[petsc-users] Using PETSc solvers and preconditioners with mfem

[Matthew Knepley]

On Tue, Oct 25, 2016 at 1:43 PM, Jed Brown <jed at jedbrown.org> wrote:

> Stefano Zampini <stefano.zampini at gmail.com> writes:
> > MATHYPRE could be a shell wrapping Hypre calls for the moment.
> > However, HypreParCSR and MATAIJ are mostly equivalent formats. As far as
> I know, the main (only?) difference resides in the fact that the diagonal
> term of the diagonal part is ordered first in the CSR.
> > For this reason, I think it should inherit from AIJ.
>
> This is more delicate.  Derived classes need to *exactly* retain the AIJ
> structure because all unimplemented methods use the parent
> implementations.  If the rows are not sorted, MatSetValues, MatGetRow,
> and the like cease to work.  You can still make MatHypreParCSR respond
> to MatMPIAIJSetPreallocation, but I don't think it can be derived from
> AIJ unless you audit *all* reachable AIJ code to remove the assumption
> of sorted rows *and* document the API change for all users that could
> observe the lack of sorting.  (I don't think that's worthwhile.)
>
> Note that the existing AIJ derived implementations merely augment the
> AIJ structure rather than modifying it.
>

<soapbox>
  Inheritance is almost never useful except in contrived textbook examples.
It was a tremendous pain to make
work in PETSc, and I think if we did it again, I would just go back and
make subobjects that packaged up lower
level behavior instead of inheriting.
</soapbox>

   Matt

> As soon  as I have time, I can start a new matrix class, but I don’t have
> much time to implement at the SetValues level yet.
>
> That's not urgent, but if you write it as a Mat implementation instead
> of some utility functions, it would be easy to add later and would not
> disrupt existing users.  There is no requirement that all Mat
> implementations include all the methods that "make sense"; it can be
> fleshed out later according to demand.
>



-- 
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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