[petsc-users] Recursive field split with MatNest

[Jed Brown]

Colin McAuliffe <cjm2176 at columbia.edu> writes:

> Dear all, I have a four field problem with dof structure stored in a DM
> shell. The matrix is a MatNest. This works fine except for when I try to
> group fields. 

Colin, MatGetSubMatrix_Nest does not implement the case of unions of
index sets.  The relevant comparison code is in the private function
MatNestFindIS(), just before the line on which your error message was
generated.  It's possible to make this function check whether the given
index set is a union of basic index sets constituting the MatNest.  But
checking for equality with every subset is not scalable in the number of
blocks, so we'd need something like an ISSubset() function.

If you just want to get your work done, I recommending not using
MatNest.  If you want to contribute an interesting feature to PETSc, we
can offer guidance on the implementation.

> For example the following commands
>
> -ksp_type gmres
>    -pc_type fieldsplit
>    -pc_fieldsplit_type multiplicative
>       -pc_fieldsplit_0_fields 0,1
>           -fieldsplit_0_ksp_type preonly
>           -fieldsplit_0_pc_type lu
>        -pc_fieldsplit_1_fields 2,3
>           -fieldsplit_1_ksp_type preonly
>           -fieldsplit_1_pc_type ilu
>
> results in the following error
>
> [0]PETSC ERROR: --------------------- Error Message
> --------------------------------------------------------------
> [0]PETSC ERROR: Arguments are incompatible
> [0]PETSC ERROR: Could not find index set
> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for
> trouble shooting.
> [0]PETSC ERROR: Petsc Release Version 3.5.2, Sep, 08, 2014
> [0]PETSC ERROR:
> /Users/Christina/Desktop/COLIN/Work/feapver83/ver83/parfeap/feap on a
> gnu-dbg named new-host.home by christina Mon Jan 12 00:36:52 2015
> [0]PETSC ERROR: Configure options --with-debugging=1
> --with-shared-libraries=0 --with-mpi-dir=/usr/local
> --with-parmetis-dir=/usr/local --with-metis-dir=/usr/local
> --with-hypre-dir=/usr/local --download-suitesparse
> --with-external-packages-dir=/usr/local
> [0]PETSC ERROR: #1 MatNestFindIS() line 310 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/mat/impls/nest/matnest.c
> [0]PETSC ERROR: #2 MatNestFindSubMat() line 371 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/mat/impls/nest/matnest.c
> [0]PETSC ERROR: #3 MatGetSubMatrix_Nest() line 387 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/mat/impls/nest/matnest.c
> [0]PETSC ERROR: #4 MatGetSubMatrix() line 7345 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/mat/interface/matrix.c
> [0]PETSC ERROR: #5 PCSetUp_FieldSplit() line 559 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> [0]PETSC ERROR: #6 PCSetUp() line 902 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/ksp/pc/interface/precon.c
> [0]PETSC ERROR: #7 KSPSetUp() line 305 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/ksp/ksp/interface/itfunc.c
> [0]PETSC ERROR: #8 KSPSolve() line 417 in
> /Users/Christina/Desktop/COLIN/Work/petsc-3.5.2/src/ksp/ksp/interface/itfunc.c
>
> My guess is that petsc is expecting a MatNest corresponding to the outer
> field groupings instead of individual submatrices. Is it possible to
> resolve this, preferably from the command line?
>
> I should also mention that grouping and recursive field split works fine
> with the same DM shell and a MATAIJ, however a significant amount of time
> is spent copying the submatrices and I'd prefer to avoid that.
>
> Thanks and all the best,
> Colin
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