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

[Sander Land]

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.

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.

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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