空闲线程，而新线程可以分配给嵌套循环 [英] Idle threads while new threads can be assigned to a nested loop

问题描述

我有两个嵌套循环：

 ！$ omp parallel 
！$ omp do 
 do i = 1,4 
 ... 
！$ omp parallel 
！$ omp do 
 do j = 1,4 
 call job（i，j）
  

我的电脑可以并行运行四个线程。对于外部循环，创建这样的四个线程。由于 i = 4 ， job 的费用是前者的四倍，所以快速完成。

现在我预计在内部并行区域，新线程共享工作。但是这种情况不会发生：CPU负载保持在1/4，就好像第四个线程在内部循环上串行工作一样。

如何分配并行CPU时间到内部并行循环？

解决方案

 ！$ omp parallel do collapse（2）
 do i = 1,4 
 do j = 1,4 
 call job（i ，j）
 end do 
 end do 
  

它应该表现得更好。 / p>

I have two nested loops:

!$omp parallel 
!$omp do
do i=1,4
  ...
  !$omp parallel 
  !$omp do
  do j=1,4
    call job(i,j)

My computer can run four threads in parallel. For the outer loop such four threads are created. The first three finish quickly since for i=4, the job is four times more expensive.

Now I expect that in the inner parallel region, new threads share the work. But this doesn't happen: The CPU load stays at 1/4, just as if the 4th thread works serially on the inner loop.

How can I allocate parallel CPU time to the inner parallel loop?

解决方案

Did you try the following approach?

!$omp parallel do collapse(2)
do i = 1,4
do j = 1,4
    call job(i,j)
end do
end do

It should behave better.

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