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-Ap... 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 <[email protected]> Sent: Thursday, April 25, 2024 7:00 PM To: [email protected] Cc: Edric Ellis <[email protected]> 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 <[email protected]<mailto:[email protected]>> Reply-To: "[email protected]<mailto:[email protected]>" <[email protected]<mailto:[email protected]>> Date: Wednesday, April 24, 2024 at 3:05 AM To: "[email protected]<mailto:[email protected]>" <[email protected]<mailto:[email protected]>> Cc: Edric Ellis <[email protected]<mailto:[email protected]>> 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 ZjQcmQRYFpfptBannerStart This Message Is From an External Sender This message came from outside your organization. ZjQcmQRYFpfptBannerEnd 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.