OpenMP：递归细分工作和线程

Question

我想知道如何使用 OpenMP 编写以下逻辑：

do_something(Job job, Threads x ... x+b){
    if(b<1)  // if only one thread is assigned
        do_real_work(job); return;

    // otherwise, divide the work in two subtasks
    // Only one thread executes this:
    Divide job into job1 and job2

    // Divide the assigned threads into two groups
    // and assign the subtasks to them.
    // The below two lines should be executed in parallel.
    do_something(job1, x ... x+b/2)
    do_something(job2, x+b/2 ... x+b)
}

上述工作流程本身就是简单的分而治之。我想以 "binary-tree" 的方式将工作分配给 n 个线程。特别是，我希望程序能够从 evn var 中获取线程数，并递归地处理除法。如果使用4个线程，则执行两级；如果使用8个线程，则执行三个级别等

我不知道如何在 OpenMP 中指定线程子集来执行并行任务。甚至可以指定线程 ID 来执行任务吗？

Answer 1

虽然可以通过omp_get_thread_num()获取线程ID，并根据该ID进行分支，但您的问题似乎更适合使用显式任务来解决。环境变量 OMP_NUM_THREADS 中设置的最大 OpenMP 线程数可以通过调用 omp_get_max_threads() 获得（即使在并行区域之外），实际数量可以通过在并行区域中调用 omp_get_num_threads() 获得活动区域。并行代码应该类似于：

do_something(Job job, Threads x ... x+b){
    if(b<1)  // if only one thread is assigned
        do_real_work(job); return;

    // otherwise, divide the work in two subtasks
    // Only one thread executes this:
    Divide job into job1 and job2

    // Divide the assigned threads into two groups
    // and assign the subtasks to them.
    // The below two lines should be executed in parallel.
    #pragma omp task
    do_something(job1, x ... x+b/2)

    #pragma omp task
    do_something(job2, x+b/2 ... x+b)
}

// Call like follows

#pragma omp parallel
{
   #pragma omp single
   {
      b = omp_get_num_threads();
      do_something(job, x, b);
   }
}

OpenMP：递归细分工作和线程

OpenMP: recursively subdividing work and threads

multithreading

openmp