[petsc-dev] Should PETSc default to a different smoother than SOR for SBAIJ matrices with blocksize > 1?

[Richard Mills]

Hi Folks,

Maybe I'm doing something wrong because I'm not that familiar with the
current workings of multigrid in PETSc, but I was playing with Jed's SNES
ex48 tutorial example (hydrostatic ice sheet flow), and was puzzled to find
that trying to run a very simple variation:

./ex48 -da_refine 1 -pc_type mg

results in a failure.  The problem appears to be that PETSc defaults to an
SOR smoother, but symmetric SOR isn't implemented for the SBAIJ matrices
(with block size > 1) that ex48 uses:

[0]PETSC ERROR: No support for this operation for this object type
[0]PETSC ERROR: SSOR for block size > 1 is not yet implemented

Is this the desired behavior, or should PETSc default to a smoother like
Jacobi that is implemented for this matrix type?

--Richard

P.S. Jed, if I just want to take the default problem and scale it up in
size (for some simple parallel performance tests), is using -da_refine
<some number> and a Jacobi smoother for the various levels reasonable?  Or
do I need to do something more sophisticated?  (Sorry if this requires
another thread.)
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