[petsc-users] Mapping between application ordering and Petsc ordering

[Mani Chandra]

Tried flipping the indices, I get a seg fault.

On Sat, Aug 8, 2015 at 3:03 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Aug 8, 2015, at 2:45 PM, Mani Chandra <mc0710 at gmail.com> wrote:
> >
> > Thanks. Any suggestions for a fix?
>
>   Just flip the meaning of the x indices and the y indices in the PETSc
> parts of the code?
>
>   Also run with a very different N1 and  N2 (instead of equal size) to
> better test the code coupling.
>
>   Barry
>
>
> >
> > Reorder the indices in arrayApplication?
> >
> > On Sat, Aug 8, 2015 at 2:19 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
> > On Sat, Aug 8, 2015 at 1:52 PM, Mani Chandra <mc0710 at gmail.com> wrote:
> > Hi,
> >
> > I'm having trouble interfacing petsc to an application which I think is
> related to the ordering of the nodes. Here's what I'm trying to do:
> >
> > The application uses a structured grid with a global array having
> dimensions N1 x N2, which is then decomposed into a local array with
> dimensions NX1 x NX2.
> >
> > I create a Petsc DMDA using
> >
> >     DMDACreate2d(MPI_COMM_WORLD,
> >                  DM_BOUNDARY_PERIODIC, DM_BOUNDARY_PERIODIC,
> >                  DMDA_STENCIL_BOX,
> >                  N1, N2,
> >                  N1/NX1, N2/NX2,
> >                  1, nghost, PETSC_NULL, PETSC_NULL,
> >                  &dmda);
> >
> > and then use this to create a vec:
> >
> >   DMCreateGlobalVector(dmda, &vec);
> >
> > Now I copy the local contents of the application array to the petsc
> array using the following:
> >
> > Let i, j be the application indices and iPetsc and jPetsc be petsc's
> indices, then:
> >
> > DMDAGetCorners(dmda, &iStart, &jStart, &kStart,
> >                                          &iSize, &jSize, &kSize
> >                               );
> >
> >
> > double **arrayPetsc;
> > DMDAVecGetArray(dmda, vec, &arrayPetsc);
> >
> > for (int j=0, jPetsc=jStart; j<NX2, jPetsc<jStart+jSize; j++, jPetsc++)
> > {
> >   for (int i=0, iPetsc=iStart; i<NX1, iPetsc<iStart+iSize; i++, iPetsc++)
> >   {
> >      arrayPetsc[jPetsc][iPetsc] = arrayApplication[j][i];
> >   }
> > }
> >
> > DMDAVecRestoreArray(dmda, vec, &arrayPetsc);
> >
> > Now if I VecView(vec, viewer) and look at the data that petsc has, it
> looks right when run with 1 proc, but if I use 4 procs it's all messed up
> (see attached plots).
> >
> > I should probably be using the AO object but its not clear how. Could
> you help me out?
> >
> > It looks like you have the global order of processes reversed, meaning
> you have
> >
> >   1   3
> >
> >   0   2
> >
> > and it should be
> >
> >   2  3
> >
> >   0  1
> >
> >   Thanks,
> >
> >       Matt
> >
> > Thanks,
> > Mani
> > --
> > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> > -- Norbert Wiener
> >
>
>
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