[petsc-dev] FieldSplit fixes in 3.3

[Jed Brown]

On Tue, Aug 21, 2012 at 8:51 PM, Matthew Knepley <knepley at gmail.com> wrote:

> I thought we agreed in this thread that we were (for now) going with
>> Matt's bastardized model of attaching the Schur null space to A11. Doesn't
>> that mean that this hunk should also be reverted (and have a comment
>> explaining this indirect effect)?
>>
>
> For future reference, this was my bastardized model in 3.3, but in
> petsc-dev I either
>
>   a) attach them to IS on input, which works beautifully
>
> or
>
>   b) Tell the DM about them
>

In either case, what happens when you switch back and forth between Schur
and, e.g. multiplicative? Does that cause there to be a different IS or a
different DM?


I fear that by including the physics in the DM, we may be obligated to have
a DMGetSchurComplement() (or, in the more general nonlinear language,
DMEliminate()). Note that some mixed discretizations have sparse Schur
complements and it could even make sense to implement a nonlinear smoother
in the reduced space.
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