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

[Matthew Knepley]

On Fri, 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.
>

Then I think it is not doing what you suppose. I am not inclined to believe
that it behaves differently on the
same matrix.

   Matt


> Regards,
> Denis.
>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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