Besides, such diverging behavior occurs only with optimized build of Petsc.
On Wed, Mar 20, 2013 at 3:13 PM, Mehrdad H Farahani <mh.farahani at gmail.com>wrote: > I am solving a Poisson equation with Neumann boundary conditions applied > along an arbitrary curve using a ghost fluid treatment. The matrix is > non-symmetric and the null space contains more than the constant vector. I > have implemented a solve for the null space itself and have previously > supplied it using MatSetNullSpace. Unfortunately the null space solve is > extremely expensive and does not play well with GAMG or ML. We've checked > that the answers we get without supplying this null space are very close to > the answers we get when we do include it. The matrix for the two solves in > the previous attachment are identical. The problem also seems to be > converging and then suddenly diverges. Is this common behavior for a > singular preconditioner? > > Thanks, > > > >> >> From: Jed Brown <jedbrown at mcs.anl.gov> >> Date: Wed, Mar 20, 2013 at 2:03 PM >> Subject: Re: [petsc-users] KSP convergence problem >> To: PETSc users list <petsc-users at mcs.anl.gov> >> >> >> On Wed, Mar 20, 2013 at 2:00 PM, Mark F. Adams <mark.adams at >> columbia.edu>wrote: >> >>> >>> On Mar 20, 2013, at 2:12 PM, Mehrdad H Farahani <mh.farahani at gmail.com> >>> wrote: >>> >>> > I'm experience a weird convergence problem with bcgsl with both ILU >>> and GAMG preconditioning. The problem converges normally (at least in the >>> preconditioned norm), but then kicks out with converged reason = -4. I've >>> attached the convergence history of the previous time-step which worked and >>> the timestep which shows problems. Could someone give me an idea on how to >>> proceed? >>> > >>> >>> Did you run with -ksp_converged_reason ? >>> >>> I don't know were this -4 comes from. Could this be from your code? >>> >> >> Some people call KSPGetConvergedReason(), but then don't compare to the >> enums and don't convert to a string using KSPConvergedReasons[reason]. >> >> >> http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/KSP/KSP_DIVERGED_DTOL.html >> >> The problem is likely that your preconditioner is singular. What sort of >> problem are you solving? >> >> > > > -- > Mehrdad > -- Mehrdad -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130320/a88b5b19/attachment.html>
