On Mar 29, 2012, at 2:40 PM, John Mousel wrote: > I'm attempting to solve a non-symmetric discretization of a 3D Poisson > problem. The problem is singular. I've attached the results of KSPView from > runs with ML and GAMG. When I run ML, I get convergence in 30 iterations. > When I attempt to use the same settings with GAMG, I'm not getting > convergence at all. The two things I notice are: > > 1. GAMG is using KSPType preonly, even though I've set it to be Richardson in > my command line options.
PETSc seems to switch the coarse grid solver to GMRES in Setup. This seems to be a bug and I unwisely decide to override this manually. I will undo this in the next checkin. This should not be the problem however. > 2. ML only coarsens down to 4 rows while GAMG coarsens to 2. My problem is > singular, and whenever I try to use LU, I get zero pivot problems. To > mitigate this, I've been using Richardson with SOR on the coarse matrix. > Could the smaller coarse grid size of GAMG be causing problems with SOR. If > so, is there a way to put a lower limit on the coarse grid size? > I'm thinking that with a 2x2 coarse grid 8 iterations of SOR is picking up the null space. Maybe try just one SOR iteration on the coarse grid. Also, can you run with options_left so that I can see your arguments. One known bug is the mat_diagaonal_scale breaks GAMG, but it should also break ML. Mark > John > > On Thu, Mar 29, 2012 at 11:03 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote: > On Thu, Mar 29, 2012 at 09:18, John Mousel <john.mousel at gmail.com> wrote: > [0]PETSC ERROR: Error in external library! > [0]PETSC ERROR: Cannot disable floating point exceptions! > > Looks like something is strange with your environment because fesetenv() is > returning an error. I have disabled the call if the trap mode is not changing. > > http://petsc.cs.iit.edu/petsc/petsc-dev/rev/352b4c19e451 > > <KSPView_GAMG.txt><KSPView_ML.txt> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120330/2c21f6c3/attachment.htm>
