[petsc-users] Question about PCFieldSplit

Tang, Qi tangqi at msu.edu
Tue Apr 20 01:29:37 CDT 2021

We were able to get field split working in DMStag with the dummy solver approach I described. Let us know when you add the capability and we will be happy to test it. Meanwhile, please take your time to merge it.


On Apr 18, 2021, at 11:51 PM, Tang, Qi <tangqi at msu.edu<mailto:tangqi at msu.edu>> wrote:

Thanks a lot, Patrick. We appreciate your help.


On Apr 18, 2021, at 11:30 PM, Patrick Sanan <patrick.sanan at gmail.com<mailto:patrick.sanan at gmail.com>> wrote:

We have this functionality in a branch, which I'm working on cleaning up to get to master. It doesn't use PETScSection. Sorry about the delay!

You can only use PCFieldSplitSetDetectSaddlePoint when your diagonal entries being zero or non-zero defines the splits correctly.

Am 17.04.2021 um 21:09 schrieb Matthew Knepley <knepley at gmail.com<mailto:knepley at gmail.com>>:

On Fri, Apr 16, 2021 at 8:39 PM Jorti, Zakariae via petsc-users <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>> wrote:


I have a DMStag grid with one dof on each edge and face center.

I want to use a PCFieldSplit preconditioner on a Jacobian matrix that I assume is already split but I am not sure how to determine the fields.

In the DMStag examples (ex2.c and ex3.c), the function PCFieldSplitSetDetectSaddlePoint is used to determine those fields based on zero diagonal entries. In my case, I have a Jacobian matrix that does not have zero diagonal entries.

Can I use that PCFieldSplitSetDetectSaddlePoint in this case?

If not, how should I do?

Should I do like this example (https://www.mcs.anl.gov/petsc/petsc-master/src/ksp/ksp/tutorials/ex43.c.html<https://urldefense.com/v3/__https://www.mcs.anl.gov/petsc/petsc-master/src/ksp/ksp/tutorials/ex43.c.html__;!!HXCxUKc!jbBwV2h9luOW4dtBcNh6n_W1ULQnSVeXpxl0Ef1752s4Hlef-nC2JcnksFSO3Q$>):

const PetscInt Bfields[1] = {0},Efields[1] = {1};



PCFieldSplitSetFields(pc,"B",1,Bfields,Bfields); PCFieldSplitSetFields(pc,"E",1,Efields,Efields);

where my B unknowns are defined on face centers and E unknowns are defined on edge centers?

That will not work.That interface only works for colocated fields that you get from DMDA.

Patrick, does DMSTAG use PetscSection? Then the field split would be automatically calculated. If not, does it maintain the
field division so that it could be given to PCFIELDSPLIT as ISes?



One last thing, I do not know which field comes first. Is it the one defined for face dofs or edge dofs.

Thank you.

Best regards,


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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