[petsc-dev] PCFIELDSPLIT MATNEST and NULL blocks

Matthew Knepley knepley at gmail.com
Sun Jul 31 16:58:46 CDT 2016

On Sun, Jul 31, 2016 at 4:56 PM, Stefano Zampini <stefano.zampini at gmail.com>

> It’s the first time I’m trying to setup field split so maybe I’m missing
> something
> I have a MatNest with NULL (1,1) block. I setup the field split using the
> row ISs from the MatNest (a note aside: why they cannot be inferred by
> FieldSplit if pc->pmat is of type MatNEST?)
> Options used are -pc_fieldsplit_type additive and
> -pc_fieldsplit_detect_saddle_point

You don't need ISes is you use detect_saddle_point, so use one or the other.

> I then get this error, because MatGetSubMatrix_Nest returns a NULL pointer.
> What is the solution? having MatGetSubMatrix_Nest return a zero-valued
> matrix of correct size? any command line option I’m not using? Or instead
> having Fieldsplit handle the case?

I think have Nest return a real matrix is the right solution, since this is
what will happen with other types.


> [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 Development GIT revision: v3.7.2-744-gb54d608  GIT
> Date: 2016-06-28 18:20:20 +0300
> [0]PETSC ERROR: Unknown Name on a debug named kl-13916.local by szampini
> Sun Jul 31 14:47:53 2016
> [0]PETSC ERROR: Configure options --download-chaco --download-ctetgen
> --download-exodusii --download-hdf5 --download-hypre --download-metis
> --download-mpich --download-mumps --download-netcdf --download-p4est
> --download-parmetis --download-ptscotch --download-scalapack
> --download-suitesparse --download-triangle --with-valgrind-dir=/usr/local/
> CC=/usr/local/bin/gcc-5 CXX=/usr/local/bin/g++-5
> FC=/usr/local/bin/gfortran-5 PETSC_ARCH=debug
> [0]PETSC ERROR: #1 MatSetOptionsPrefix() line 654 in
> /Users/szampini/software/petsc/src/mat/interface/matrix.c
> [0]PETSC ERROR: #2 PCSetUp_FieldSplit() line 558 in
> /Users/szampini/software/petsc/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> [0]PETSC ERROR: #3 PCSetUp() line 968 in
> /Users/szampini/software/petsc/src/ksp/pc/interface/precon.c

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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