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

[Jorti, Zakariae]

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]));

KSPSetOperators(subksp[1],T,T);

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<mailto: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/>
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