[petsc-users] Question about PCFieldSplit

[Patrick Sanan]

Hi Zakariae -

The usual way to do this is to define an IS (index set) with the degrees of freedom of interest for the rows, and another one for the columns, and then use MatCreateSubmatrix [1] .

There's not a particularly convenient way to create an IS with the degrees of freedom corresponding to a particular "stratum" (i.e. elements, faces, edges, or vertices) of a DMStag, but fortunately I believe we have some code to do exactly this in a development branch. 

I'll track it down and see if it can quickly be added to the main branch.


[1]: https://petsc.org/release/docs/manualpages/Mat/MatCreateSubMatrix.html

> Am 22.06.2021 um 22:29 schrieb Jorti, Zakariae <zjorti at lanl.gov>:
> 
> Hello,
> 
> I am working on DMStag and I have one dof on vertices (let us call it V), one dof on edges (let us call it E), one dof on faces ((let us call it F)) and one dof on cells (let us call it C).
> I build a matrix on this DM, and I was wondering if there was a way to get blocks (or sub matrices) of this matrix corresponding to specific degrees of freedom, for example rows corresponding to V dofs and columns corresponding to E dofs.
> I already asked this question before and the answer I got was I could call PCFieldSplitSetDetectSaddlePoint with the diagonal entries being of the matrix being zero or nonzero. 
> That worked well. Nonetheless, I am curious to know if there was another alternative that does not require creating a dummy matrix with appropriate diagonal entries and solving a dummy linear system with this matrix to define the splits. 
> 
> Many thanks.
> 
> Best regards,
> 
> Zakariae
> From: petsc-users <petsc-users-bounces at mcs.anl.gov> on behalf of Tang, Qi <tangqi at msu.edu>
> Sent: Sunday, April 18, 2021 11:51:59 PM
> To: Patrick Sanan
> Cc: petsc-users at mcs.anl.gov; Tang, Xianzhu
> Subject: [EXTERNAL] Re: [petsc-users] Question about PCFieldSplit
>  
> Thanks a lot, Patrick. We appreciate your help.
> 
> Qi
> 
> 
> 
>> 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:
>>> Hello,
>>> 
>>> 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$>): 
>>> const PetscInt Bfields[1] = {0},Efields[1] = {1};
>>> KSPGetPC(ksp,&pc);
>>> PCFieldSplitSetBlockSize(pc,2);
>>> PCFieldSplitSetFields(pc,"B",1,Bfields,Bfields); PCFieldSplitSetFields(pc,"E",1,Efields,Efields); 
>>> where my B unknowns are defined on face centers and E unknowns are defined on edge centers?
>>> That will not work.That interface only works for colocated fields that you get from DMDA.
>>> 
>>> Patrick, does DMSTAG use PetscSection? Then the field split would be automatically calculated. If not, does it maintain the
>>> field division so that it could be given to PCFIELDSPLIT as ISes?
>>> 
>>>   Thanks,
>>> 
>>>      Matt
>>> One last thing, I do not know which field comes first. Is it the one defined for face dofs or edge dofs.
>>> 
>>> Thank you.
>>> Best regards,
>>> 
>>> Zakariae
>>> 
>>> 
>>> 
>>> -- 
>>> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
>>> -- Norbert Wiener
>>> 
>>> https://www.cse.buffalo.edu/~knepley/ <https://urldefense.com/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!HXCxUKc!jbBwV2h9luOW4dtBcNh6n_W1ULQnSVeXpxl0Ef1752s4Hlef-nC2JcmGgSwfag$>
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