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? > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120614/a8361e78/attachment.html>
