Non repeatability issue
Matthew Knepley
knepley at gmail.com
Tue Mar 18 08:00:40 CDT 2008
To me this looks like
1) You have way too many Newton steps. Newton is quadratically
convergent, so if
you have 100+ steps, it means you are very very far from the
solution when you
begin. In this region, Newton is a really bad algorithm and can
be very very
sensitive to perturbations. I would never expect it to be
reproducible. From the
well-known fracint demo, it can be chaotic.
2) I would also question whether the system is actually stable.
Instability can result
in occasional divergence based upon small perturbations.
Thus, I am not sure you are asking the correct question with this
solve. I don't think
reproducible Newton over 100s of steps makes sense theoretically. Maybe Barry
thinks differently.
Matt
On Tue, Mar 18, 2008 at 5:57 AM, Aldo Bonfiglioli
<aldo.bonfiglioli at unibas.it> wrote:
> >
> > 1) Have you made runs where you require, say -ksp_rtol 1.e-12 to
> > eliminate the effects of
> > not solving the linear systems accurately?
>
>
> I have performed two runs with ksp_rtol = 1.e-12. The relevant plots are
> enclosed
> where comparisons are made with PETSc's default for ksp_rtol.
> In one of these two runs Newton even diverges.
> It should however be mentioned that
> at least for some Newton steps, the linear solver does not meet the
> convergence criterion in 2000 linear iterations (I reduced the default).
>
> > I have added the argument -vecscatter_reproduce
> > that will cause the receives to always be processed in the same order
> > (though order or
> > operations in the MPI reductions may still result in slightly
> > different convergence histories.)
> >
> > Hope this helps clear things up,
>
> I am not sure "-vecscatter_reproduce " has changed the situation much.
> Out of 4 subsequent runs, 2 converge
> while 2 enter a limit cycle I had not seen previously
> (I mean without the -vecscatter_reproduce option).
> The initial solution is the same as that shown in the other plot.
>
> Aldo
>
> --
> Dr. Aldo Bonfiglioli
> Dip.to di Ingegneria e Fisica dell'Ambiente (DIFA)
> Universita' della Basilicata
> V.le dell'Ateneo lucano, 10 85100 Potenza ITALY
> tel:+39.0971.205203 fax:+39.0971.205160
>
>
--
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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