Setting tolerances for multigrid

[Knut Erik Teigen]

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
> >
> >
> 
>