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

[Matthew Knepley]

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
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