[mpiwg-p2p] Ordering of P2P messages in multithreaded applications

[Balaji, Pavan]

Hi Dan,

> On Nov 23, 2018, at 4:11 AM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk> wrote:
> However, it is *also* a correct implementation choice to ignore that “physical order” even in this case because the MPI library does not know, and cannot determine, *why* that “physical order” happened.

I don't think this is a correct implementation and I'm not sure what part of the chapter is causing you to interpret this as a correct implementation.  If there's algorithmic logic in the application to guarantee an order, then those operations are not logically concurrent.  Although I'm happy to help clarify something that's unclear in the standard, I'm at a loss as to what is unclear here.

FWIW, every implementation of MPI that I know of interprets the standard the way I stated it, i.e., those operations are not concurrent and the MPI library has to process them in the order that it sees it.  Whether that is an explicit scheduling done by the user or is an accidental schedule created by the OS cannot be determined by the MPI library, so it better respect the order that it sees.

  -- Pavan