[petsc-users] Question about MatCreateSubMatrix

[Manav Bhatia]

Got it! 

Thanks,
Manav

> On Mar 29, 2018, at 4:13 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> Manav Bhatia <bhatiamanav at gmail.com> writes:
> 
>> Thanks, Matt. 
>> 
>> I also read page 72 in the manual about creating and assembling matrices. 
>> 
>> Just so that I understand correctly, MatGetLocalSubMatrix is a generic call to create submatrices in the split-local or split-global space? So, it is primarily a matter of the isrow and iscol? 
> 
> Yes.
> 
>> The manual says that this provides a “view” matrix, so any updates to this submatrix should show up in the global? 
>> 
>> How is MatCreateSubMatrixVirtual different? Is this different from a “view” matrix? 
> 
> That is also a view, but specified in terms of the global space rather
> than the local space.  It's normally used by solvers and other places
> where you need the action of the submatrix while the local submatrix is
> generally used for assembly.
> 
>> 
>>> On Mar 29, 2018, at 3:10 PM, Matthew Knepley <knepley at gmail.com> wrote:
>>> 
>>> On Thu, Mar 29, 2018 at 3:28 PM, Manav Bhatia <bhatiamanav at gmail.com <mailto:bhatiamanav at gmail.com>> wrote:
>>> Hi,
>>> 
>>>  I have quick questions about MatCreateSubMatrix:
>>> 
>>> — Once the sub matrix is created, do I need to keep the IS (isrow and iscol) in memory till this matrix is active, or can I destroy it immediately after the creation of the matrix and continue to use the matrix?
>>> 
>>> You can destroy it.
>>> 
>>> — Is the assembly begin/end supposed to be called on the submatrix or the original full matrix? Is it a problem is I call this on both?
>>> 
>>> The SubMatrix is not a view. Its a fully independent matrix.
>>> 
>>> — If I modify values in the sub matrix, would these be reflected in the original matrix after assembly begin/end, or do I need to do something else for that to happen?
>>> 
>>> No. Do you want something like this
>>> 
>>>  http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatGetLocalSubMatrix.html <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatGetLocalSubMatrix.html>
>>> 
>>>  Thanks,
>>> 
>>>    Matt
>>> 
>>> Thanks,
>>> Manav
>>> 
>>> 
>>> 
>>> 
>>> 
>>> -- 
>>> 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.caam.rice.edu/~mk51/>