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

[Matthew Knepley]

You can see the block size in the output

                  Mat Object:                   1 MPI processes
                    type: seqaij
                    rows=12, cols=12, bs=6
                    total: nonzeros=144, allocated nonzeros=144
                    total number of mallocs used during MatSetValues calls
=0
                      using I-node routines: found 3 nodes, limit used is 5
              linear system matrix = precond matrix:
              Mat Object:               1 MPI processes
                type: seqaij
                rows=12, cols=12, bs=6
                total: nonzeros=144, allocated nonzeros=144
                total number of mallocs used during MatSetValues calls =0
                  using I-node routines: found 3 nodes, limit used is 5

  Thanks,

    Matt

On Thu, Mar 3, 2016 at 10:28 AM, Sander Arens <Sander.Arens at ugent.be> wrote:

> Sure, here it is.
>
> Thanks,
> Sander
>
> On 3 March 2016 at 15:33, Matthew Knepley <knepley at gmail.com> wrote:
>
>> 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
>>
>
>


-- 
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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