[petsc-users] help for beginner :-)

[robert]

Am Sonntag, den 06.11.2011, 14:28 +0100 schrieb 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


I have to correct my last entry: 

I get the following output:

  0 KSP preconditioned resid norm 9.794488229448e+17 true resid norm
9.794488229448e+17 ||Ae||/||Ax|| 9.424443120337e-03

  1 KSP preconditioned resid norm 0.000000000000e+00 true resid norm
9.794488229448e+17 ||Ae||/||Ax|| 9.424443120337e-03
Linear solve converged due to CONVERGED_ATOL iterations 1


Isn't it strange to have a preconditioned residual norm of exactly 0
(that's why I thought there might be a problem somewhere)???

Thank you, 
Robert