Idle threads while new threads can be assigned to a nested loop

问题

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?

回答1:

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.

来源：https://stackoverflow.com/questions/39019299/idle-threads-while-new-threads-can-be-assigned-to-a-nested-loop