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

[Barry Smith]

  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 <davydden at gmail.com> wrote:
> 
> 
>> 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.
> 
>