[mpich-discuss] Non-blocking collectives

[Jed Brown]

rreddypsc at gmail.com writes:

> I apologize for creating a new thread, I have accidentally deleted the 
> original message.
>
> Not to split hairs, can you please clarify the following:
>
> After doing a non-blocking collective:
>
> The MPI_Wait would be satisfied as soon as the other ranks in the 
> communicator have called the non-blocking collective (and may not yet have 
> called MPI_Wait), right?

Yes.

> Or do they wait until all of them have called MPI_Wait?

No, that would make the MPI_Wait synchronizing, mostly defeating any
potential benefit of non-blocking collectives.  Note that the
non-blocking request can complete in other ways, e.g., via MPI_Test.

> On the contrary, if one of the ranks calls MPI_Wait (without calling the 
> non-blocking collective, which is an incorrect program), will the MPI_Wait 
> be satisfied?

This doesn't even make sense.  Where would you be getting an MPI_Request
on which to call MPI_Wait?
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