[petsc-users] Block Jacobi for Matrix-free

Timothée Nicolas timothee.nicolas at gmail.com
Thu Jan 7 07:49:47 CST 2016

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 ?



[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
[0]PETSC ERROR: #2 PCSetUp_BJacobi() line 126 in
[0]PETSC ERROR: #3 PCSetUp() line 982 in
[0]PETSC ERROR: #4 KSPSetUp() line 332 in
[0]PETSC ERROR: #5 KSPSolve() line 546 in
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