[petsc-users] help for beginner :-)

[Jed Brown]

On Sat, Nov 5, 2011 at 12:28, robert <robert.bodner at unil.ch> wrote:

> question 1:
> -----------
> How can I get an output of the reason value to the console via a command
> line option - in the manual only the petsc-function/method for it is
> given???
>

-ksp_converged_reason


>
> question 2:
> ----------
> I have tried the option -ksp_monitor_true_residual but I don't know how
> to interpret the output.
>

 [...]


>
>
Did I get it right that the resid norm and true resid norm should be
> more or less the same value?
>

The preconditioned residual is after preconditioning where as the true
residual does not include the preconditioner. Libmesh examples enforce
Dirichlet boundary conditions using penalties which makes the true residual
very poorly scaled, in which case the preconditioned residual is preferable
for convergence tests (so long as the preconditioner is non-singular, but
that should not be a problem for diffusion).


> ||Ae||/||Ax||: Ax is clear, but what does the e in Ae stand for? How do
> I interpret this value? Is this a comparison beteen the 'real' solution
> and the calculated solution. I recognized that the iterations stopped
> when this value got smaller than 1e-12.
>

You should update to petsc-3.2. The string is more clear
there: ||r(i)||/||b|| where r(i)=Ax_i-b is the residual.

question 3:
> -----------
> I am sure that there are many people out there who have a lot of
> experience with my kind of systems - I mean diffusional heat tranfer is
> a standard problem in many different fields.
> Which solution algorithm (solver, preconditioner) would you suggest for
> solving in parallel? Which ones do I have to avoid (e.g. not stable)???
>

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.
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