3.2.3 OMP_MAX_ACTIVE_LEVELS

环境变量 OMP_MAX_ACTIVE_LEVELS 可控制嵌套活动并行区域的最大数量。如果由包含多个线程的组执行并行区域，则该并行区域处于活动状态。如果未设置，则使用缺省值 4。

请注意，设置该环境变量仅控制嵌套活动并行区域的最大数量，并不启用嵌套并行操作。要启用嵌套并行操作，必须将 OMP_NESTED 设置为 TRUE，或者必须使用求值结果为 true 的参数调用 omp_set_nested()。

以下样例代码将创建 4 级嵌套并行区域。

#include <omp.h>
#include <stdio.h>
#define DEPTH 4
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);
}

以下输出显示将 DEPTH 设置为 4 时编译和运行样例代码可能产生的结果。实际结果取决于操作系统调度线程的方式。

% setenv OMP_NESTED TRUE
% 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

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。以下示例显示一个可能的结果。

% setenv OMP_NESTED TRUE
% 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