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

[Gideon Simpson]

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”?


-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 <mailto: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
> 

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