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

Bland, Wesley wesley.bland at intel.com
Tue Apr 26 15:11:09 CDT 2016

Apologies if we’ve covered this and I’ve now forgotten, but it’s something that came up in the FTWG con call today. If I’m not in a set, am I still able to get the group it describes?

An example is because the set names are easily serialized (because they’re strings), I could pass a set name around so that someone who wasn’t in the set could have access to it and therefore ask the runtime to create a group from it. In the group world, this is allowed:
> Section 6.4.2 (MPI 3.1 p. 230 line 4-5) - … a process may also define a group that does not include itself. 
This has advantages and disadvantages. The advantage is that we could do things like create inter-communicators without the bridge_comm as long as you have some other way of getting the set name. We decided on the last call that this was important. In the context of fault tolerance, it *might* be possible to do in-place replacement if we have the ability to touch all sets from anywhere (HUGE might there).

The downside is that you have to be able to get the definition of any set from anywhere, which could have scalability issues (having to keep track of all the sets and how they overlap would be an O(p*s) problem, which we’re trying to avoid).


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