[petsc-dev] programming model for PETSc

[Jed Brown]

On Thu, Nov 24, 2011 at 18:30, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:

> In any event, this business of storing explicit ranks/offsets for the
> ghost indices is sort of antithetical to the current
> PetscLayout-based approach of indices having parallel semantics from the
> outset.  A lot of conceptual difficulties
> in Sieve and MOAB (and the need for the extra objects such as Overlap and
> ParallelComm (sic!)) stem from a lack of
> a global address space.
>

Global addresses and (rank, offset) are equivalent modulo MPI_Scan() which
costs essentially the same as MPI_Reduce(). Not free, but not huge when
done in setup. I think global indices are usually easier for users to play
with (if they need anything that has global semantics, local indices
combined with a local-to-global map is even better when sufficient), but
they eventually need to be converted to (rank, offset) for communication
(MPI does not know about a "global index" and I don't think it should.)
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