Re: [petsc-users] dmplex overlap questions

[Lawrence Mitchell]

Hi Blaise,

On Thu, 16 Feb 2023 at 15:17, Blaise Bourdin <bour...@mcmaster.ca> wrote:
>
> Hi,
>
> I am trying to implement a non-local finite elements reconstruction operator 
> in parallel.
>
> Given a dmplex distributed with an overlap, is there a way to figure out 
> which cells are in the overlap and which are not?

Yes. Get the pointSF of the DM, and the cell chart

DMPlexGetPointSF(dm, &sf);
DMPlexGetHeightStratum(dm, 0, &cstart, &cend);

Now get the graph (specifically ilocal of the sf):

PetscSFGetGraph(sf, NULL, &nleaves, &ilocal,  NULL);

Now any value of ilocal that lies in [cstart, cend) is a cell which is
not owned by this process (i.e. in the overlap). Note that ilocal can
be NULL which just means it is the identity map [0, ..., nleaves), so
you just intersect [cstart, cend) with [0, nleaves) in that case to
find the overlap cells.

But that is very unlikely to be true, so:

for (PetscInt i = 0; i < nleaves; i++) {
    if (cstart <= ilocal[i] && ilocal[i] < cend) {
       // i is an overlap cell
    }
}
> Alternatively, suppose that I distribute the same DM with and without an 
> overlap. Is there any warranty that the distributions are compatible (i.e. 
> coincide when the overlap is ignored)? If this is the case, can I assume that 
> the non-overlap cells are numbered first in the overlapped dm?

If you do:

DMPlexDistribute(dm, 0, &migrationSF, &paralleldm);
DMPlexDistributeOverlap(paralleldm, 1, &migrationSF2, &overlapdm);

Then paralleldm and overlapdm will be compatible, and I think it is
still the case that the overlap cells are numbered last (and
contiguously).

If you just do DMPlexDistribute(dm, 1, &migrationSF, &overlapdm) then
you obviously don't have the non-overlapped one to compare, but it is
in this case still true that the overlap cells are numbered last.

Thanks,

Lawrence