用一个线程做一个部分，用多线程做一个for循环 [英] Doing a section with one thread and a for-loop with multiple threads

问题描述

我正在使用OpenMP，并且希望生成线程，以便一个线程执行一段代码并完成操作，并与N个线程并行运行一个并行for循环.

I am using OpenMP and I want to spawn threads such that one thread executes one piece of code and finishes, in parallel with N threads running the iterations of a parallel-for loop.

执行应如下所示:

Section A (one thread)       ||      Section B (parallel-for, multiple threads)
         |                   ||        | | | | | | | | | |
         |                   ||        | | | | | | | | | |
         |                   ||        | | | | | | | | | |
         |                   ||        | | | | | | | | | |
         |                   ||        | | | | | | | | | |
         V                   ||        V V V V V V V V V V

我不能只用#pragma omp once编写并发，因为我不希望执行A节的线程执行for循环.

I cannot just write a parallel-for with a #pragma omp once because I do not want the thread that executes section A to execute the for-loop.

我已经尝试过了:

#pragma omp parallel sections {
    #pragma omp section
    {
         // Section A
    }

    #pragma omp section
    {
         // Section B;
         #pragma omp parallel for
         for (int i = 0; i < x; ++i)
             something();
    }
}

但是，parallel-for总是只用一个线程执行(我知道，因为我将循环的主体打印为omp_get_thread_num()，并且它们都是相同的数字，根据执行的是两个线程是1还是0第二个并行部分).

However, the parallel-for always executes with only one thread (I know because I made the body of the loop print omp_get_thread_num() and they are all the same number, either 1 or 0 depending on which thread of the two executed the second parallel section).

我也尝试过

#pragma omp sections {
    #pragma omp section
    {
         // Section A
    }

    #pragma omp section
    {
         // Section B;
         #pragma omp parallel for
         for (int i = 0; i < x; ++i)
             something();
    }
}

这允许for循环使用多个线程执行，但它使各部分不并行，并且第一个部分在第二个部分之前顺序执行.

Which allows the for-loop to execute with multiple threads, but it makes the sections non-parallel, and the first section is executed sequentially before the second section.

我需要的是两种方法的组合，其中for循环的每次迭代和第一部分都并行运行.

What I need is a combination of the two approaches, where each iteration of the for-loop and the first section are all run in parallel.

推荐答案

嵌套并行性必须明确设置，因为大多数实现中默认情况下将其禁用.遵循OpenMP 4.0标准，您必须设置OMP_NESTED环境变量:

Nested parallelism must be explicitly set, as it is disabled by default in most implementations. Standing to the OpenMP 4.0 standard, you must set the OMP_NESTED environment variable:

OMP_NESTED环境变量通过以下方式控制嵌套并行性 设置Nest-var ICV的初始值.这个的价值 环境变量必须为true或false.如果环境 变量设置为true，启用嵌套并行性；如果设置为 错误，嵌套并行性被禁用.该程序的行为是 如果OMP_NESTED的值不为true也不是，则定义实现 错误.

The OMP_NESTED environment variable controls nested parallelism by setting the initial value of the nest-var ICV. The value of this environment variable must be true or false. If the environment variable is set to true, nested parallelism is enabled; if set to false, nested parallelism is disabled. The behavior of the program is implementation defined if the value of OMP_NESTED is neither true nor false.

以下行应适用于bash:

The following line should work for bash:

 export OMP_NESTED=true

Futhermore，如@HristoIliev在下面的注释中所述，很可能您想设置OMP_NUM_THREADS环境变量来调整性能.引用标准:

Futhermore, as noted by @HristoIliev in the comment below, it's very likely that you want to set the OMP_NUM_THREADS environment variable to tune performance. Quoting the standard:

此环境变量的值必须为正数列表 整数值.列表的值将线程数设置为 用于相应嵌套级别的并行区域.

The value of this environment variable must be a list of positive integer values. The values of the list set the number of threads to use for parallel regions at the corresponding nested levels.

这意味着应该设置与n,n-1类似的OMP_NUM_THREADS值，其中n是CPU内核数.例如:

This means that one should set the value of OMP_NUM_THREADS similar to n,n-1 where n is the number of CPU cores. For instance:

export OMP_NUM_THREADS=8,7

对于8核系统(示例摘自下面的注释).

for an 8-core system (example copied from the comment below).

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