[petsc-dev] future of PetscSubcomm

[Barry Smith]

https://bitbucket.org/petsc/petsc/issue/46/replace-petscsubcomm-with-the-information


On Jul 31, 2013, at 10:59 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Barry Smith <bsmith at mcs.anl.gov> writes:
> 
>>  Jed,
>> 
>>    Hong and I were discussing MatGetRedundantMatrix() etc and she
>>    mention that you don't like PetscSubcomm. I suggested removing the
>>    PetscSubcomm object from PETSc but retaining the functionality of
>>    PetscSubcomm by putting the subcomm information into an MPI
>>    attribute inside an MPI_Comm (so this new comm behaves like a
>>    MPI_Comm subclass with extra data over a regular MPI_Comm) and
>>    having something like PetscCommSublimate(comm, …., &subcomm with
>>    attribute). Then in all the places in PETSc that take PetscSubcomm
>>    they would instead take a MPI_Comm and check for the attribute and
>>    use the additional information if it is there. 
> 
> I would be inclined to remove dependence on this extra information
> whenever possible.  For example, I think it is counter-productive in
> MatGetRedundantMatrix because it imposes a distribution that has poor
> locality.  But if we need that extra information, or can do something
> better if it is there, I prefer attaching it as an attribute to having a
> wrapper (PetscSubcomm).
> 
>>    Managing destruction will be tricky due to possible reference
>>    loops but presumably we can figure out a way to do it.
> 
> What is missing in the MPI attribute callbacks to implement reference
> counting?  (We already use it for this purpose.)