Exact numbers of iterations are sensitive to details of computations, so you need to check
a) that the solvers are exactly the same using -ksp_view b) the convergence history, which can be different in parallel due to non-commutativity of floating point arithmetic Matt On Thu, Sep 23, 2010 at 7:51 AM, 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/a86a3fa4/attachment.htm>
