Copying a MPI vector into a small MPI vector

[liu chang]

On 12/6/06, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>   This is because the default when you do NOT provide iy (you have PETSC_NULL)
> is not what you expect. You need instead to explicitly provide the iy, in this case
> since you want to put them in the first slots of w, just pass the wv2w again
> as the iy.
>
>   Barry
>
>
> On Wed, 6 Dec 2006, liu chang wrote:
>
> > Hi, I'm fairly new to PETSc so excuse me if the problem seems obvious.
> >
> > I'm trying to copy the first m entries of a MPI vector of size n (m <
> > n) into another MPI vector of size m. From the Petsc documentation I
> > think I need to use VecScatter, so I wrote the following code
> > test.cpp:
> >
> > int main(int argc, char *argv[])
> > {
> >    int info;
> >    PetscInitialize(&argc, &argv, PETSC_NULL, PETSC_NULL);
> >
> >    Vec w, wv;
> >    info = VecCreateMPI(PETSC_COMM_WORLD, PETSC_DECIDE, 100, &w);
> > CHKERRQ(info);
> >    info = VecCreateMPI(PETSC_COMM_WORLD, PETSC_DECIDE, 110, &wv);
> > CHKERRQ(info);
> >
> >    IS wv2w;
> >    VecScatter s_wv2w;
> >    info = ISCreateStride(PETSC_COMM_WORLD, 100, 0, 1, &wv2w); CHKERRQ(info);
> >    info = VecScatterCreate(wv, wv2w, w, PETSC_NULL, &s_wv2w); CHKERRQ(info);
> >
> >    // actual copying skipped
> >
> >    PetscFinalize();
> >    return 0;
> > }
> >
> > The code runs fine on a single process, but when I run it over 2 processes:
> >
> > mpirun -np 2 -machinefile ~/machines.LINUX test
> >
> > The program crashes on the VecScatterCreate() statement:
> >
> > ------------------------------------------------------------------------
> > Petsc Release Version 2.3.1, Patch 15, Thu Jul  6 21:32:01 CDT 2006 HG
> > revision: dc39078b37aa6becf2c4f8b06facf07566bf5454
> > See docs/changes/index.html for recent updates.
> > See docs/faq.html for hints about trouble shooting.
> > See docs/index.html for manual pages.
> > ------------------------------------------------------------------------
> > ... Wed Dec  6 14:52:09 2006
> > Libraries linked from ...
> > Configure run at Sat Jul  8 15:09:13 2006
> > Configure options --download-f-blas-lapack=1 --with-mpi-dir=/opt/mpich
> > -with-clanguage=C++ --with-shared
> > ------------------------------------------------------------------------
> > [0]PETSC ERROR: VecScatterCreate() line 1350 in src/vec/vec/utils/vscat.c
> > [0]PETSC ERROR: Nonconforming object sizes!
> > [0]PETSC ERROR: Local scatter sizes don't match!
> > [0]PETSC ERROR: User provided function() line 20 in test.cpp
> > ------------------------------------------------------------------------
> > Petsc Release Version 2.3.1, Patch 15, Thu Jul  6 21:32:01 CDT 2006 HG
> > revision: dc39078b37aa6becf2c4f8b06facf07566bf5454
> > See docs/changes/index.html for recent updates.
> > See docs/faq.html for hints about trouble shooting.
> > See docs/index.html for manual pages.
> > ------------------------------------------------------------------------
> > ... Wed Dec  6 14:52:39 2006
> > Libraries linked from ...
> > Configure run at Sat Jul  8 15:09:13 2006
> > Configure options --download-f-blas-lapack=1 --with-mpi-dir=/opt/mpich
> > -with-clanguage=C++ --with-shared
> > ------------------------------------------------------------------------
> > [1]PETSC ERROR: VecScatterCreate() line 1350 in src/vec/vec/utils/vscat.c
> > [1]PETSC ERROR: Nonconforming object sizes!
> > [1]PETSC ERROR: Local scatter sizes don't match!
> > [1]PETSC ERROR: User provided function() line 20 in test.cpp
> >
> >
> > So the problem is `local scatter sizes don't match'. I guess it's
> > because some entries need to be passed from one process to the other
> > due to the different layouts of the two vectors. So if VecScatter
> > refuses to do it for me, how can it be done? Any help is greatly
> > appreciated.
> >
> > Liu Chang
> >

That works great! Thank you.

Liu Chang