[petsc-users] Uninterpolating a distributed mesh

[Justin Chang]

Hello,

When I call DMPlexUninterpolate(...) on a distributed mesh (say 2
processors), it seems to overwrite the "ghost" points (i.e., the points not
locally owned by the processor) and treats all points as if they are local
to the processor.

Say I have this 2D cell-vertex mesh:

14----15-----16
| \  5  | \ 7   |
|   \   |   \   |
|  4  \ |  6  \ |
11----12-----13
| \  1  | \ 3   |
|   \   |   \   |
|  0  \ |  2  \ |
8------9------10

Which results in the following DM:

DM Object: 2 MPI processes
  type: plex
DM_0x84000004_0 in 2 dimensions:
  0-cells: 9 0
  2-cells: 8 0
Labels:
  marker: 1 strata of sizes (8)
  depth: 2 strata of sizes (9, 8)

I proceed by interpolating this DM:

DM Object: 2 MPI processes
  type: plex
DM_0x84000004_1 in 2 dimensions:
  0-cells: 9 0
  1-cells: 16 0
  2-cells: 8 0
Labels:
  marker: 1 strata of sizes (16)
  depth: 3 strata of sizes (9, 16, 8)

Then distributing across 2 processors:

DM Object:Parallel Mesh 2 MPI processes
  type: plex
Parallel Mesh in 2 dimensions:
  0-cells: 6 6
  1-cells: 9 9
  2-cells: 4 4
Labels:
  marker: 1 strata of sizes (9)
  depth: 3 strata of sizes (6, 9, 4)

I have the option of uniformly refining the mesh here but I choose not to
for now. If my dofs are vertex based, then the global size of my DM vector
is 9 and the local sizes for ranks 0 and 1 are 3 and 6 respectively.
However, if I choose to uninterpolate the mesh by calling
DMPlexUninterpolate(...), I get this:

DM Object: 2 MPI processes
  type: plex
DM_0x84000004_2 in 2 dimensions:
  0-cells: 6 6
  2-cells: 4 4
Labels:
  marker: 1 strata of sizes (5)
  depth: 2 strata of sizes (6, 4)

And the global size of my DM vector becomes 12 and the local size for both
ranks is 6. It looks like the ghost points in rank 0 have been duplicated,
which is not suppose to happen.

Is there a way to capture the ghost point information when uninterpolating
the DM?

Thanks,

-- 
Justin Chang
PhD Candidate, Civil Engineering - Computational Sciences
University of Houston, Department of Civil and Environmental Engineering
Houston, TX 77004
(512) 963-3262
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