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