If it is a true mass matrix in the finite element sense of the word then it should be very well conditioned and one definitely would not use something like GAMG on. Jacobi + CG or maybe SSOR + CG should converge rapidly
Barry > On Nov 6, 2015, at 9:29 AM, Denis Davydov <[email protected]> wrote: > > >> On 6 Nov 2015, at 16:22, Matthew Knepley <[email protected]> wrote: >> >> Is it possible that the matrix is rank deficient? Jacobi will just chug >> along and sometimes work, but >> AMG will fail spectacularly in that case. > > It should not. It is just a mass (overlap) matrix coming from linear FEs with > zero Dirichlet BC assembled in deal.II. > Due to elimination of some algebraic constraints on DoFs there are lines with > only diagonal element, > but it should still be SPD. > > More interestingly is that it does not fail immediately (i.e. the first time > it’s used in SLEPc solvers), > but only on the 4th step. So 3 times SLEPc worked just fine to solve GHEP > with Gamg and zero shift. > > Regards, > Denis. > >
