[petsc-users] CG+GAMG convergence issues in GHEP Krylov-Schur for some MPI runs

[Denis Davydov]

> On 6 Nov 2015, at 16:22, Matthew Knepley <knepley at gmail.com> 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.