On 10/10/2025 16:39, Matthew Knepley wrote:
On Fri, Oct 10, 2025 at 9:48 AM Matteo Semplice via petsc-users
<[email protected]> 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
