[petsc-users] [EXTERNAL] Re: Question about PCFieldSplit

Mon Jan 31 10:01:02 CST 2022

On Mon, Jan 31, 2022 at 10:48 AM Jorti, Zakariae via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Yes, exactly.
>
If you already have the FieldSplit for the 2 dofs on vertices, you can
easily recursively split this into 1 dof per block, since
those are laid out like a colocated field. You just give another
PCFIELDSPLIT of size 2 and the split will be automatic.

  Thanks,

     Matt

> ------------------------------
> *From:* Patrick Sanan <patrick.sanan at gmail.com>
> *Sent:* Monday, January 31, 2022 8:47:23 AM
> *To:* Jorti, Zakariae
> *Cc:* petsc-users at mcs.anl.gov; Tang, Xianzhu; Tang, Qi
> *Subject:* Re: [EXTERNAL] Re: [petsc-users] Question about PCFieldSplit
>
> The current behavior is that a single IS is returned for each stratum, so
> if you have 2 unknowns on vertices, it should still return a single IS
> including both of those unknowns per vertex. Am I understanding that that's
> working as expected but you need *two* ISs in that case?
>
>
> Am Mo., 31. Jan. 2022 um 16:29 Uhr schrieb Jorti, Zakariae <
> zjorti at lanl.gov>:
>
>> Hi Patrick,
>>
>>
>> Thanks for your recent updates on DMStag.
>> After getting the Finite Difference Coloring to work with our solver, I
>> was testing that DMCreateFieldDecomposition routine that you added last
>> week.
>> It seems to work fine when there is only one unknown per location (i.e.
>> one unknown on vertices and/or one unknown on edges and/or one unknown
>> on faces and/or one unknown on elements).
>> That being said, when there is more than one unknown in some location
>> (let's say 2 unknowns on vertices for instance), I could not get the ISs
>> for those two unknowns with that routine.
>> Should I still rely on PCFieldSplitSetDetectSaddlePoint in this case?
>> Many thanks once again.
>>
>>
>> Kind regards,
>>
>>
>> Zakariae
>> ------------------------------
>> *From:* Patrick Sanan <patrick.sanan at gmail.com>
>> *Sent:* Tuesday, January 25, 2022 5:36:17 AM
>> *To:* Matthew Knepley
>> *Cc:* Jorti, Zakariae; petsc-users at mcs.anl.gov; Tang, Xianzhu; Dave May
>> *Subject:* [EXTERNAL] Re: [petsc-users] Question about PCFieldSplit
>>
>> Here is an MR which intends to introduce some logic to support
>> DMCreateFieldDecomposition(). It doesn't use the PetscSection approach,
>> which might be preferable, but nonetheless is a necessary component so It'd
>> like to get it in, even if it has room for further optimization. Hopefully
>> this can be followed fairly soon with some more examples and tests using
>> PCFieldSplit itself.
>>
>> https://gitlab.com/petsc/petsc/-/merge_requests/4740
>>
>> Am Mi., 23. Juni 2021 um 12:15 Uhr schrieb Matthew Knepley <
>> knepley at gmail.com>:
>>
>>> On Wed, Jun 23, 2021 at 12:51 AM Patrick Sanan <patrick.sanan at gmail.com>
>>> wrote:
>>>
>>>> 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.
>>>>
>>>
>>> Note that an easy way to keep track of this would be to create a section
>>> with the different locations as fields. This Section could then
>>> easily create the ISes, and could automatically interface with
>>> PCFIELDSPLIT.
>>>
>>>   Thanks,
>>>
>>>      Matt
>>>
>>>
>>>>
>>>> [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>
>>>> 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>:
>>>>
>>>> On Fri, Apr 16, 2021 at 8:39 PM Jorti, Zakariae via petsc-users <
>>>> 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$>
>>>>> ):
>>>>> const PetscInt Bfields[1] = {0},Efields[1] = {1};
>>>>> KSPGetPC(ksp,&pc);
>>>>> PCFieldSplitSetBlockSize(pc,2);
>>>>> PCFieldSplitSetFields(pc,"B",1,Bfields,Bfields);
>>>>> PCFieldSplitSetFields(pc,"E",1,Efields,Efields);
>>>>> where my B unknowns are defined on face centers and E unknowns are
>>>>> defined on edge centers?
>>>>>
>>>> That will not work.That interface only works for colocated fields that
>>>> you get from DMDA.
>>>>
>>>> Patrick, does DMSTAG use PetscSection? Then the field split would be
>>>> automatically calculated. If not, does it maintain the
>>>> field division so that it could be given to PCFIELDSPLIT as ISes?
>>>>
>>>>   Thanks,
>>>>
>>>>      Matt
>>>>
>>>>> One last thing, I do not know which field comes first. Is it the one
>>>>> defined for face dofs or edge dofs.
>>>>>
>>>>> Thank you.
>>>>> Best regards,
>>>>>
>>>>> Zakariae
>>>>>
>>>>>
>>>>
>>>> --
>>>> What most experimenters take for granted before they begin their
>>>> experiments is infinitely more interesting than any results to which their
>>>> experiments lead.
>>>> -- Norbert Wiener
>>>>
>>>> https://www.cse.buffalo.edu/~knepley/
>>>> <https://urldefense.com/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!HXCxUKc!jbBwV2h9luOW4dtBcNh6n_W1ULQnSVeXpxl0Ef1752s4Hlef-nC2JcmGgSwfag$>
>>>>
>>>>
>>>>
>>>
>>> --
>>> 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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