[petsc-users] DMPlex overlap

[Matthew Knepley]

On Sat, Mar 27, 2021 at 9:27 AM Nicolas Barral <
nicolas.barral at math.u-bordeaux.fr> wrote:

> Hi all,
>
> First, I'm not sure I understand what the overlap parameter in
> DMPlexDistributeOverlap does. I tried the following: generate a small
> mesh on 1 rank with DMPlexCreateBoxMesh, then distribute it with
> DMPlexDistribute. At this point I have two nice partitions, with shared
> vertices and no overlapping cells. Then I call DMPlexDistributeOverlap
> with the overlap parameter set to 0 or 1, and get the same resulting
> plex in both cases. Why is that ?
>

The overlap parameter says how many cell adjacencies to go out. You should
not get the same
mesh out. We have lots of examples that use this. If you send your small
example, I can probably
tell you what is happening.


> Second, I'm wondering what would be a good way to handle two overlaps
> and associated local vectors. In my adaptation code, the remeshing
> library requires a non-overlapping mesh, while the refinement criterion
> computation is based on hessian computations, which require a layer of
> overlap. What I can do is clone the dm before distributing the overlap,
> then manage two independent plex objects with their own local sections
> etc. and copy/trim local vectors manually. Is there a more automatic way
> to do this ?
>

DMClone() is a shallow copy, so that will not work. You would maintain two
different Plexes, overlapping
and non-overlapping, with their own sections and vecs. Are you sure you
need to keep around the non-overlapping one?
Maybe if I understood what operations you want to work, I could say
something more definitive.

  Thanks,

     Matt


> Thanks
>
> --
> Nicolas
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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