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

[William Gropp]

What is the size of the variable dummy?  Since you are using  
MPI_MAXLOC, it needs to be at least 2*all in size  
(2*imax*jmax*kmax) .  Did you mean to use MPI_MAX instead?

Bill

On Jul 1, 2008, at 4:32 AM, Elliot Parkin wrote:

> 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.
>>>
>>> ----------------------------------------------------------------
>>> This message was sent using IMP, the Internet Messaging Program.
>>>
>>>
>>
>>
>
>

William Gropp
Paul and Cynthia Saylor Professor of Computer Science
University of Illinois Urbana-Champaign


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