[petsc-users] dmplex overlap questions

[Matthew Knepley]

On Thu, Feb 16, 2023 at 1:09 PM Lawrence Mitchell <wence at gmx.li> wrote:

> On Thu, 16 Feb 2023 at 16:43, Matthew Knepley <knepley at gmail.com> wrote:
> >
> > On Thu, Feb 16, 2023 at 10:54 AM Lawrence Mitchell <wence at gmx.li> wrote:
> >>
> >> Hi Blaise,
> >>
> >> On Thu, 16 Feb 2023 at 15:17, Blaise Bourdin <bourdin at 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:
> >
> >
> > Note that you can use
> >
> >   PetscFindInt(nleaves, ilocal, cell, &loc);
>
> Modulo argument order, is it not PetscFindInt(cell, nleaves, ilocal, &loc)?
>

You are right.


> I guess one should probably document that PetscSFSetGraph ensures that
> ilocal is sorted.
>

Yes, I thought it was already there, but does not seem so.

  Thanks,

    Matt


> Lawrence

-- 
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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