> > > The most common cause of this is inaccurate eigenvalue bounds. You can > try using more iterations for estimation. > > I have also found that CG converges much faster than GMRES (default) so if you are SPD, I would always use: '-gamg_est_ksp_type cg' . Also, '-pc_gamg_verbose 2' will print out the eigenvalues that GAMG computes for smoothing. With -ksp_view you will see the eigenvalues that Cheby is using (GAMG sets these so they should be the same if you use GAMG with pc_gamg_nsmooths>0). The largest eigenvalue should be around 2-3. If it is too low (<2) then the instability that Jed is referring to would be the problem.
You can also use more iterations, and see if it has converged, with -gamg_est_ksp_max_it N. I think the default is 5.
