[petsc-dev] DMPlex label distribution

[Adrian Croucher]

hi

I'm seeing some slightly puzzling behaviour when I set up a DMLabel on a 
DMPlex, then distribute the DMPlex using DMPlexDistribute().

Specifically, if I make the label values unique to start with (i.e. each 
mesh cell has a unique value), this is no longer always the case after 
distribution. In some cases two cells (not ghost cells) end up with the 
same label value, and one of the original values goes missing.

I'm running the 'next' branch.

The attached minimal example shows it. In this example the label values 
are just the cell indices before distribution, so they're unique.  There 
are 900 mesh cells, so if running on 2 processors, each rank gets 450 of 
them.

If I run it using

mpiexec -np 2 testlabel > out

and then go

grep '(45)' out

I get:

[0]: 0 (45)
[1]: 409 (45)
[1]: 450 (45)

So there are 3 cells ending up with the label value 45. The first one is 
the one I expect to get this label value.

The third one has cell index > 449, so I figure it's a ghost cell on 
rank 1 (actually the same cell as above on rank 0). So that's probably 
OK too.

But I don't know where the second one has come from. Its index indicates 
it isn't a ghost cell.

Based on how the other cells are labelled, I would expect this cell 409 
on rank 1 to have label value 814. But it turns out there is no cell in 
the distributed mesh with that label value- even though there was before 
distribution (I checked).


Am I misunderstanding something or is this unexpected behaviour?

Cheers, Adrian

-- 
Dr Adrian Croucher
Senior Research Fellow
Department of Engineering Science
University of Auckland, New Zealand
email: a.croucher at auckland.ac.nz
tel: +64 (0)9 923 84611

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