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

[Sander Land]

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 ?


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
>
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