[petsc-dev] [petsc-users] unreliable AMG in PETSc

[Matthew Knepley]

On Mon, Oct 27, 2014 at 4:53 PM, Jed Brown <jed at jedbrown.org> wrote:

> Barry Smith <bsmith at mcs.anl.gov> writes:
> >    How can we insure that that the smoother is SPD? Which are known to
> be. In his case it was Richardson + SSOR on a bunch of processes.
>
> The Richardson scaling cannot be too ambitious and the SSOR needs to be
> an SPD operator.  I think this is true any time the forward and back
> sweep is stable, which is guaranteed for an M-matrix, for example.
>
> I'm not aware of an inexpensive way to guarantee stability/SPD smoother
> for a general matrix, and even if you could choose parameters to make it
> so, you might end up with such aggressive under-relaxation that the
> solver configuration is not useful.
>
> Focus on debuggability or automation?
>

Definitely debuggability. It would be great if we automatically diagnose
the failure of definiteness (I assume
all these retain symmetry). Maybe we are already doing this. Then we should
improve the error message
to give the user easy options for fixing it.

  Matt

-- 
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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