Hi Barry, Thank you very much for the explanation. It is good to know there are more options to start with.
Yan On Tue, Sep 15, 2009 at 8:32 PM, Barry Smith <bsmith at mcs.anl.gov> wrote: > > On Sep 15, 2009, at 6:04 PM, Ryan Yan wrote: > > Thanks Jed. >> >> And in the term ||Ae||/||Ax||, PETSc impilictly assume that we use zero >> as initial guess and here the x really means exact solution, right? >> > > If you call KSPSetInitialGuessNonzero() then PETSc will use the initial > value in x as the initial guess. The default convergence tests > use the ratio of || r_current||/ || b ||. Thus if you start with a great > initial guess it may not iterate much at all. You can use also > -ksp_converged_use_initial_residual_norm > or KSPDefaultConvergedSetUIRNorm() to have it use the ratio of || > r_current||/|| r_initial|| for the relative convergence test. > > You can also provide your own convergence test with > KSPSetConvergenceTest(). > > Barry > > > > >> Yan >> >> >> >> >> On Tue, Sep 15, 2009 at 6:41 PM, Jed Brown <jed at 59a2.org> wrote: >> Ryan Yan wrote: >> > But can you say a little more about what happened in the first scenario, >> I >> > did not quite fellow you. Let's say, If I set -ksp_rtol 1e-2, what does >> this >> > mean in the left PC case. More specifically, which term will be >> considered >> > as a stop creteria for the KSP solve. >> >> Look at the first column (preconditioned residual norm). Notice that >> this is decreasing by 1e-2 and 1e-3 respectively in your examples. >> >> Jed >> >> >> > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20090915/05c52cb6/attachment.htm>