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

[Edric Ellis]

Hm, I attempted to attach my full code, but maybe it didn’t get through correctly. Weirdly though when I look at the web view of this discussion group, *only* the code shows up (initially):

https://urldefense.us/v3/__https://lists.mpich.org/pipermail/discuss/2024-April/006578.html__;!!G_uCfscf7eWS!dMPBNIcfC4U_UYaGzoh3rU-i7QhBDnAdh1zo0sC8SnytRIlGThRVjCONSrYU_SSHHqdH9riWwFtj_3c$ 

Hopefully that is visible? (I think the problem shows up only when using 2 hosts)

(On looking again at that code, there’s a stray logging message about MPI_Comm_free – initially I was using separate communicators for the ISend and the Barrier – but that seems to make no difference).

Cheers,
Edric.

From: Raffenetti, Ken <raffenet at anl.gov>
Sent: Thursday, April 25, 2024 7:00 PM
To: discuss at mpich.org
Cc: Edric Ellis <eellis at mathworks.com>
Subject: Re: [mpich-discuss] Differences between ch3:nemesis and ch4:ofi:tcp with MPI_Barrier before completion of MPI_Isend

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<mailto:discuss at mpich.org>>
Reply-To: "discuss at mpich.org<mailto:discuss at mpich.org>" <discuss at mpich.org<mailto:discuss at mpich.org>>
Date: Wednesday, April 24, 2024 at 3:05 AM
To: "discuss at mpich.org<mailto:discuss at mpich.org>" <discuss at mpich.org<mailto:discuss at mpich.org>>
Cc: Edric Ellis <eellis at mathworks.com<mailto: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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