OPENMP F90/95 Nested DO loops - problems getting improvement over serial implementation

问题

I've done some searching but couldn't find anything that appeared to be related to my question (sorry if my question is redundant!). Anyway, as the title states, I'm having trouble getting any improvement over the serial implementation of my code. The code snippet that I need to parallelize is as follows (this is Fortran90 with OpenMP):

do n=1,lm     
  do m=1,jm   
    do l=1,im      
      sum_u = 0
      sum_v = 0
      sum_t = 0
      do k=1,lm
       !$omp parallel do reduction (+:sum_u,sum_v,sum_t) 
        do j=1,jm  
          do i=1,im
            exp_smoother=exp(-(abs(i-l)/hzscl)-(abs(j-m)/hzscl)-(abs(k-n)/vscl))
            sum_u = sum_u + u_p(i,j,k) * exp_smoother
            sum_v = sum_v + v_p(i,j,k) * exp_smoother
            sum_t = sum_t + t_p(i,j,k) * exp_smoother

            sum_u_pert(l,m,n) = sum_u
            sum_v_pert(l,m,n) = sum_v
            sum_t_pert(l,m,n) = sum_t          

            end do
          end do
       end do      
    end do
  end do  
end do

Am I running into race condition issues? Or am I simply putting the directive in the wrong place? I'm pretty new to this, so I apologize if this is an overly simplistic problem.

Anyway, without parallelization, the code is excruciatingly slow. To give an idea of the size of the problem, the lm, jm, and im indexes are 60, 401, and 501 respectively. So the parallelization is critical. Any help or links to helpful resources would be very much appreciated! I'm using xlf to compile the above code, if that's at all useful.

Thanks! -Jen

回答1:

The obvious place to put the omp pragma is at the very outside loop.

For every (l,m,n), you're calculating a convolution between your perturbed variables and an exponential smoother. Each (l,m,n) calculation is completely independant from the others, so you can put it on the outermost loop. So for instance the simplest thing

!$omp parallel do private(n,m,l,i,j,k,exp_smoother) shared(sum_u_pert,sum_v_pert,sum_t_pert,u_p,v_p,t_p), default(none)
do n=1,lm
  do m=1,jm
    do l=1,im
      do k=1,lm
        do j=1,jm
          do i=1,im
            exp_smoother=exp(-(abs(i-l)/hzscl)-(abs(j-m)/hzscl)-(abs(k-n)/vscl))
            sum_u_pert(l,m,n) = sum_u_pert(l,m,n) + u_p(i,j,k) * exp_smoother
            sum_v_pert(l,m,n) = sum_v_pert(l,m,n) + v_p(i,j,k) * exp_smoother
            sum_t_pert(l,m,n) = sum_t_pert(l,m,n) + t_p(i,j,k) * exp_smoother
          end do
        end do
      end do
    end do
  end do
end do

gives me a ~6x speedup on 8 cores (using a much reduced problem size of 20x41x41). Given the amount of work there is to do in the loops, even at the smaller size, I assume the reason it's not an 8x speedup involves memory contension or false sharing; for further performance tuning you might want to explicitly break the sum arrays into sub-blocks for each thread, and combine them at the end; but depending on the problem size, having the equivalent of an extra im x jm x lm sized array might not be desirable.

It seems like there's a lot of structure in this problem you aught to be able to explot to speed up even the serial case, but it's easier to say that then to find it; playing around on pen and paper nothing comes to mind in a few minutes, but someone cleverer may spot something.

回答2:

What you have is a convolution. This can be done with a Fast Fourier Transform in N log2(N) time. Your algorithm is N^2. If you use FFT, one core will probably be enough!

来源：https://stackoverflow.com/questions/6257887/openmp-f90-95-nested-do-loops-problems-getting-improvement-over-serial-impleme

标签

loops

fortran

openmp