On Fri, May 20, 2016 at 5:26 PM, Barry Smith <[email protected]> wrote:
> > > On May 20, 2016, at 5:17 PM, Juha Jäykkä <[email protected]> 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, [email protected] | > > | 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
