[petsc-users] DMDAGetElements returns unexpected values

[Barry Smith]

  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
> 
>