[petsc-dev] Cell overlap in DMPlexDistribute

[Matthew Knepley]

On Tue, Jan 28, 2014 at 8:57 AM, Michael Lange <michael.lange at imperial.ac.uk
> wrote:

> Hi,
>
> I noticed that the cell overlap created during DMPlexDistribute does not
> include cells that only share a vertex but no edge with an owned cell. This
> causes problems when performing local assembly
> (MAT_IGNORE_OFF_PROC_ENTRIES) based on information from the plex, because
> one contribution to the shared vertex is missing.
>
> As an example consider the 2x2 square with 8 cells (attached). When
> partitioned across two ranks with overlap=1 each rank owns 4 cells and in
> the current version knows about 2 halo cells, giving a total of 6 cells.
> The central vertex, however, touches 3 owned and 3 non-owned cells, one of
> which doesn't share an edge with any owned cells. So, in order to correctly
> assemble the central vertex locally, the rank owning it needs to know about
> 7 cells in total.
>
> I have attached a patch that fixes this problem by going over the inverse
> closure of all vertices associated with a given cell instead of using the
> provided edge graph. Please tell me what you think and whether there might
> be an easier way to fix this.
>

This is true, but unfortunately not what everyone wants. FVM people want
just what is there now. This
same choice comes up in preallocation. There I have put in the "center
dimension" which says what
kind of point do you use for the center, vertex or face? I guess we need
that here as well.

   Matt


> Kind regards
> Michael Lange
>



-- 
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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