Add thread-safety information to the standard

[raffenet]

Overview

The PMIx Standard should have information on what users can expect regarding the thread-safety of APIs

Motivation

OpenPMIx today is thread-safe. Consumers of PMIx likely already expect thread-safety as standard.

Discussion Items

Is there any special initialization needed for thread-safety? Can users query what level of thread-safety is supported? Or what thread runtimes are expected to work?

[rhc54]

FWIW: OpenPMIx is always thread safe, so no special initialization required. There is no query for thread-safety support as it is always "on" - but it is done using an internal progress thread and thread-shifting from the user's thread to that progress thread. We therefore serialize all operations via the progress thread - which we judged to be "okay" since PMIx is not intended to be a performant capability like an MPI library.

We only support the pthread library. There were some initial offers to add support for user-level thread runtimes, but that work never materialized. We remain open to it, but there is no way I have time to provide it myself - it would have to be a 3rd party contribution.

[rhc54]

I should have added - there are APIs by which a user can:

• externally drive the OpenPMIx progress thread - OpenPMIx will internally operate the same as always.
• provide their own libevent event base for OpenPMIx to use when thread-shifting. In this case, the user assumes all responsibility for deadlocks

However, both of those options still internally depend upon the pthread library.