[petsc-users] pcfieldsplit for a composite dm with multiple subfields

[Matthew Knepley]

On Tue, Sep 8, 2015 at 10:00 PM, Gideon Simpson <gideon.simpson at gmail.com>
wrote:

> Ok, again, I’m ignorant, where do I put the VecView command to get it to
> save at some particular iterate?  Also, what do you mean by “halves”?
>

Well, you should be able to just give -snes_monitor_residual, but it does
not take arguments nicely (ugh). I can fix that, but in the meantime
you can just put VecViewFromOptions(v, NULL, "-res_vec_view") in a custom
monitor.

By halves I meant the two different problems. I want to see the residual
norms for both parts of the combined problem.

  Matt


> -gideon
>
> On Sep 8, 2015, at 10:53 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Tue, Sep 8, 2015 at 9:48 PM, Gideon Simpson <gideon.simpson at gmail.com>
> wrote:
>
>> Your comment about the different pieces scaling different made me think
>> of something, and I made a slight modification.  It still fails, but the
>> conditioning looks much better, no?
>>
>
> This is great. Now its clear that the nonlinear convergence is crappy.
>
> Lets start with looking at the distribution of residuals. Take iterate 50,
> which is as converged as you get and save the residual (VecView with
> binary). Then we can load it up and play with it. I recommend
>
>   a) Norms for both halves
>
>   b) Plot of the big half
>
> What usually happens is that some small part is screwing up, but you
> have to iterate the whole system, and finding the descent direction is
> hard. If you select the problem variables, Newton will converge, and then
> the big system will converge once the problem is removed. This is the
> classic
> "coarse basis selection" problem in the nonlinear regime.
>
>    Matt
>
>
>


-- 
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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