[petsc-dev] why exist named global and local vectors

[Jed Brown]

Vecs hold a reference to their DM.  If the Vec is also composed to the
DM, then you have a reference counting loop.  Discovering and breaking
general reference counting loops is hard; the named vectors are a
restrictive feature so that it is not hard.  I don't have a problem with
other solutions, but that was the rationale.

You can see the original commit.

commit dfe153156869f5931ee6b299eecdcfd8470c0dd0                           
Author: Jed Brown <jed at 59A2.org>
Date:   Sun Mar 25 20:17:10 2012 -0500

    Strip out bad idea of holding vec_sol in SNESDM, implement DMGetNamedGlobalVector() and implement restriction with it
    
    Hg-commit: 0040de20b98becd00af00473b94f3d32a4299b3d

 include/petsc-private/dmimpl.h   | 10 ++++
 include/petsc-private/snesimpl.h |  6 ---
 include/petscdm.h                |  2 +
 src/dm/impls/da/dalocal.c        | 94 +++++++++++++++++++++++++++++++++++
 src/dm/interface/dm.c            | 10 ++++
 src/snes/interface/snes.c        | 40 +++++++++++++--
 src/snes/utils/dmsnes.c          | 74 +--------------------------
 7 files changed, 154 insertions(+), 82 deletions(-)

"Smith, Barry F." <bsmith at mcs.anl.gov> writes:

>   Why have 
>
> PETSC_EXTERN PetscErrorCode DMHasNamedGlobalVector(DM,const char*,PetscBool*);
> PETSC_EXTERN PetscErrorCode DMGetNamedGlobalVector(DM,const char*,Vec*);
> PETSC_EXTERN PetscErrorCode DMRestoreNamedGlobalVector(DM,const char*,Vec*);
> PETSC_EXTERN PetscErrorCode DMHasNamedLocalVector(DM,const char*,PetscBool*);
> PETSC_EXTERN PetscErrorCode DMGetNamedLocalVector(DM,const char*,Vec*);
> PETSC_EXTERN PetscErrorCode DMRestoreNamedLocalVector(DM,const char*,Vec*);
>
>   when one can simply using the already existing PetscObjectCompose() and PetscObjectQuery() to attach named vectors to a given DM (or any other object in fact)?
>
>    Barry