[petsc-users] Matrix indexing for distributed DMPlex

[Matthew Knepley]

On Fri, Nov 6, 2015 at 6:35 PM, Matthew Kury <mkury at berkeley.edu> wrote:

> Dear All,
>
> I have been trying to figure out how to appropriately index a matrix that
> was created from a DMPlex with a section defined for it.
>
>  I created the matrix with DMCreateMatrix() and I tried to index the
> entries by using the ISLocalToGlobalMapping obtained from
> DMGetLocalToGlobalMapping, with the local indices being those obtained from
> PetscSectionGetOffset() with the appropriate points from the D.A.G. However
> this does not seem to work.
>

It should work. However, you can always get the global section using
DMGetDefaultGlobalSection() which directly gives global offsets,
although nonlocal offsets are stored as -(off+1).


> In particular, with the  ISLocalToGlobalMapping, what I understand is that
> it gives the relationship of the local indexing to the global indexing,
> however there are negative numbers in this mapping which I fail to find an
> explanation of in the documentation. In addition, there do not appear to be
> redundant indexes to the global matrix as one would expect in a distributed
> DMPlex because there can be some of the same points on different
> processors.
>

Yes, the negative numbers are -(off+1) for nonlocal offsets. I am not sure
I understand your point about redundant offsets.

  Thanks,

    Matt


> Any help is greatly appreciated.
>
> Thank you for your time,
>
>
> Matthew W. Kury
> Ph.D. Candidate
> CMRL UC Berkeley
> M.S. Mechanical Engineering UC Berkeley, 2014
>
>
>
>
>
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>


-- 
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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