[petsc-users] ml precondioner

[Matthew Knepley]

On Fri, Jun 15, 2012 at 3:35 AM, Nakib Haider Protik <nprot048 at uottawa.ca>wrote:

> I am running 3.0.0-p12. Upgrading to 3.3 has proven difficult for me.
>

This is 5 years old. You will notice that we have an email list to help
people
with upgrade problems. We have not received any mails about problems.


> In my version I used -mg_levels_ksp_type chebychev and the error is
> greater than both ordinary gmres with lu, and ml with gmres and lu on each
>

The solver has nothing to do with the error. This is a matter for the
iterative
tolerance and condition number of the problem.

   Matt


> level. Also, it takes a very long time to converge for even a 200 x 200
> grid.
>
> Thanks.
>
> > On Thu, Jun 14, 2012 at 1:09 PM, Nakib Haider Protik
> > <nprot048 at uottawa.ca>wrote:
> >
> >> I have a 2 dimensional Poisson problem and the Laplacian is defined on a
> >> mesh that has both uniform and non-uniform regimes. I am using the
> >> following command:
> >>
> >> -pc_type ml -mg_levels_ksp_type gmres -mg_levels_pc_type lu
> >>
> >
> > This does a direct solve on every level, of course it's going to be slow.
> >
> >
> >>
> >> This gives a good result. (However, if I don't specify the
> >> -mg_level_ksp_type and -mg_levels_pc_type to gmres and lu respectively,
> >> the default are used (richardson and sor respectively). These lead to
> >> wrong results and the solver is extremely slow.)
> >>
> >
> > Are you using petsc-3.3?
> >
> > Run with -mg_levels_ksp_type chebyshev (and only that). Does that
> converge
> > better?
> >
> >
> >>
> >> Now, the following method:
> >>
> >> -ksp_type gmres -pc_type lu
> >>
> >> gives a slightly better result and is faster. The speed difference is
> >> quite conspicuous for a 501 x 1001 grid size.
> >>
> >> According to my limited knowledge, asymmetric matrices are better dealt
> >> with by gmres and lu. However, multigrid methods are supposed to have
> >> O(n)
> >> complexity. Why is the non-multigrid method doing better in both speed
> >> and
> >> accuracy?
> >>
> >
>
>
> --
> Nakib :)
>



-- 
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
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