[petsc-users] multigrid questions

[Matthew Knepley]

On Wed, Feb 15, 2017 at 10:42 AM, Matt Landreman <matt.landreman at gmail.com>
wrote:

> 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?
>

Yes.


> 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.)
>

Yes, this is true.


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

I don't know if Barry has something special, but you could easily do it
using PCFIELDSPLIT with the default splitting and
Jacobi on each block. Line relaxation is similar, with different blocking
and Block-Jacobi.

  Matt


> Thanks,
> Matt Landreman
>



-- 
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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