[petsc-dev] DMPlex label distribution

[Matthew Knepley]

On Wed, Mar 16, 2016 at 9:19 PM, Adrian Croucher <a.croucher at auckland.ac.nz>
wrote:

> 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).
>

Yep, that looks wrong. I can reproduce what you show.

I setup a small test with 8 cells and everything works as expected, so this
is not simple. Will track it down.

  Thanks,

     Matt


>
> 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
>
>


-- 
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
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