作时运行相同程序的输出结果：


% setenv OMP_NESTED FALSE
% a.out
Level 1: number of threads in the team - 2
Level 2: number of threads in the team - 1
Level 3: number of threads in the team - 1
Level 2: number of threads in the team - 1
Level 3: number of threads in the team - 1