[petsc-dev] DMSetUseNatural() and DMPlexConstructGhostCells()

Matthew Knepley knepley at gmail.com
Mon Jul 20 17:39:50 CDT 2015

On Mon, Jul 20, 2015 at 5:24 PM, Adrian Croucher <a.croucher at auckland.ac.nz>

> On 20/07/15 22:51, Matthew Knepley wrote:
> Thats a bug. CreateGhostCells() makes an entirely new DM, and the
> SetNatural is a piece of state which did not get carried over. I will
> fix it.
> OK, that's what I suspected. Thanks.
>  I have a question about this strategy. The design philosophy for the
> PETSc discretization stuff is to make as much
> mesh-independent as possible. I usually think of boundary and initial
> conditions as analytic functions that are
> applied to a given geometric region on the mesh, and then projected into a
> function space once I choose my
> discretization. Why doesn't that work in this case?
> Yeah, I noticed that's the way all the example problems work, and I'd do
> it that way too if I could.
> However my initial conditions aren't defined by analytic functions. Often
> they are taken from the end result of a previous model run, so they're
> defined as values on particular cells. In most such cases I should be able
> to load them in from HDF5 files, but even then I gather I'll still need
> this natural ordering stuff to make them end up on the right cells in the
> mesh, especially as the number of processors won't necessarily be the same
> between runs.

It is really seductive to view the solution as a bunch of FEM coefficients,
but I think that destroys code modularity. I would rather
endure (at least in the short term) the overhead of point location and
represent my solution as a real function, than hope that I am
using exactly the same mesh/discretization the next time around and have to
do parallel fixups like this.



> - 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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