[mpich-discuss] Differences between ch3:nemesis and ch4:ofi:tcp with MPI_Barrier before completion of MPI_Isend

[Raffenetti, Ken]

Hi Edric,

I don’t see anything wrong in your pseudo code. I believe it is a correct pattern. I ran some experiments myself on some local machines and could not cause a hang, so if you come up with a reproducer please send it along.

Ken

From: Edric Ellis via discuss <discuss at mpich.org>
Reply-To: "discuss at mpich.org" <discuss at mpich.org>
Date: Wednesday, April 24, 2024 at 3:05 AM
To: "discuss at mpich.org" <discuss at mpich.org>
Cc: Edric Ellis <eellis at mathworks.com>
Subject: [mpich-discuss] Differences between ch3:nemesis and ch4:ofi:tcp with MPI_Barrier before completion of MPI_Isend

I'm trying to understand if a change in behaviour I'm seeing is expected or not. My code initiates an MPI_Isend on rank==1, and before waiting for completion of that send, all ranks perform an MPI_Barrier. This works fine on ch3: nemesis. It
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I'm trying to understand if a change in behaviour I'm seeing is expected or not. My code initiates an MPI_Isend on rank==1, and before waiting for completion of that send, all ranks perform an MPI_Barrier. This works fine on ch3:nemesis. It works fine on ch4:ofi when the message is small (presumably using the "eager" protocol). When using ch4:ofi (the embedded tcp provider) and the message is large (presumably switching to "rendezvous" protocol), rank 0 never leaves the MPI_Barrier call. (I think the SHM piece of ch4 does not show the problem)



Should this work? I cannot find anything in the MPI standard that says it should not, but perhaps I'm not looking in the right place.



I'm using mpich-4.1.2 in both cases, either in "--with-device=ch3:nemesis" mode or "--with-libfabric=embedded --with-device=ch4:ofi:tcp".



Here's a sketch of the problematic section of code, I'll attempt to attach a full reproduction (but I'm not sure if that works?)



// setup code...

if (rank == 1) {

    MPI_Isend(data, count, MPI_INT, 0, TAG, comm, &req);

} else {

    std::this_thread::sleep_for(std::chrono::seconds(1));

}

MPI_Barrier(comm);

MPI_Barrier(comm);

// cleanup code, receive the message etc...



Cheers,

Edric.
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