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

[Matthew Knepley]

On Thu, Mar 3, 2016 at 8:05 AM, Sander Arens <Sander.Arens at ugent.be> wrote:

> 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.
>

Hmm, then something is wrong because the block size should be printed for
the matrix at the end of the solver in -snes_view, Can you
send that to me?

  Thanks,

     Matt


> 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
>>
>
>


-- 
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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