Hi.
I am using a DMPLex to store a grid (at present 2d symplicial but will
need to work more in general), but after distributing it, I am finding
it hard to organize my loops over the cells/faces.
First things first: the mesh distribution. I do
DMPlexSetAdjacencyUseCone(dm, PETSC_TRUE);
DMPlexSetAdjacencyUseClosure(dm, PETSC_FALSE);
DMPlexDistribute(dm, 1, NULL, &dmDist);
The 1 in DMPlexDistribute is correct to give me 1 layer of ghost cells,
right?
Using 2 instead of 1 should add more internal ghosts, right? Actually,
this errors out with petsc3.7.7 from debian/stable... If you deem it
worth, I will test again with petsc3.8.
Secondly, looping over cells. I do
DMPlexGetHeightStratum(dm, 0, &cStart, &cEnd);
DMPlexGetHybridBounds(dm, &cPhysical, NULL, NULL, NULL);
for (int c = cStart; c < cEnd; ++c) {
if ((cPhysical>=0)&&(c>=cPhysical))
{code for ghost cells outside physical domain boundary}
else if ( ??? )
{code for ghost cells}
else
{code for cells owned by this process}
}
What should I use as a test for ghost cells outside processor
boundary? I have seen that (on a 2d symplicial mesh) reading the ghost
label with DMGetLabelValue(dm, "ghost", c, &ghost) gives the value -1
for inner cells and domain boundary ghosts but 2 for processor boundary
ghosts. Is this the correct test? What is the meaning of ghost=2? In
general, should I test for any value >=0?
Lastly, since I will do finite volumes, I'd like to loop over the faces
to compute fluxes.
DMPlexGetHeightStratum(dm, 1, &fStart, &fEnd)
gives me the start/endpoints for faces, but they include also faces
between ghost cells that are therefore external to the cells owned by
the processor. I see that for faces on processor boundary, ghost=-1 on
one process and ghost=2 on the other process, which I like. However
ghost=-1 also on faces between ghosts cells and therefore to compute
fluxes I should test for the ghost value of the face and for the ghost
value of the cells on both sides... Is there a better way to loop over
faces?
Thanks in advance for any suggestion.
Matteo
