[mpich-discuss] Problems with multiple calls to MPI_Reduce?

[Elliot Parkin]

Thanks for the reply guys, very much appreciated.

Just to set the scene of how and why I'm using MPI_Allreduce:

I'm using MPI to parallelize a hydrodynamics code. The hydro grid is  
split into sections and to output a data file containing the whole  
grid requires the separate parts to be pieced together. The separate  
parts of the grid pass their part to a dummy array and then  
MPI_Allreduce fits this together and broadcasts the parts back to all  
processors. Here's a chunk of the code:

c Multiple calls to glue for various parameters

          call glue(lzro,zro)    ! density
          call glue(lzpr,zpr)    ! pressure
          call glue(lzco,zco)    ! colour
          call glue(lzux,zux)    ! x-velocity
          call glue(lzuy,zuy)    ! y-velocity
          call glue(lzuz,zuz)    ! z-velocity

c The glue subroutine looks like this:

       all = (imax*jmax*kmax)

       do k=1,kmax
          do j=1,jmax
             do i=1,imax
                dummy(i,j,k) = -1.0e50
             end do
          end do
       end do

       do k=1,mpikmax
          do j=1,mpijmax
             do i=1,mpiimax
                a = j + rank*mpijmax
                dummy(i,a,k) = input(i,j,k)
             end do
          end do
       end do

       call MPI_Allreduce (dummy,output,all,MPI_2DOUBLE_PRECISION,
      :     MPI_MAXLOC,comm,ierr)


Michael Ahlmann suggested that the problem could be due to using  
non-unique tags for the calls. At the minute the calls all use the  
same communicator, but I have to apologise about my lack of knowledge  
about MPI because I don't really know if that would be a problem or not.

Thanks again,

Elliot Parkin


Quoting Rajeev Thakur <thakur at mcs.anl.gov>:

> Can you send us a small code fragment?
>
> Rajeev
>
>> -----Original Message-----
>> From: owner-mpich-discuss at mcs.anl.gov
>> [mailto:owner-mpich-discuss at mcs.anl.gov] On Behalf Of Elliot Parkin
>> Sent: Friday, June 27, 2008 6:28 AM
>> To: mpich-discuss at mcs.anl.gov
>> Subject: [mpich-discuss] Problems with multiple calls to MPI_Reduce?
>>
>> Hello everyone,
>>
>> I've ran into some difficulties using MPI_Reduce. It seems
>> that after
>> it has been called some of the parameters in the code get set to
>> bizarre values. Also, if I call MPI_Reduce consecutively (to fill an
>> array with the maximum values across the processes) it
>> doesn't appear
>> to work the third time and sets all values of the array to zero. Has
>> anybody else had this problem, and if so is there any way to
>> explain/fix this?
>>
>> Cheers,
>>
>> Elliot Parkin.
>>
>> ----------------------------------------------------------------
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>>
>>
>
>