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Subject: [OMPI users] best way to ALLREDUCE multi-dimensional arrays in Fortran?
From: Greg Fischer (greg.a.fischer_at_[hidden])
Date: 2009-09-24 17:44:43


(I apologize in advance for the simplistic/newbie question.)

I'm performing an ALLREDUCE operation on a multi-dimensional array. This
operation is the biggest bottleneck in the code, and I'm wondering if
there's a way to do it more efficiently than what I'm doing now. Here's a
representative example of what's happening:

   ir=1
   do ikl=1,km
     do ij=1,jm
       do ii=1,im
         albuf(ir)=array(ii,ij,ikl,nl,0,ng)
         ir=ir+1
       enddo
     enddo
   enddo
   agbuf=0.0
   call
mpi_allreduce(albuf,agbuf,im*jm*kmloc(coords(2)+1),mpi_real,mpi_sum,ang_com,ierr)
   ir=1
   do ikl=1,km
     do ij=1,jm
       do ii=1,im
         phim(ii,ij,ikl,nl,0,ng)=agbuf(ir)
         ir=ir+1
       enddo
     enddo
   enddo

Is there any way to just do this in one fell swoop, rather than buffering,
transmitting, and unbuffering? This operation is looped over many times.
Are there savings to be had here?

Thanks,
Greg