[petsc-users] Block Jacobi for Matrix-free

[Timothée Nicolas]

Ok, so it should be sufficient. Great, I think I can do it.

Best

Timothée

2016-01-07 23:06 GMT+09:00 Matthew Knepley <knepley at gmail.com>:

> On Thu, Jan 7, 2016 at 7:49 AM, Timothée Nicolas <
> timothee.nicolas at gmail.com> wrote:
>
>> Hello everyone,
>>
>> I have discovered that I need to use Block Jacobi, rather than Jacobi, as
>> a preconditioner/smoother. The linear problem I am solving at this stage
>> lives in a subspace with 3 degrees of freedom, which represent the 3
>> components of a 3D vector. In particular for multigrid, using BJACOBI
>> instead of JACOBI as a smoother changes everything in terms of efficiency.
>> I know it because I have tested with the actual matrix in matrix format for
>> my problem. However, eventually, I want to be matrix free.
>>
>> My question is, what are the operations I need to provide for the
>> matrix-free approach to accept BJACOBI ? I am confused because when I try
>> to apply BJACOBI to my matrix-free operator; the code asks for
>> MatGetDiagonalBlock (see error below). But MatGetDiagonalBlock, in my
>> understanding, returns a uniprocessor matrix representing the diagonal part
>> of the matrix on this processor (as defined in the manual). Instead, I
>> would expect that what is needed is a routine which returns a 3x3 matrix at
>> the grid point (that is, the block associated with this grid point,
>> coupling the 3 components of the vector together). How does this work ? Do
>> I simply need to code MatGetDiagonalBlock ?
>>
>
> Just like Jacobi does not request one diagonal element at a time,
> Block-Jacobi does not request one diagonal block at a time. You
> would need to implement that function, or write a custom block Jacobi for
> this matrix.
>
>   Thanks,
>
>     Matt
>
>
>>
>> Thx
>> Best
>>
>> Timothée
>>
>> [0]PETSC ERROR: --------------------- Error Message
>> --------------------------------------------------------------
>> [0]PETSC ERROR: No support for this operation for this object type
>> [0]PETSC ERROR: Matrix type shell does not support getting diagonal block
>> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html
>> for trouble shooting.
>> [0]PETSC ERROR: Petsc Release Version 3.6.1, Jul, 22, 2015
>> [0]PETSC ERROR: ./miips on a arch-linux2-c-debug named Carl-9000 by
>> timothee Thu Jan  7 22:41:13 2016
>> [0]PETSC ERROR: Configure options --with-cc=gcc --with-cxx=g++
>> --with-fc=gfortran --download-fblaslapack --download-mpich
>> [0]PETSC ERROR: #1 MatGetDiagonalBlock() line 166 in
>> /home/timothee/Documents/petsc-3.6.1/src/mat/interface/matrix.c
>> [0]PETSC ERROR: #2 PCSetUp_BJacobi() line 126 in
>> /home/timothee/Documents/petsc-3.6.1/src/ksp/pc/impls/bjacobi/bjacobi.c
>> [0]PETSC ERROR: #3 PCSetUp() line 982 in
>> /home/timothee/Documents/petsc-3.6.1/src/ksp/pc/interface/precon.c
>> [0]PETSC ERROR: #4 KSPSetUp() line 332 in
>> /home/timothee/Documents/petsc-3.6.1/src/ksp/ksp/interface/itfunc.c
>> [0]PETSC ERROR: #5 KSPSolve() line 546 in
>> /home/timothee/Documents/petsc-3.6.1/src/ksp/ksp/interface/itfunc.c
>>
>
>
>
> --
> 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
>
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