[petsc-users] snes failures

[Matthew Knepley]

On Fri, May 20, 2016 at 5:26 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On May 20, 2016, at 5:17 PM, Juha Jäykkä <juhaj at iki.fi> wrote:
> >
> >>>>  1) Grid sequencing: This is easy if you use a DMDA. You just use
> >>>> -snes_grid_sequence
> >>>>      and its automatic. Since you report that smaller grids converge,
> >>> this
> >>>> is usually enough.
> >>> Unfortunately, no.
> >> Isn't this a 1D problem with no geometry? You should use DMDA. It would
> make
> >> it easier.
> >
> > I think you misunderstood my too-short-a-comment. What I meant was
> > unfortunately -snes_grid_sequence was not enough to make it converge. I
> went
> > up to -snes_grid_sequence 10.
>
>    Interesting. How much is the solution changing in each refinement? Is
> there a singularity in the solution at one end point, or elsewhere?  If you
> know the form of the singularity perhaps you can subtract it out from the
> solution (and this hence changes the function) to get a simpler problem
> without a singularity that Newton works better on.


This is very counter-intuitive. I would now really like to see the code. I
would be very very very very
surprised to see grid sequencing fail on such as smooth solution.

    Matt


>
>    Barry
>
> >
> > I always use DMDA and yes, the only geometry in the problem is a
> straight line
> > of points from -X to +X, where preferably X = infinity, but numerically
> of
> > course not.
> >>> I'm not sure what you mean here. Do you mean I should run normal newton
> >>> line
> >>> search snes with SNESSetNPC(snes_of_type_gs)?
> >> Yes.
> >
> > Will do that once I get back to this next week.
> >
> >>> I wish it was so easy: I did try all the snes types before posting the
> >>> first
> >>> post. Also all KSP types. Except those that need things I don't have,
> of
> >>> course.
> >> Not sure what you mean here. To use FAS, you can either use DMDA, or
> provide
> >> interpolation operators between grids.
> >
> > Yes, but FAS does not converge either. Neither does any other snes type
> except
> > ngs, and changing to any other KSP type makes no difference either –
> though
> > why would it as the KSP seems to converge nicely anyway. I have also
> tried all
> > snes LS types, to no avail.
> >
> > A correction: funnily enough, newtontr claims it converges, but it does
> not
> > really. It ends up with the same "solution" as almost everything else
> does and
> > thinks it is a solution (CONVERGED_SNORM_RELATIVE) whereas I know full
> well it
> > is not a solution, not even close. But I always found newtontr fiddly
> anyway:
> > it tends to be too trigger happy to shout "convergence" and finding the
> right
> > parameters to make it not-so-trigger-happy is hard.
> >
> > Cheers,
> > Juha
> >
> > --
> >                -----------------------------------------------
> >               | Juha Jäykkä, juhaj at iki.fi                     |
> >               | http://koti.kapsi.fi/~juhaj/                  |
> >                -----------------------------------------------
>
>


-- 
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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