[petsc-users] MatGetDiagonalBlock and shell matrices

[Steven Dargaville]

Hi Dave

Thanks for the response. I'm actually using fortran and I wasn't sure that
PetscObjectComposeFunction would be accessible, and if so, what sort of
fortran magic I might need to call this function (possibly an interface,
with or without c_opt).

Do you know if it is possible to call that routine directly from fortran?

Thanks
Steven

On Fri, Aug 26, 2016 at 1:35 PM, Dave May <dave.mayhem23 at gmail.com> wrote:

>
>
> On 26 August 2016 at 14:34, Dave May <dave.mayhem23 at gmail.com> wrote:
>
>>
>>
>> On 26 August 2016 at 14:14, Steven Dargaville <
>> dargaville.steven at gmail.com> wrote:
>>
>>> Hi all
>>>
>>> I'm just wondering if there is any plans in the future for
>>> MatGetDiagonalBlock to support shell matrices by registering a
>>> user-implemented MATOP? MatGetDiagonal supports MATOP, but the block
>>> version of this does not.
>>>
>>> I found a previous query on the user list which touched on this and
>>> mentioned that it would be easy to add:
>>>
>>> http://lists.mcs.anl.gov/pipermail/petsc-users/2011-May/008700.html
>>>
>>> I have implemented a matrix-free multigrid algorithm using shell
>>> operations in PETSc, and it would be very convenient to be able to provide
>>> a local shell Mat so I could apply local GMRES (or other matvec-based
>>> solvers) as a local block smoother.
>>>
>>
>> It looks like the thing you need to do is use
>> PetscObjectComposeFunction() and not MatShellSetOperation()
>>
>> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/
>> Sys/PetscObjectComposeFunction.html#PetscObjectComposeFunction
>>
>> with your MatShell object.
>>
>> That is, instead of calling MatShellSetOperation(), call
>> ierr = PetscObjectComposeFunction(myshell,"MatGetDiagonalBlock_C",
>> MatGetDiagonalBlock_MyShell);CHKERRQ(ierr);
>>
>
> Oops - I forgot the cast, the above should be
>
> ierr = PetscObjectComposeFunction((PetscObject)myshell,"MatGetDia
> gonalBlock_C", MatGetDiagonalBlock_MyShell);CHKERRQ(ierr);
>
>
>>
>> where MatGetDiagonalBlock_MyShell is a function pointer to your method to
>> get the diagonal block.
>>
>> Important detail is that you don't change the string
>> "MatGetDiagonalBlock_C". The method MatGetDiagonalBlock() does a function
>> pointer query using this string. See
>> http://www.mcs.anl.gov/petsc/petsc-current/src/mat/interface
>> /matrix.c.html#MatGetDiagonalBlock
>>
>>
>> Thanks
>> Dave
>>
>>
>>
>>
>>
>>
>>>
>>> Thanks!
>>> Steven
>>>
>>>
>>>
>>
>
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