c在并行区域内openmp并行 [英] c openmp parallel for inside a parallel region

问题描述

我的问题就像这个一个。但我想做一些不同的事情...例如，在我的并行区域内，我想在4个线程上运行我的代码。当每个线程进入for循环，我想在8个线程上运行我的代码。就像

  #pramga omp parallel num_threads（4）
 {
 //在4个线程上执行某些操作
 #pragma omp parallel for num_threads（2）
 for（int i = 0; i <2; i ++）{
 //在8个线程中执行总计
} 
 
 
 
 
 
 $ b 
那么，是否有办法将每个（4）运行的线程拆分两个（新的）线程，所以在for循环内更多（8）个线程正在运行？
解决方案
 href =http://docs.oracle.com/cd/E19205-01/819-5270/aewbc/index.html =nofollow noreferrer>嵌套并行，一个并行部分在另一个内部 - 被大多数当前支持OpenMP的编译器支持，但通常默认情况下是关闭的。您需要将 OMP_NESTED 环境变量设置为 TRUE ，或者在程序中调用 omp_set_nested（1）。请参阅这个答案。
 
 
 
回答您的后续问题在注释中提出问题，在OpenMP并行for循环结束时不需要屏障;除非您使用  nowait 子句，在for循环结束时已经有一个显式的同步屏障。而且你不能在for循环里有一个障碍。如果循环迭代没有被线程均分，会发生什么？你最终会遇到一些线程卡住在其他线程无法访问的障碍。
 
my question is like this one. but i'd like to do something different...

for instance, inside my parallel region i'd like to run my code on 4 threads. when each thread enters the for loop, i'd like to run my code on 8 threads. something like
#pramga omp parallel num_threads(4)
{
    //do something on 4 threads
    #pragma omp parallel for num_threads(2)
    for(int i=0;i<2;i++){
        //do something on 8 threads in total
    }
}
so, is there a way to "split" each (4) running threads into two (new) threads so inside the for loop more (8) threads are running ?
解决方案
What you have here - nested parallelism, with one parallel section inside another - is supported by most current OpenMP-enabled compilers, but is normally turned off by default. You'll need to set the OMP_NESTED environment variable to TRUE, or in your program call omp_set_nested(1). See, eg, this answer.

To answer your followup question in comments, you don't need a barrier at the end of OpenMP parallel for loops; unless you use the nowait clause, there already is an explicit barrier for synchronization at the end of your for loops.  And you can't have a barrier inside the for loop; what happens if the loop iterations aren't evenly divided by threads?  You'd end up with some threads being "stuck" waiting at a barrier none of the other threads would get to.

                        
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