[petsc-users] How to access the KSP objects from a PCFIELDSPLIT Schur preconditioner

[Thomas Witkowski]

Am 24.10.2012 21:33, schrieb Matthew Knepley:
> On Wed, Oct 24, 2012 at 3:23 PM, Thomas Witkowski 
> <thomas.witkowski at tu-dresden.de 
> <mailto:thomas.witkowski at tu-dresden.de>> wrote:
>
>     I want to play around with some special type of a Schur complement
>     preconditioner. It perfectly fits into PETSc's fieldsplit
>     preconditioner (schur - full factorization). But for the inner
>     solvers, I would like to create some MatShells. So I need to
>     access both KSP objects, for the upper block (KSP(A_00)) and for
>     the Schur complement (KSP(S)). How can I get these objects in my
>     source code?
>
>
> First you have to make them different. This happens when you give 
> options for
>
>   -fieldsplit_1_upper_*
>   -fieldsplit_1_inner_*
>
> This is how I customize the solvers to get things like SIMPLE. Once 
> they are created, you
> can get them by pulling apart the FS. The upper is fs->kspupper, and 
> the inner is the KSP
> from the MATSCHUR from fs->schur.
I don't understand your solution. Maybe I should put some lines of code 
to emphasize my question. The relevant part looks as follows:

PCSetType(pc, PCFIELDSPLIT);
PCFieldSplitSetSchurFactType(pc, PC_FIELDSPLIT_SCHUR_FACT_FULL);
...
PCFieldSplitSetIS(pc, "velocity", is0);
PCFieldSplitSetIS(pc, "pressure", is1);
...
KSP velocity;
KSP schur;
....

Okay, I don't solve any Stokes/Navier Stokes equation, but to make the 
names somehow convenient I choose here the names velocity and pressure 
for the two system blocks. My question is now, how to figure out the two 
inner solver objects from the field split preconditioner.

Matt, if your answer is still appropriate to my question, can you make 
it clear on the above piece of code? Thank you very much.

Thomas

>
>    Matt
>
>     Best regards,
>
>     Thomas
>
>
>
>
> -- 
> 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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