[petsc-users] Fieldsplit PC null pointer error on getksp

Tue Jul 14 11:56:49 CDT 2015

On Tue, Jul 14, 2015 at 11:37 AM, Sander Land <sander.land at gmail.com> wrote:

> Hi Matt,
>
> Doing things to the PC like this has worked for me for a long time. It
> just happened to be formerly KSPPREONLY/PCLU. Setting things like
> GMRES/PCNONE works as well with no errors, though it converges far too
> slowly.
>
> The actual preconditioner I am trying to form is one from recent
> literature and uses a slightly modified Schur complement split embedded in
> another Schur complement split with multiple solves of the inner system and
> very specific choices for sub-preconditioners. I don't think command line
> options will work very well here.
>
> I've since gone the way of just using PCSHELL and setting things up more
> explicitly. This seems to work, but i'm not quite sure how to set up
> boomeramg for one of the sub-problems.
>
> boomeramg is used on the matrix A derived from:
> MatGetSubMatrix(J,is_x ,is_x ,MAT_INITIAL_MATRIX,&ctxp->A);
> The degrees of freedom for J are xxxyyyzzzpw, with the size of xxxyyyzzz
> around 20k, p around 1k, and w 8 for my 'small' test problem.
>
> I can see that HYPRE_BoomerAMGSetNumFunctions takes its value from the
> matrix block size. This is 1 for J as it doesn't have a consistent block
> size. However, the number of functions for boomeramg should be 3.
> Changing the block size is not allowed by petsc, and there doesn't seem to
> be another way to reach HYPRE_BoomerAMGSetNumFunctions.
> What is the best way to handle this, and does it matter that the dofs are
> ordererd xxxyyyzzz rather than xyzxyzxyz ?
>

If you want to explicitly setup the near null space, I recommend using an
agglomeration multigrid, which can
take that in, like ML or GAMG, and using MatSetNearNullSpace(), which
functions just like MatSetNullSpace().

  Thanks,

     Matt

> Thanks,
> Sander
>
>
>
> On Fri, Jul 10, 2015 at 7:44 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Tue, Jun 30, 2015 at 3:10 PM, Sander Land <sander.land at gmail.com>
>> wrote:
>>
>>> This gives
>>> [0]PETSC ERROR: Object is in wrong state
>>> [0]PETSC ERROR: Matrix must be set first
>>>
>>> The PC is from a SNESGetKSP, and I have called SNESSetJacobian before
>>> this.
>>>
>>
>> If this is embedded in a SNES, it is very messy to yank it out and do
>> thing manually. However, if you
>> set the matrix in the PC manually, it might work.
>>
>> However, I would encourage you to try an alternate scheme and see if you
>> think it is easier. We have
>> started to use the DM object to hold this kind of information. For
>> FieldSplit, I think it is easier to tell
>> a DMShell about your field division, and then have it pass this on to
>> PCFIELDSPLIT. So you would
>>
>>   a) Create a DMSHELL
>>
>>   b) Create a PetscSection and call DMSetDefaultSection()
>>
>>   c) Call SNESSetDM()
>>
>>   d) use command line options to configure the split
>> -pc_fieldsplit_0_fields 0 -pc_fieldsplit_1_fields 1
>>
>> Then everything should work, and can work in any embedded context. Does
>> this make sense?
>>
>>   Thanks,
>>
>>     Matt
>>
>> Thanks,
>>> Sander
>>>
>>> On Tue, Jun 30, 2015 at 2:09 PM, Matthew Knepley <knepley at gmail.com>
>>> wrote:
>>>
>>>> On Tue, Jun 30, 2015 at 11:43 AM, Sander Land <sander.land at gmail.com>
>>>> wrote:
>>>>
>>>>> I am trying to use the Schur complement preconditioner in petsc, but
>>>>> am encountering a null argument error calling PCFieldSplitGetSubKSP.
>>>>> This only happens on PC_COMPOSITE_SCHUR, the multiplicative/additive
>>>>> options do return a KSP array.
>>>>>
>>>>> Error message:
>>>>>
>>>>> [0]PETSC ERROR: --------------------- Error Message
>>>>> --------------------------------------------------------------
>>>>> [0]PETSC ERROR: Null argument, when expecting valid pointer
>>>>> [0]PETSC ERROR: Null Object: Parameter # 1
>>>>> [0]PETSC ERROR: See
>>>>> http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble
>>>>> shooting.
>>>>> [0]PETSC ERROR: Petsc Release Version 3.6.0, Jun, 09, 2015
>>>>> [0]PETSC ERROR: ./elecmech on a arch-linux2-cxx-debug
>>>>> [0]PETSC ERROR: Configure options --with-shared-libraries=1
>>>>> --with-debugging=1 --download-openmpi=1 --with-clanguage=c++
>>>>> --download-fblaslapack=1 --download-scalapack=1 --download-blacs=1
>>>>> --download-suitesparse=1 --download-pastix=1 --download-superlu=1
>>>>> --dowload-essl=1 --download-ptscotch=1 --download-mumps=1 --download-lusol=1
>>>>> [0]PETSC ERROR: #1 MatSchurComplementGetKSP() line 317 in
>>>>> petsc-3.6/src/ksp/ksp/utils/schurm.c
>>>>> [0]PETSC ERROR: #2 PCFieldSplitGetSubKSP_FieldSplit_Schur() line 1264
>>>>> in petsc-3.6/src/ksp/pc/impls/fieldsplit/fieldsplit.c
>>>>> [0]PETSC ERROR: #3 PCFieldSplitGetSubKSP() line 1668 in
>>>>> petsc-3.6/src/ksp/pc/impls/fieldsplit/fieldsplit.c
>>>>>
>>>>> Code snippet:
>>>>>
>>>>>
>>>>> ISCreateGeneral(PETSC_COMM_SELF,xp_dofs,&ii[0],PETSC_COPY_VALUES,&is_xp);
>>>>> ISCreateGeneral(PETSC_COMM_SELF,all_dofs -
>>>>> xp_dofs,&ii[xp_dofs],PETSC_COPY_VALUES,&is_wk);
>>>>>
>>>>> PCSetType(pc,PCFIELDSPLIT);
>>>>> PCFieldSplitSetType(pc,PC_COMPOSITE_SCHUR);
>>>>> PCFieldSplitSetIS(pc,"xp",is_xp);
>>>>> PCFieldSplitSetIS(pc,"wk",is_wk);
>>>>> int n;
>>>>> KSP* ksps;
>>>>> PC   subpc;
>>>>> PCFieldSplitGetSubKSP(pc,&n,&ksps);
>>>>>
>>>>>
>>>>> ii here is simply an array with ii[i] = i. There is probably a better
>>>>> way to simply indicate two blocks of different size, but I couldn't find it.
>>>>>
>>>>
>>>> It looks like the PC is not setup. Can you try calling PCSetUp(pc)
>>>> before GetSubKSP()?
>>>>
>>>>   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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