[petsc-users] snes failures

Juha Jäykkä juhaj at iki.fi
Fri May 20 17:17:09 CDT 2016

> > >   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.

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.


		| Juha Jäykkä, juhaj at iki.fi			|
		| http://koti.kapsi.fi/~juhaj/			|
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