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

[Sander Arens]

And how can I do this? Because when I look at all the options with -help I
can strangely enough only find -fieldsplit_pressure_mat_block_size and not
-fieldsplit_displacement_mat_block_size.

Thanks,
Sander

On 3 March 2016 at 14:21, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Mar 3, 2016 at 6:20 AM, Sander Arens <Sander.Arens at ugent.be>
> wrote:
>
>> Ok, I forgot to call SNESSetJacobian(snes, J, P, NULL, NULL) with J != P,
>> which caused to write the mass matrix into the (otherwise zero) (1,1) block
>> of the Jacobian and which was the reason for the linesearch to fail.
>> However, after fixing that and trying to solve it with FieldSplit with LU
>> factorization for the (0,0) block it failed because there were zero pivots
>> for all rows.
>>
>> Anyway, I found out that attaching the mass matrix to the Lagrange
>> multiplier field also worked.
>>
>> Another related question for my elasticity problem: after creating the
>> rigid body modes with DMPlexCreateRigidBody and attaching it to the
>> displacement field, does the matrix block size of the (0,0) block still
>> have to be set for good performance with gamg? If so, how can I do this?
>>
>
> Yes, it should be enough to set the block size of the preconditioner
> matrix.
>
>   Matt
>
>
>> Thanks,
>> Sander
>>
>> On 2 March 2016 at 12:25, Matthew Knepley <knepley at gmail.com> wrote:
>>
>>> 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
>>>
>>
>>
>
>
> --
> 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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