[petsc-users] help for beginner :-)

[robert]

>         
> 
> Diffusion is as "nice" as possible in terms of stability and
> well-established theory, multigrid will normally work very well. In
> the latest release of PETSc, you could try -pc_type gamg.
> Otherwise/alternatively, configure using --download-ml or
> --download-hypre, then run with -pc_type ml or -pc_type hypre.

Thanks for your reply. 

Do you have experience with lumping of the mass matrix?

The following options semm to work fine if I don't use lumping: 

mpiexec -np 4 ./ThermoPaine3d-opt -ksp_type fgmres -pc_type ksp
-ksp_ksp_type cg -ksp_pc_type jacobi -ksp_monitor_true_residual
-ksp_converged_reason

(any comments to the options???)

However, for heat diffusion, especially for non-steady starting
conditions, oscillations seem to be a common problem (AFAIK).
If I apply lumping of the mass matrix - which means I put the row-sum of
the mass matrix on the main diagonal - the solver doesn't converge any
more. Before it converged after 2 to 5 iterations, now I was running
several hundreds. 


Maybe some of you - definitely more experienced users - have some
ideas/suggestions?

P.S.: I have tried a lot of different combinations of solvers and
preconditioners (including those of the last reply)..

Thank you, 
Robert