[petsc-users] snes failures

[Matthew Knepley]

On Fri, May 20, 2016 at 10:11 AM, Juha Jaykka <juhaj at iki.fi> wrote:

> > not. This usually indicates a bad guess for Newton. I would now advocate
>
> Oh, did I forget to mention, that other initial conditions DO converge. The
> funny thing is the initial condition which does NOT converge is actually
> CLOSER to the solution than ones which DO. Had it been the other way
> around, I
> probably would not have thought twice of it.
>
> >   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.


>
> >   2) NPC: If the above fails, I would try preconditioning Newton with GS.
> > You can do this
> >       just using multiplicative composition, or left preconditioning.
>
> 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.


> If so, does the inner snes_of_type_gs and the outer snes_of_type_newtonls
> get
> all the same routines? I guess so, but just checking.


The outer SNES get the same routines, and the inner one is setup
automatically.


> > If 1 works, then its likely that FAS would work even better.
>
> 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.

  Thanks,

     Matt


> Cheers,
> Juha
>



-- 
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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