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

[Sander Arens]

Yes, the matrix is created by one of the assembly functions in Plex, so
that answers my question. I was confused by this because I couldn't see
this information when using -snes_view.

Thanks for the helpful replies,
Sander

On 3 March 2016 at 14:52, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Mar 3, 2016 at 7:49 AM, Sander Arens <Sander.Arens at ugent.be>
> wrote:
>
>> 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.
>>
>
> Maybe I am missing something here. The matrix from which you calculate the
> preconditioner using GAMG must be created somewhere. Why
> is the block size not specified at creation time? If the matrix is created
> by one of the assembly functions in Plex, and the PetscSection has
> a number of field components, then the block size will already be set.
>
>   Thanks,
>
>      Matt
>
>
>> 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
>>>
>>
>>
>
>
> --
> 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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