On Mar 30, 2012, at 10:52 AM, John Mousel wrote: > Mark, > > I've run GAMG twice with different coarse grid sizes of 2 and 8 with 1 sweep > of SOR on the coarse grid. For a size of 8 it converges nicely, but for a > size of 2, I think the null space is causing too many problems.
YEs, the iterative method is seeing the null space because of floating point error. > If GAMG were to coarsen to a size of 1, then there would be no hope because > only the null space would remain, right? This doesn't ever seem to occur with > ML because there are at least as many rows as processors. Yes that seems like a good assumption. The right thing to do here would probably be to do and SVD and filter out the very low modes explicitly. For now I guess tweaking -pc_gamg_coarse_eq_limit n is al that can be done. Not very satisfying. We will think about this ... any thoughts anyone? Mark > > John > > On Fri, Mar 30, 2012 at 8:42 AM, Mark F. Adams <mark.adams at columbia.edu> > wrote: > > 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> > > > <KSPView_ML.txt><KSPView_GAMG.txt> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120330/2832a82e/attachment.htm>
