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
