test

Oracle Solaris Studio 12.2：OpenMP API 用户指南

4.2.3 OMP_MAX_ACTIVE_LEVELS

环境变量 OMP_MAX_ACTIVE_LEVELS 可控制需要多个线程的嵌套活动并行区域的最大深度。

活动嵌套深度大于此环境变量值的任何活动并行区域将仅由一个线程来执行。如果并行区域没有 if 子句，或者其 if 子句计算为 true，会将此区域视为活动区域。活动嵌套级别的缺省最大数量是 4。

以下代码将创建 4 级嵌套并行区域。如果将 OMP_MAX_ACTIVE_LEVELS 设置为 2，嵌套深度为 3 和 4 的嵌套并行区域将由单个线程来执行。


#include <omp.h>
#include <stdio.h>
#define DEPTH 5
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());
    }
}
void nested(int depth)
{
    if (depth == DEPTH)
        return;

    #pragma omp parallel num_threads(2)
    {
        report_num_threads(depth);
        nested(depth+1);
    }
}
int main()
{
    omp_set_dynamic(0);
    omp_set_nested(1);
    nested(1);
    return(0);
}

使用最大嵌套级别 4 来编译和运行此程序会产生以下可能的输出。（实际结果取决于操作系统调度线程的方式。）


% setenv OMP_MAX_ACTIVE_LEVELS 4
% a.out |sort 
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
Level 4: number of threads in the team - 2
Level 4: number of threads in the team - 2
Level 4: number of threads in the team - 2
Level 4: number of threads in the team - 2
Level 4: number of threads in the team - 2
Level 4: number of threads in the team - 2
Level 4: number of threads in the team - 2
Level 4: number of threads in the team - 2

使用设置为 2 的嵌套级别来运行会产生以下可能的结果：


% setenv OMP_MAX_ACTIVE_LEVELS 2
% a.out |sort 
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 - 1
Level 3: number of threads in the team - 1
Level 3: number of threads in the team - 1
Level 3: number of threads in the team - 1
Level 4: number of threads in the team - 1
Level 4: number of threads in the team - 1
Level 4: number of threads in the team - 1
Level 4: number of threads in the team - 1

此外，这些示例只显示了一些可能的结果。实际结果取决于操作系统调度线程的方式。