Setting tolerances for multigrid

[Matthew Knepley]

This does not sound right to me. I think something must be
configured incorrectly. All MG does is accelerate the solution
of the fine grid problem. I think something must be wrong with
the specification here. It is easy to check. Run with -dmmg_nlevels 1
which just solves on the fine grid.

  Matt

On 6/15/07, Knut Erik Teigen <knutert at stud.ntnu.no> wrote:
> Thanks, Matthew. It seems like the tolerances weren't my problem,
> though. Even with a tolerance of 1e-10, the computation diverges after
> only a few time steps when using multigrid. With ordinary linear
> solvers, I can use a tolerance of 1e-4 and still get a satisfactory
> solution.
> Could it be that the Galerkin approximation for the coarser levels is
> too inaccurate for my problem?
>
> -Knut Erik-
>
> On Thu, 2007-06-14 at 11:32 -0500, Matthew Knepley wrote:
> > On 6/14/07, Knut Erik Teigen <knutert at stud.ntnu.no> wrote:
> > > Hello,
> > >
> > > How do I set the tolerances for the KSP solvers when using the
> > > DMMG routines without using command line arguments? It seems that
> > > I can use the argument -ksp_rtol on the command line, but in the code I
> > > can't use KSPSetTolerances since I don't have access to the KSP solver
> > > context?
> >
> > You can use DMMGGetKSP() for the finest level, and dmmg[level]->ksp for
> > any others.
> >
> > > Also, I see there is a reference to the function DMMGSetUseGalerkin
> > > in the source code, but I can't find the documentation for it.
> > > Does this function not exist yet? Again I would like to use
> > > -dmmg_galerkin, but setting it in the code instead of on the command
> > > line.
> >
> > I think you want this:
> >
> > http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/DA/DMMGSetUseGalerkinCoarse.html
> >
> >   Matt
> >
> > > Regards,
> > > Knut Erik Teigen
> > >
> > >
> >
> >
>
>


-- 
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