On Thu, Sep 23, 2010 at 10:41 AM, Leo van Kampenhout < lvankampenhout at gmail.com> wrote:
> > Thank you both. The solvers are the same, I double checked that. It could > be the case that the type of partitioning plays a role here, since i'm > indeed using a DA. However, why is it that for example a run on 2 processors > the number of iterations is higher than on 8? Both use DA-partitioning in > this case. To specify subdomains manually, where do i start? > It is an open secret that Krylov methods are incredibly sensitive to orderings, especially when combined with incomplete factorization preconditioners. Since the ordering depends on the division (see tutorials for a picture of "petsc" orderings which are just contiguous per process), you can get non-intuitive effects. Matt > Leo > > > 2010/9/23 Jed Brown <jed at 59a2.org> > >> The latter is using the partition provided by the DA (or user) which looks >> to be better than the one computed in the serial run. If you have Parmetis, >> then it will be used by PCBJACOBI, otherwise the partition is naive. You can >> specify subdomains manually if you want. >> >> Jed >> >> On Sep 23, 2010 1:51 PM, "Leo van Kampenhout" <lvankampenhout at gmail.com> >> wrote: >> >> Hi all, >> >> With p number of processors in the communicator, the block preconditioner >> PC_BJACOBI will by default use p blocks. So far, so good. However, in order >> to compare this algorithmic efficiency decrease (since the bigger p, the >> less efficient the preconditioner), i ran the commands >> >> mpirun -n 1 ./program -pc_bjacobi_blocks 8 >> mpirun -n 8 ./program -pc_bjacobi_blocks 8 >> >> I expected the preconditioning to be equally efficient in this case. >> However, GMRES makes more iterations in the first case (30 against 28) which >> I cannot explain. Are there more subtle differences about the preconditioner >> or the KSP that i'm overlooking here? >> >> regards, >> >> Leo >> >> >> >> >> >> > -- 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/20100923/a12721a3/attachment-0001.htm>
