[petsc-users] MatGetDiagonalBlock and shell matrices

[Barry Smith]

   I have added support for this operation in the branch barry/add-matshell-matgetdiagonalblock-fortran

   Barry

Because this required changing the handling of the method MatGetDiagonalBlock() the addition cannot be back ported to PETSc 3.7.x 

> On Aug 26, 2016, at 8:22 AM, Dave May <dave.mayhem23 at gmail.com> wrote:
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> On Friday, 26 August 2016, Steven Dargaville <dargaville.steven at gmail.com> wrote:
> 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?
> 
>  I don't know. I'll have appeal to the other petsc users for an answer to these questions.
> 
> Thanks,
>   Dave
>  
> Thanks
> Steven
> 
> On Fri, Aug 26, 2016 at 1:35 PM, Dave May <dave.mayhem23 at gmail.com> wrote:
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> On 26 August 2016 at 14:34, Dave May <dave.mayhem23 at gmail.com> wrote:
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> 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
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>  
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> Thanks!
> Steven
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