[petsc-users] extract arbitrary subset of a DMDA

[Matteo Semplice]

On 10/10/2025 16:39, Matthew Knepley wrote:
> On Fri, Oct 10, 2025 at 9:48 AM Matteo Semplice via petsc-users 
> <petsc-users at mcs.anl.gov> wrote:
>
>     Dear all,
>
>          I am wondering if there is a way to extract a subset of a
>     DMDA and
>     use it as a mesh. The use case is to program a finite-difference
>     method
>     in which the domain is defined by a levelset function: if I could
>     completely ignore the parts of the background DMDA that are "far
>     away"
>     from the object, I guess I would avoid some cores having almost no
>     workload. I figure that I could setup a DMDA, load/compute the
>     levelset
>     on the entire box, then mark the nodes to be retained, extract the
>     submesh and repartition it. I would also need a mean to transfer some
>     Vec data from the DMDA to the new mesh.
>
>     I guess that the extracted mesh would then become a DMPlex and it
>     would
>     not retain any DMDA flavour (like notions of which are the grid nodes
>     sitting on top/bottom, left/right of a given node), right?
>
>
> If you are planning on extracting a Plex anyway, I think it would be 
> easier to just
> start with a Cartesian Plex, instead of a DA, and use 
> DMPlexCreateSubmesh().

Hmmm... doable, but I have a couple of questions.

By Cartesian Plex you mean a Plex created by DMPlexCreateBoxMesh with 
simplex=false, right? And, could you point me to the routines that can 
perform data tranfer from Vecs associated to the DM to the ones 
asscoiated to the subDM? Is DMPlexGetSubpointIS the way to go?

Next, I will load the levelsets from the output of another code that is 
DA-based and that I'd really like to reuse some code in the setup phase 
which relies on the DA indexing. So maybe I'd rather, create the DMDA 
and the associated Vecs, do the setup phase, then DMConvert the DMDA to 
a "large" DMPlex that covers the entire box, transfer the DA Vecs to the 
"large" Plex vectors and then extract the submesh. Would this be 
feasible? If so, can you point me to the routines to transfer the vecs 
from the dmda to the large plex?

Thanks

     Matteo
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