[petsc-dev] PCFIELDSPLIT MATNEST and NULL blocks

[Stefano Zampini]

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

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?

[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

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