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

[Matthew Knepley]

On Fri, Nov 6, 2015 at 9:15 AM, Denis Davydov <davydden at gmail.com> wrote:

> Hi Hong,
>
> > On 6 Nov 2015, at 16:09, Hong <hzhang at mcs.anl.gov> wrote:
> >
> > Denis:
> > Do you use shift-and-invert method for solving eigenvalue problem?
> no, it’s just shift with zero value. So for GHEP one inverts B-matrix.
>
> > If so, the linear problems would be extremely ill-conditioned, for which
> the direct solver, such LU or Cholesky are usually the only working option.
> Depends on the shift, i would say.
> In any case the same problem works with jacobi preconditioner no no other
> changes,
> so i would not relate it to any settings on SLEPc part.


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.

   Matt


>
> > You may run your petsc/slepc code with option '-ksp_monitor' to observe
> convergence behavior.
> Will do, thanks.
>
> 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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