[petsc-users] gamg and zero pivots

[Matthew Knepley]

On Sat, Sep 19, 2015 at 6:10 PM, Gideon Simpson <gideon.simpson at gmail.com>
wrote:

> It gives zero pivots on:
>

Send -ksp_view

  Matt


> -ksp_type gmres -pc_type gamg -mg_coarse_sub_pc_factor_shift_type NONZERO
>
> It has no problem with:
>
> -ksp_type gmres -pc_type bjacobi -sub_pc_factor_shift_type NONZERO
>
> -gideon
>
> > On Sep 19, 2015, at 12:37 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >
> >> On Sep 18, 2015, at 10:42 PM, Gideon Simpson <gideon.simpson at gmail.com>
> wrote:
> >>
> >> I’m getting a zero pivot in LU factorization error when I use the gamg
> preconditioner.  I know there is some issue with my matrix, as I get the
> same error if I use pc_type lu in serial, or if I use pc_type bjacobi in
> parallel, both of which are corrected by adding -pc_factor_shift_type
> NONZERO -sub_pc_factor_shift_type NONZERO, respectively.  However, I can’t
> quite seem to figure out the analogous command to use for the gamg
> preconditioner.
> >
> >  You need to figure out the prefix to the option that is added by gamg
> to set it. One way is to run with the gamg options and -help and grep for
> factor_shift.
> >
> >   The option is like something like -mg_coarse_pc_factor_shift_type
> nonzero or -mg_coarse_sub_pc_factor_shift_type
> >
> >  Barry
> >
> >>
> >> -gideon
> >>
> >
>
>


-- 
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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