[mpich-discuss] Why stuck in MPI_Finalize?

[Zhou, Hui]

Hi Erik,

It is not just a barrier in MPI_Finalize. It also needs to make sure pending communications are completed. Sometimes this is the user's fault, such as mismatched communication or partially freed communicators. But sometimes this is due to lower-level network library. For example, some libfabric provider does not always flush send. We tried to detect and prevent all cases that result in hanging, but there always seem to be cases that escape our solution. If you can drill down some simple reproducible (even with 5% chance) cases, please create a github issue and we'll track it down.

By the way, if you have pointers on setting up Julia CI testing, we can try to setup nightly testing on our end as well. Catching the errors when it appear always makes troubleshooting easier.

--
Hui
________________________________
From: Erik Schnetter via discuss <discuss at mpich.org>
Sent: Monday, November 29, 2021 12:30 PM
To: discuss at mpich.org <discuss at mpich.org>
Cc: Erik Schnetter <schnetter at gmail.com>
Subject: [mpich-discuss] Why stuck in MPI_Finalize?

I have a Julia test case on macOS where MPICH randomly gets stuck in
MPI_Finalize (with about a 5% chance). See e.g.
https://github.com/JuliaParallel/MPI.jl/runs/4357341818

Can you advise under what circumstances MPICH could get stuck there?
The respective run uses 3 processes, and all 3 processes call into
MPI_Finalize, but no process returns.

I assume that MPI_Finalize contains internally the equivalent to an
MPI_Barrier, but that should succeed here. Are there other actions
taken in MPI_Finalize that would require some kind of consistent state
across the application? For example, if a communicator was created on
all processes, but freed only on some processes, could this cause such
a deadlock?

-erik

--
Erik Schnetter <schnetter at gmail.com>
http://www.perimeterinstitute.ca/personal/eschnetter/
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