[petsc-users] dmplex overlap questions

[Lawrence Mitchell]

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)?

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

Lawrence