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

Sander Land sander.land at gmail.com
Thu Jul 16 13:35:03 CDT 2015 

Hadn't even thought of IS being the place to look for this, I see there is
ISBlockGetSize and will give that a try tomorrow.
Thanks,
Sander

On Thu, Jul 16, 2015 at 5:10 PM, Mark Adams <mfadams at lbl.gov> wrote:

>
>
> On Thu, Jul 16, 2015 at 6:33 AM, Sander Land <sander.land at gmail.com>
> wrote:
>
>> Yes, I figured I'd need to rewrite my dirichlet bc implementation to zero
>> out the row rather than remove the dof completely. For now I'm just
>> avoiding partial bcs.
>> Using MatSetBlockSize is a problem because the matrix has stuff after the
>> xyzxyz (hydrostatic pressure dofs and a few coupled things).
>> MatGetSubMatrix inherits the block size of 1, and I can't change it
>> afterwards.
>>
>
> Humm, this needs to work.  Are you saying you get the (sub) matrix with an
> IS of the non-pressure part?  You should be able to set that block size to
> 3.  If this get submatrix happens inside of FieldSplit then I think there
> is a solution for this.
>
>
>
>>
>>
>> Sander
>>
>> On Thu, Jul 16, 2015 at 12:26 AM, Mark Adams <mfadams at lbl.gov> wrote:
>>
>>> Note, you want to tell the matrix that it has these blocks with
>>> MatSetBlockSize, and do not remove a single DOF (like for BCs). That is,
>>> xyzxyxyz is not allowed.  The block size is critical for GAMG and would
>>> think HYPRE uses it also because it does OK on elasticity.
>>>
>>> On Wed, Jul 15, 2015 at 2:52 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>>
>>>>
>>>> > On Jul 15, 2015, at 5:43 AM, Sander Land <sander.land at gmail.com>
>>>> wrote:
>>>> >
>>>> > Hi Barry,
>>>> >
>>>> > I have some structures set up that map dofs to their respective
>>>> matrix indices, and it wasn't difficult to change them.
>>>> >
>>>> > However, now this call:
>>>> > MatMultAdd(ctxp->B,xp,bx,xxtmp);
>>>> > is directed to "MatMultAdd_SeqAIJ_Inode" and gives a vector lock
>>>> error at line 601 of mat/impls/aij/seq/inode.c
>>>> > ierr = VecGetArray(zz,&z);
>>>> > I think this is a bug and should be VecGetArrayRead as it doesn't
>>>> need to be written to.
>>>>
>>>>     Oh yes, this is a bug. I have fixed it in maint, master and next
>>>> >
>>>> > In the old xxxyyyzzz ordering this call went to MatMultAdd_SeqAIJ,
>>>> which (via VecGetArrayPair) does use VecGetArrayRead. I'm not sure what is
>>>> causing the different matrix formats.
>>>>
>>>>    When the values are interlaced the matrix has "identical nodes"
>>>> (i-nodes) which means certain rows have identical nonzero structure with
>>>> there neighbors and the AIJ matrix code automatically switches to the more
>>>> efficient Inode operations. This is why you only saw the bug with
>>>> interlacing.
>>>> >
>>>> > I'm also still not sure at what point I can change the matrix block
>>>> size to get the correct BoomerAMGSetNumFunctions.
>>>> >
>>>> > Thanks,
>>>> > Sander
>>>> >
>>>> >
>>>> >
>>>> >
>>>> >
>>>> > On Tue, Jul 14, 2015 at 6:34 PM, Barry Smith <bsmith at mcs.anl.gov>
>>>> wrote:
>>>> >
>>>> > > On 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 ?
>>>> >
>>>> >    I don't think BoomerAMG supports xxxyyyzzz, I think it will just
>>>> get very confused trying to do algebraic multigrid with this form. Read up
>>>> on it but my guess is you need to rewrite the code that prepares the system
>>>> to do the interlacing.
>>>> >
>>>> >   Barry
>>>> >
>>>> > >
>>>> > >
>>>> > > 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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