On Thu, Nov 24, 2011 at 6:41 PM, Jed Brown <[email protected]> wrote:

On Thu, Nov 24, 2011 at 18:30, Dmitry Karpeev <[email protected]> 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.)

I think the idea of a local point of view is fundamental, not just for parallelism. We use LocalToGlobal mappings in FS so that yu do not have to know about other fields, just the same as you should not have to know about other processes. Matt -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener