[petsc-users] KSPSetOperators/sub-preconditioner type

[Paul T. Bauman]

On Tue, Jul 10, 2012 at 1:34 PM, Paul T. Bauman <ptbauman at gmail.com> wrote:

> Greetings,
>
> This question is somewhat related to this thread:
> http://lists.mcs.anl.gov/pipermail/petsc-users/2011-August/009583.html but
> instead of resurrecting that thread, I decided to start a new one.
>
> Summary: In the libMesh wrapper of PETSc's KSP, a call is made to
> KSPSetOperators; I gather this is in order to allow reusing the same
> preconditioner, if desired. However, I've noticed that when I use the
> following options, the sub-preconditioner type gets reset:
>
> -pc_type bjacobi -sub_pc_type ilu -sub_pc_factor_mat_solver_package superlu
>
> In particular, the first linear solve uses what I asked for, but in all
> following linear solves, the petsc ilu solver is used instead of superlu.
> If I remove the KSPSetOperators call, everything is peachy.
>
> My question: Does a call need to be made to PCSetFromOptions (or something
> similar...) to keep the superlu ilu solver around or would that wipe out
> the KSPSetOperators effect? Is this a PETSc bug? Other?
>
> This is with petsc-3.3-p1.
>

Sorry to reply to myself, but I read this and realized I was rather
imprecise.

The KSPSetOperators call I referred to is in the solve step. In particular,
KSPSetFromOptions is called during initilization, only once, but
KSPSetOperators is called each time solve is called. I was able to mimic my
problem in the attached, slightly modified ksp/ex5.c, running with:

mpiexec -np 6 ./ex5 -ksp_type gmres -pc_type bjacobi -sub_pc_type ilu
-ksp_view -ksp_monitor -sub_pc_factor_mat_solver_package superlu
-sub_pc_factor_levels 1

Is the correct solution just to call KSPSetFromOptions every time? Will
that be noticeable at all? I hope this was less vague than my original
message. Thanks again.

Best,

Paul
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