[petsc-users] PetscDSSetJacobianPreconditioner causing DIVERGED_LINE_SEARCH for multi-field problem

[Matthew Knepley]

On Wed, Mar 2, 2016 at 5:13 AM, Sander Arens <Sander.Arens at ugent.be> wrote:

> Hi,
>
> I'm trying to set a mass matrix preconditioner for the Schur complement of
> an incompressible finite elasticity problem. I tried using the command
> PetscDSSetJacobianPreconditioner(prob, 1, 1, g0_pre_mass_pp, NULL, NULL,
> NULL) (field 1 is the Lagrange multiplier field).
> However, this causes a DIVERGED_LINE_SEARCH due to to Nan or Inf in the
> function evaluation after Newton iteration 1. (Btw, I'm using the next
> branch).
>
> Is this because I didn't use PetscDSSetJacobianPreconditioner for the
> other blocks (which uses the Jacobian itself for preconditioning)? If so,
> how can I tell Petsc to use the Jacobian for those blocks?
>

1) I put that code in very recently, and do not even have sufficient test,
so it may be buggy

2) If you are using FieldSplit, you can control which blocks come from A
and which come from the preconditioner P


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCFieldSplitSetDiagUseAmat.html#PCFieldSplitSetDiagUseAmat

http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCFieldSplitSetOffDiagUseAmat.html#PCFieldSplitSetOffDiagUseAmat


> I guess when using PetscDSSetJacobianPreconditioner the preconditioner is
> recomputed at every Newton step, so for a constant mass matrix this might
> not be ideal. How can I avoid recomputing this at every Newton iteration?
>

Maybe we need another flag like


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESSetLagPreconditioner.html

or we need to expand


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESSetLagJacobian.html

to separately cover the preconditioner matrix. However, both matrices are
computed by one call so this would
involve interface changes to user code, which we do not like to do. Right
now it seems like a small optimization.
I would want to wait and see whether it would really be maningful.

  Thanks,

    Matt


> Thanks,
> Sander
>



-- 
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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