[petsc-users] [EXTERNAL] Re: Problem with PCFIELDSPLIT

[Matthew Knepley]

On Wed, Jul 7, 2021 at 2:33 PM Jorti, Zakariae <zjorti at lanl.gov> wrote:

> Hi Matt,
>
>
> Thanks for your quick reply.
>
> I have not completely understood your suggestion, could you please
> elaborate a bit more?
>
> For your convenience, here is how I am proceeding for the moment in my
> code:
>
>
> TSGetKSP(ts,&ksp);
>
> KSPGetPC(ksp,&pc);
>
> PCSetType(pc,PCFIELDSPLIT);
>
> PCFieldSplitSetDetectSaddlePoint(pc,PETSC_TRUE);
>
> PCSetUp(pc);
>
> PCFieldSplitGetSubKSP(pc, &n, &subksp);
>
> KSPGetPC(subksp[1], &(subpc[1]));
>
I do not like the two lines above. We should not have to do this.

> KSPSetOperators(subksp[1],T,T);
>
 In the above line, I want you to use a separate preconditioning matrix M,
instead of T. That way, it will provide
the preconditioning matrix for your Schur complement problem.

  Thanks,

      Matt

> KSPSetUp(subksp[1]);
>
> PetscFree(subksp);
>
> TSSolve(ts,X);
>
>
> Thank you.
>
> Best,
>
>
> Zakariae
> ------------------------------
> *From:* Matthew Knepley <knepley at gmail.com>
> *Sent:* Wednesday, July 7, 2021 12:11:10 PM
> *To:* Jorti, Zakariae
> *Cc:* petsc-users at mcs.anl.gov; Tang, Qi; Tang, Xianzhu
> *Subject:* [EXTERNAL] Re: [petsc-users] Problem with PCFIELDSPLIT
>
> On Wed, Jul 7, 2021 at 1:51 PM Jorti, Zakariae via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
>> Hi,
>>
>>
>> I am trying to build a PCFIELDSPLIT preconditioner for a matrix
>>
>> J =  [A00  A01]
>>
>>        [A10  A11]
>>
>> that has the following shape:
>>
>>
>> M_{user}^{-1} = [I   -ksp(A00) A01] [ksp(A00)           0] [I
>>            0]
>>
>>                           [0                        I]  [0
>> ksp(T)] [-A10 ksp(A00)  I ]
>>
>>
>> where T is a user-defined Schur complement approximation that replaces
>> the true Schur complement S:= A11 - A10 ksp(A00) A01.
>>
>>
>> I am trying to do something similar to this example (lines 41--45 and
>> 116--121):
>> https://www.mcs.anl.gov/petsc/petsc-current/src/snes/tutorials/ex70.c.html
>>
>>
>> The problem I have is that I manage to replace S with T on a
>> separate single linear system but not for the linear systems generated by
>> my time-dependent PDE. Even if I set the preconditioner M_{user}^{-1}
>> correctly, the T matrix gets replaced by S in the preconditioner once I
>> call TSSolve.
>>
>> Do you have any suggestions how to fix this knowing that the matrix J
>> does not change over time?
>>
>> I don't like how it is done in that example for this very reason.
>
> When I want to use a custom preconditioning matrix for the Schur
> complement, I always give a preconditioning matrix M to the outer solve.
> Then PCFIELDSPLIT automatically pulls the correct block from M, (1,1) for
> the Schur complement, for that preconditioning matrix without
> extra code. Can you do this?
>
>   Thanks,
>
>     Matt
>
>> Many thanks.
>>
>>
>> Best regards,
>>
>>
>> Zakariae
>>
>>
>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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