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

[Smith, Barry F.]

  ok, sounds reasonable


> On Oct 5, 2018, at 11:26 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> 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