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

[Thomas Witkowski]

Thanks, exactly what I've searched for.

Thomas

Am 24.10.2012 22:19, schrieb Jed Brown:
> You are looking for PCFieldSplitGetSubKSP().
>
> Matt's comment was about distinguishing between the KSP living inside 
> of S = D - C A^{-1} B and the one used directly by PCFIeldSplit when 
> solving with A.
>
> On Wed, Oct 24, 2012 at 3:04 PM, Thomas Witkowski 
> <thomas.witkowski at tu-dresden.de 
> <mailto:thomas.witkowski at tu-dresden.de>> wrote:
>
>     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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