Matt, We are confident that the modified Schur complement works well based on our subksp and ksp tests. Let me summarize what you suggest us to do regarding to our original question of TSSolve. Instead of calling subksp, you suggest we should provide two matrices J and Jpre through TSSetIJacobian, where J is the original Jacobian and Jpre is the one we modified and both are 2x2 blocks. Then if we call fieldsplit, petsc will automatically use Jpre to construct its Schur complement in the preconditioner stage. Is that what you suggested?
Thanks a lot! Qi On Jul 7, 2021, at 3:54 PM, Matthew Knepley <[email protected]<mailto:[email protected]>> wrote: On Wed, Jul 7, 2021 at 2:33 PM Jorti, Zakariae <[email protected]<mailto:[email protected]>> 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 <[email protected]<mailto:[email protected]>> Sent: Wednesday, July 7, 2021 12:11:10 PM To: Jorti, Zakariae Cc: [email protected]<mailto:[email protected]>; 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 <[email protected]<mailto:[email protected]>> 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<https://urldefense.com/v3/__https://www.mcs.anl.gov/petsc/petsc-current/src/snes/tutorials/ex70.c.html__;!!HXCxUKc!hoEfgnaraTfQoSgAiplsc6GJ_HuPXN88m5AJVy1gb7WVMNkGENDnJ3zToOGlhw$> The problem I have is that I manage to replace S with T on a separate single linear system but not for the linear systems generated by my time-dependent PDE. Even if I set the preconditioner M_{user}^{-1} correctly, the T matrix gets replaced by S in the preconditioner once I call TSSolve. Do you have any suggestions how to fix this knowing that the matrix J does not change over time? I don't like how it is done in that example for this very reason. When I want to use a custom preconditioning matrix for the Schur complement, I always give a preconditioning matrix M to the outer solve. Then PCFIELDSPLIT automatically pulls the correct block from M, (1,1) for the Schur complement, for that preconditioning matrix without extra code. Can you do this? Thanks, Matt Many thanks. Best regards, Zakariae -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener https://www.cse.buffalo.edu/~knepley/<https://urldefense.com/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!HXCxUKc!hoEfgnaraTfQoSgAiplsc6GJ_HuPXN88m5AJVy1gb7WVMNkGENDnJ3wB3dcMFw$> -- 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/<https://urldefense.com/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!HXCxUKc!hoEfgnaraTfQoSgAiplsc6GJ_HuPXN88m5AJVy1gb7WVMNkGENDnJ3wB3dcMFw$>
