Need to scatter from a few of the processors only to the zero processor

[Matthew Knepley]

Well, I can think of one way right away:

  1) Define global vector, which may have size 0 on some procs

  2) Put your local calculations in this vector. You may need a few vectors for
      handling coordinates, and then values, but I will describe just one.

  3) Use VecScatterCreateToZero() to send it to proc 0:
     http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Vec/VecScatterCreateToZero.html

There is sample code on that webpage for using it.

   Matt

On 12/13/06, Randall Mackie <randy at geosystem.us> wrote:
> At the end of my large 3D computation, I compute the results at, say, 20 locations
> in the entire 3D volume. This calculation doesn't happen on every processor, but
> only those processors that own that part of the 3D volume.
>
> I want to get those results to the zero processor for output, but I'm unsure
> about the best way to do this. My question is can I define a parallel vector
> that is smaller than the number of processors, or does that even make sense?
>
> Right now, I'm doing the following, and I'm wondering how to do this with
> VecCreateMPI and scattering the results to the zero processor:
>
> !
> ! See if computation point is owned by this processor:
> !
>          IF (ii >= xs .and. ii <= xs+xm-1 .and.
>       .      jj >= ys .and. jj <= ys+ym-1 .and.
>       .      kk >= zs .and. kk <= zs+zm-1 ) THEN
>
>            flg=.true.
>
>            hyijk=xh(idx+3*row+2)
>            hyijf=xh(idx+3*(row+gxm*gym)+2)
>            hzijk=xh(idx+3*row+3)
>            hziek=xh(idx+3*(row+gxm)+3)
>
> etc, do some computations
>
>         END IF
>
>          IF (flg .and. rank /= 0) THEN
>
>            call MPI_Send(esxInterp(iTx,1), 1, MPI_DOUBLE_COMPLEX, 0, 1,
>       .                  MPI_COMM_WORLD, ierr)
>
>          ELSE IF (.not. flg .and. rank == 0) THEN
>
>            call MPI_Recv(esxInterp(iTx,1), 1, MPI_DOUBLE_COMPLEX,
>       .         MPI_ANY_SOURCE, 1, PETSC_COMM_WORLD, mpi_status, ierr)
>
>          END IF
>
>
> --
> Randall Mackie
> GSY-USA, Inc.
> PMB# 643
> 2261 Market St.,
> San Francisco, CA 94114-1600
> Tel (415) 469-8649
> Fax (415) 469-5044
>
> California Registered Geophysicist
> License No. GP 1034
>
>


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