[petsc-users] Uninterpolating a distributed mesh

[Matthew Knepley]

On Sat, Apr 11, 2015 at 6:55 PM, Justin Chang <jchang27 at uh.edu> wrote:

> 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.
>

Yes, I wrote Uninterpolate() just for testing, and do not currently handle
the SF. I put it on my TODO list.

Its not hard if you want to try. You just filter out any points that are
not cells and vertices from the SF, so

  PetscSFGetGraph()
  for (leaves)
    if leaf not a cell or vertex, skip
  PetscSFSetGraph()

  Thanks,

    Matt


> 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
>



-- 
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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