[petsc-users] MatGetDiagonalBlock and shell matrices

[Dave May]

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,"
MatGetDiagonalBlock_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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