Emend the point-to-point progress rule 'on two processes'

[Wee-Free-Scot]

Problem

Reported by Jasper Larsson Träff.

The current expression of the point-to-point progress rule starts "If a pair of matching send and receives have been initiated on two processes, ..."

The "on two processes" is not required and is confusing. A send-to-self, receive-from-self pattern is specifically excluded by this expression of the progress rule but should not be. I do not believe it was ever the intent to exclude the progress guarantee for this communication pattern. This communication pattern is possible even for blocking operations with multi-threading.

Proposal

Clarify to include the communicate-with-self pattern in the progress guarantee.

Changes to the Text

§3.5 "Progress" 1st line (MPI-4.0-RC-FEB, p54, line 42)
Remove the words "on two processes".

The process(es) involved in the communication is/are already unambiguously specified by the "source" and "dest" parameters in the point-to-point MPI procedures. The word "matching" requires that the receiving process is specified as "dest" at the sender process and that the sender process is specified (or at least covered by, in the case of a wildcard) by the "source" at the sender process. There is no loss of intended specification detail; only the potentially conflicting superfluous phrase is removed.

Impact on Implementations

Additional clarity.

Impact on Users

Additional clarity.

References and Pull Requests

Fixed by PR: https://github.com/mpi-forum/mpi-standard/pull/589 (closed prematurely)
Fixed by PR: https://github.com/mpi-forum/mpi-standard/pull/605 (replacement targeting mpi-4.x branch)

[wesbland]

This passed an errata vote on 2021-09-10:

Yes	No	Abstain
28	0	2