[petsc-users] gamg and zero pivots

[Gideon Simpson]

It gives zero pivots on:

-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
>> 
>