[petsc-users] DMDAGetElements returns unexpected values

[Mikhail Artemyev]

Barry,

Thank you for the explanation! I finally got where I was wrong - I mixed 
up ghost nodes and cells, so that trying to have 0 ghost cells I set up 
0 ghost nodes which was wrong as you pointed out.

Best regards,
Mikhail


On 07/15/2015 07:19 PM, Barry Smith wrote:
>    Mikhail
>
> DMDAGetElements - Gets an array containing the indices (in local coordinates)
>                   of all the local elements
>
>    Since it returns the "local indices" of the vertices of the elements it does not make sense for a stencil width of zero (with parallel DMDA) since the vertex of the last element on each process is actually owned by the next process so it is a ghost vertex on the process but with a stencil width of zero there are no ghost vertices hence no "local index" to represent that vertex.
>
>    I have changed the code to return an error instead of "garbage" values if the stencil width is zero.
>
>    So you should just use a stencil width of 1 if you want to do finite elements (or finite differences actually also :-). Stencil width of 0 is only for strange situations and doesn't make much sense for PDEs.
>
>    Thanks for reporting the problem.
>
>    Barry
>
>
>> On Jul 15, 2015, at 10:46 AM, Mikhail Artemyev <artemiev.mikhail at gmail.com> wrote:
>>
>> Dear all,
>>
>> Here is a minimal working example that I'm testing:
>>
>> #include "petscsys.h"
>> #include "petscdmda.h"
>>
>> int main(int argc, char **argv)
>> {
>>   PetscInitialize(&argc, &argv, 0, 0);
>>
>>   PetscInt rank;
>>   MPI_Comm_rank(PETSC_COMM_WORLD, &rank);
>>
>>   DM da;
>>   DMDACreate1d(PETSC_COMM_WORLD, DM_BOUNDARY_NONE, -9, 1, 0, NULL, &da);
>>
>>   PetscInt n_local_cells, n_cell_nodes;
>>   const PetscInt *local_indices;
>>   DMDAGetElements(da, &n_local_cells, &n_cell_nodes, &local_indices);
>>
>>   PetscSynchronizedPrintf(PETSC_COMM_WORLD,
>>                           "rank %d n_local_cells %d n_cell_nodes %d\n",
>>                           rank, n_local_cells, n_cell_nodes);
>>   PetscSynchronizedFlush(PETSC_COMM_WORLD, PETSC_STDOUT);
>>
>>   DMDARestoreElements(da, &n_local_cells, &n_cell_nodes, &local_indices);
>>
>>   PetscFinalize();
>>   return 0;
>> }
>>
>>
>> I believe it creates a 1D DM object, and outputs the number of local cells assigned to each process.
>>
>> Here is what I have as an output:
>>
>> $ mpirun -np 1 ./test
>> rank 0 n_local_cells 8 n_cell_nodes 2     // OK
>>
>> $ mpirun -np 2 ./test
>> rank 0 n_local_cells 4 n_cell_nodes 2     // OK
>> rank 1 n_local_cells 3 n_cell_nodes 2     // I expected 4 local cells here
>>
>> $ mpirun -np 4 ./test
>> rank 0 n_local_cells 2 n_cell_nodes 2     // OK
>> rank 1 n_local_cells 1 n_cell_nodes 2     // I expected 2 local cells here
>> rank 2 n_local_cells 1 n_cell_nodes 2     // I expected 2 local cells here
>> rank 3 n_local_cells 1 n_cell_nodes 2     // I expected 2 local cells here
>>
>>
>> What am I missing?
>>
>> Thank you.
>>
>> Best,
>> Mikhail
>>
>>