[petsc-users] Wrong Global Vector size when default section is built after dmdistribute?

[Matthew Knepley]

On Sat, Feb 23, 2019 at 10:48 AM Blaise A Bourdin via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Hi,
>
> My student Alex and I are trying to build an example combining natural to
> global ordering and constraints.
> Constraints information do not seem to be distributed,
>

Well dang. That is true. I never considered doing it the other way since
all my specifications come from compact things
like an FEM discretization and BC labels, which get distributed. But yes,
this is a big hole. I think it could be fixed without
too much work since the constraints are just another section+IS to
distribute, and the IS does not even have to be renumbered.
I think we can just call
https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMPLEX/DMPlexDistributeFieldIS.html
on it.


> so the most rational thing to do seems to be creating the default section
> after distribution, then figuring out a way to reconstruct the natural to
> global stuff.
>

That is what I do, but we could do it the other way.


> In the example attached, we do this, then create a global vector, but for
> some reason that I do not understand, the global vector size is 0 (should
> be 18) when run on more than 1 CPU. If we make 2 calls to
> DMGetGlobalVector, we get a vector of the proper size. What are we doing
> wrong?
>

At the beginning, you turn on

  DMSetUseNatural(dm, PETSC_TRUE);

so during Distribute() it tries to make the mapping. This process creates a
global vector, which has size 0, because
the Section is empty. The reason it works when you call it again is that it
creates a new vector this time, now with
the new Section. So, to do it this way, don't set this flag at the
beginning. You have to setup the GlobalToNatural() afterwards.

Also, this shows that I should probably invalidate the global/local vector
cache after a new Section is set.

  Thanks,

    Matt


> Regards,
> Blaise
>
>
> --
> Department of Mathematics and Center for Computation & Technology
> Louisiana State University, Baton Rouge, LA 70803, USA
> Tel. +1 (225) 578 1612, Fax  +1 (225) 578 4276
> http://www.math.lsu.edu/~bourdin
>
>
>
>
>
>
>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20190223/949526e0/attachment.html>