Setting tolerances for multigrid

[Knut Erik Teigen]

The computation runs just fine with -dmmg_nlevels 1.
I use a grid of 65x129, with three levels, so that the coarsest grid is
17x33. 
I've diffed the solutions, and they are equal for the first two time
steps, but then they start to differ somewhat. Here is an excerpt:
-dmmg_nlevels 1:
65.0427492820557        65.0216231814457      64.9793945321386        
-dmmg_nlevels 3:
65.0920864319067        65.0711253401585      65.0292265582817

Using 2 or 4 levels produce the same result as 3 levels.

-Knut Erik-



On Fri, 2007-06-15 at 10:21 +0200, Matthew Knepley wrote:
> 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
> > > >
> > > >
> > >
> > >
> >
> >
> 
>