[petsc-dev] KSP in DMPlex

[Matthew Knepley]

On Sun, Nov 17, 2019 at 12:15 PM Smith, Barry F. via petsc-dev <
petsc-dev at mcs.anl.gov> wrote:

>
>
> > On Nov 14, 2019, at 12:30 PM, Faibussowitsch, Jacob via petsc-dev <
> petsc-dev at mcs.anl.gov> wrote:
> >
> > Hello,
> >
> > So I am trying to make a simple 5 pt stencil finite difference laplacian
> operator (
> https://akmitra.public.iastate.edu/aero361/design_web/Laplace.pdf) work
> on a dmplex mesh, but all of the examples that I have found in
> ksp/examples/tutorials all use DMDA. The only way that I have managed to
> get the KSP to even run is by creating the operator matrix myself by hand.
> The workflow for the working implementation is as follows:
> >
> > DMPlexMakeBoxMesh -> KSPCreate -> KSPsetDM -> KSPDeactiveDM -> Make the
> matrix by hand ->  KSPSetOperator(matrix) -> KSPSetComputeRHS -> Solve
> >
> > Given that I tether KSP to the DM by KSPSetDM, but then immediately
> deactivate it (because then it complains about mallocs), this doesn’t work
> when reading in an actual unstructured mesh. Does KSP just not support
> dmplex?
>
>    Nothing to do with KSP supporting DMPLEX.
>
>    For a given DMDA the matrix structure is completely determined when
> using finite differences, thus DMDA can always create an empty sparse
> matrix ready for KSP. Then one can either fill in the sparse matrix (as do
> many of the examples) with values for a particular PDE or have the
> MatFDColoring code compute the matrix entries from the provided RHS
> function.
>
>    For DMPLEX the flex object itself does not have enough information to
> create a sparse matrix; it would be ill-defined since the matrix depends on
> the finite element space, boundary conditions etc. So it would really be
> DMPLEX + PETSCFE and a bunch of stuff to fully define the matrix.
>
>    See for example snes/examples/tutorials/ex12.c this is a way over
> complicated with lots of options, but it is one of the most basic for use
> DMPLEX for a PDE.  See SetupDiscretization() for how it creates the FE and
> DS objects.
>

Barry is right, also about ex12 being too complicated for this kind of a
question. So I made ex13, which is the same thing, but
with far fewer variants, so its easier to see what is happening.

  Thanks,

    Matt


> >
> > Best,
> >
> > Jacob
> >
>
>

-- 
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-dev/attachments/20191117/1533da00/attachment.html>