[petsc-users] Distribution of DMPlex for FEM

[Matthew Knepley]

On Wed, Jul 13, 2016 at 3:57 AM, Morten Nobel-Jørgensen <mono at dtu.dk> wrote:

> I’m having problems distributing a simple FEM model using DMPlex. For test
> case I use 1x1x2 hex box elements (/cells) with 12 vertices. Each vertex
> has one DOF.
> When I distribute the system to two processors, each get a single element
> and the local vector has the size 8 (one DOF for each vertex of a hex box)
> as expected.
>
> My problem is that when I manually assemble the global stiffness matrix (a
> 12x12 matrix) it seems like my ghost values are ignored. I’m sure that I’m
> missing something obvious but cannot see what it is.
>
> In the attached example, I’m assembling the global stiffness matrix using
> a simple local stiffness matrix of ones. This makes it very easy to see if
> the matrix is assembled correctly. If I run it on one process, then global
> stiffness matrix consists of 0’s, 1’s and 2’s and its trace is 16.0. But if
> I run it distributed on on two, then it consists only of 0's and 1’s and
> its trace is 12.0.
>
> I hope that somebody can spot my mistake and help me in the right
> direction :)
>

This is my fault, and Stefano Zampini had already tried to tell me this was
broken. I normally use DMPlexMatSetClosure(), which handles global indices
correctly.
I have fixed this in the branch

  knepley/fix-plex-l2g

which is also merged to 'next'. I am attaching a version of your sample
where all objects are freed correctly. Let me know if that works for you.

  Thanks,

     Matt


> Kind regards,
> Morten
>



-- 
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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