[petsc-dev] future of PetscSubcomm

[Jed Brown]

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.)
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