[petsc-users] multigrid questions

[Matt Landreman]

Hi, Petsc developers,

A few basic questions about geometric and algebraic multigrid in petsc:

Based on the output of -ksp_view, I see that for both -pc_type mg and
-pc_type gamg, the smoothing on each multigrid level is implemented using a
ksp with default type chebyshev, associated with a pc of default type sor.
I also see in section 4.4.5 of the petsc manual that switching to
-mg_levels_ksp_type richardson is sometimes recommended, and I see in the
ksp examples makefile that ksp ex28.c uses -mg_levels_ksp_type gmres.

1. Does “textbook” Gauss-Seidel smoothing correspond in petsc to
 -mg_levels_ksp_type richardson?

2. It says in
http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPCHEBYSHEV.html
that PETSc’s chebyshev iteration only works for symmetric matrices. So to
confirm, for any problem with non-symmetric matrices such as
convection-diffusion, setting -mg_levels_ksp_type to some non-default value
like richardson or gmres is *mandatory* then?  (If so, it would be helpful
if this were stated in the manual.)

3. For smoothing with geometric multigrid, is there a way in petsc to do
red-black Gauss Seidel? Line relaxation?

Thanks,
Matt Landreman
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20170215/86f36d08/attachment.html>