[petsc-users] Question about MatGetSubMatrix

[Patrick Sanan]

Hi Qi -

I just opened a PR to make DMStagStencilToIndexLocal() public
https://gitlab.com/petsc/petsc/-/merge_requests/4180

(Sorry for my inattention - I think I may have missed some communications in processing the flood of PETSc emails too quickly - I still plan to get some more automatic DMStag fieldsplit capabilities into main, if it's not too late).

> Am 20.07.2021 um 02:47 schrieb Tang, Qi <tangqi at msu.edu>:
> 
> Hi,
> As a part of implementing this process by ourself, we would like to eliminate boundary dofs. By reading DMStag code, we guess we can use
> DMStagStencilToIndexLocal
> MatZeroRowsLocal
> 
> We note that DMStagStencilToIndexLocal is not explicitly defined in the header file. Is this function ready to use? And will we be able to eliminate the dofs using the above functions?
> 
> Thanks,
> Qi
> 
> 
> 
>> On Jul 16, 2021, at 8:17 PM, Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>> wrote:
>> 
>> 
>>   Zakariae,
>> 
>>     MatGetSubMatrix() was removed a long time ago, the routine is now MatCreateSubMatrix() but it does not work in way you had hoped. There is currently no mechanism to move values you put into the sub matrix back into the original larger matrix (though perhaps there should be?). 
>> 
>>     Please look at MatCreateSubMatrixVirtual() and also MatCreateNest() to see if either of those approaches satisfy your needs. 
>> 
>>     Please let us know if there are extensions that would be useful for you to accomplish what you need. 
>> 
>>   Barry
>> 
>> 
>>> On Jul 16, 2021, at 7:45 PM, Jorti, Zakariae via petsc-users <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>> wrote:
>>> 
>>> Hello,
>>> 
>>> I have a matrix A = [A00 , A01 ; A10, A11]. 
>>> I extract the submatrix A11 with MatGetSubMatrix.
>>> I only know the global IS is1 and is2, so to get A11 I call: 
>>> MatGetSubMatrix(A,is2,is2,MAT_INITIAL_MATRIX,&A11);
>>> I want to modify A11 and update the changes on the global matrix A but I could not find any MatRestoreSubMatrix routine.
>>> Is there something similar to VecGetSubVector and VecRestoreSubVector for matrices that uses only global indices?
>>> Many thanks.
>>> Best regards,
>>> 
>>> Zakariae 
>> 
> 

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