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

[Mani Chandra]

Thanks. Any suggestions for a fix?

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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