[petsc-dev] DMPlex label distribution

[Matthew Knepley]

On Thu, Mar 17, 2016 at 12:41 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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.
>

I believe I have fixed your bug:

  https://bitbucket.org/petsc/petsc/branch/knepley/fix-plex-distribute-label

I merged to next, and it will get to master before the release.

  Thanks,

    Matt


>   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
>



-- 
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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