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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120615/6c4ef398/attachment-0001.html>
