On Apr 17, 2013, at 4:26 PM, Jozsef Bakosi <jbakosi at lanl.gov> wrote:
> >> Mark F. Adams mark.adams at columbia.edu >> Wed Apr 17 14:25:04 CDT 2013 >> >> 2) If you get "Indefinite PC" (I am guessing from using CG) it is because the >> preconditioner >> really is indefinite (or possible non-symmetric). We improved the checking >> for this in one >> of those releases. >> >> AMG does not guarantee an SPD preconditioner so why persist in trying to use >> CG? >> >> >> AMG is positive if everything is working correctly. >> >> Are these problems only semidefinite? Singular systems can give erratic >> behavior. > > It is a Laplace operator from Galerkin finite elements. And the PC is fine on > ranks 1, 2, 3, and 5 -- indefinite only on 4. I think we can safely say that > the > same PC should be positive on 4 as well. > > Can you guys please CC jbakosi at lanl.gov? Thanks, J > I assume that this is not a Neumann problem ? you can try -pc_type gamg and -pc_type hypre. And PETSc is testing for essentially < 0 which is a numerically finicky thing to do. These solvers are not bitwise identical wrt number of processors so getting different results for this test is not unreasonable.
