[mpiwg-sessions] Can I query a set even if I'm not in it?

Dan Holmes dholmes at epcc.ed.ac.uk
Thu Apr 28 12:40:33 CDT 2016

Hi Jeff,

Inline comments.


> On 28 Apr 2016, at 18:17, Jeff Squyres (jsquyres) <jsquyres at cisco.com> wrote:
> In principle, I don't think I have a problem with a process asking for a group for a set of which it is not a member.  However:
> 1. You're correct about the scalability issues.
> --> This exposes an ambiguity in MPI_Session_get_names(): does that return *all* set names, or just sets to which the querying process belongs?  I'm in favor of the latter.

What about supplying ocean_set and atoms_set on the command-line - should ocean processes only see ocean_set and not atoms_set? Can I do multi-physics couple simulations where  communication happens occasionally between the groups but very often within the groups? I’d need ocean_set -> comm_ocean and mpi://WORLD -> filtered to include some ocean and some not ocean -> comm_exchange. With a 2-way partition then WORLD disjoint ocean gives atmos.
> 2. There's also an issue about the uniqueness of set names.  E.g., we defined that every process is part of mpi://SELF, but each of those sets are distinct from each other.  E.g., when process X refers to mpi://SELF, is it referring to its *own* mpi://SELF, or the mpi://SELF relative to some other process?

Isn't mph://SELF just not a special case like HEAD in git? mph://WORLD is sort of like master in git (pretty much always defined but not always where the main action is). I think there’s some mileage in exploring the {set names, set members} -> {git branches, git commits} analogy.

> Point #2 alone may dictate that set names are only meaningful in the process in which they were queried / returned by the runtime.

Set names don’t necessarily mean the same thing in all processes but that doesn’t stop them being meaningful.

> -- 
> Jeff Squyres
> jsquyres at cisco.com
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