[New-bugs-announce] [issue45714] test_multiprocessing_spawn hangs sometimes

[Skip Montanaro]

New submission from Skip Montanaro <skip.montanaro at gmail.com>:

I find that test_multiprocessing_spawn frequently hangs. Hitting Ctl-C then rerunning "make test" generally works. Still, this behavior makes it problematic to run testing unattended. I don't think I have an unusual environment (XUbuntu 20.04, GCC 9.3.0). Here's some output from the currently running/hanging process, just after the last other unit test completed.

0:03:42 load avg: 4.96 [421/422/21] test_concurrent_futures passed (2 min 57 sec) -- running: test_multiprocessing_spawn (2 min 59 sec)
0:04:12 load avg: 3.19 running: test_multiprocessing_spawn (3 min 29 sec)
0:04:42 load avg: 1.93 running: test_multiprocessing_spawn (3 min 59 sec)
0:05:12 load avg: 1.23 running: test_multiprocessing_spawn (4 min 29 sec)
0:05:42 load avg: 0.74 running: test_multiprocessing_spawn (4 min 59 sec)
...
0:22:12 load avg: 0.42 running: test_multiprocessing_spawn (21 min 29 sec)
0:22:42 load avg: 0.27 running: test_multiprocessing_spawn (21 min 59 sec)
0:23:12 load avg: 0.37 running: test_multiprocessing_spawn (22 min 29 sec)

I'm currerntly mostly messing around with the nogil code and 3.9 tip, but I've noticed this problem on other branches as well. The fact that this seems not to have been reported before suggests that I'm somehow an outlier. If it was common, my guess is that the buildbots would experience this problem on a regular basis.

Here is the current python process info (output of pgrep -fla python).

1572195 /home/skip/src/python/cpython/python -u -W default -bb -E -m test -r -w -j 0 -u all,-largefile,-audio,-gui
1575193 /home/skip/src/python/cpython/python -bb -E -Wdefault -u -m test.regrtest --worker-args [{"testdir": null, "verbose": 0, "quiet": false, "exclude": false, "single": false, "randomize": true, "fromfile": null, "findleaks": 1, "use_resources": ["curses", "network", "decimal", "cpu", "subprocess", "urlfetch"], "trace": false, "coverdir": "coverage", "runleaks": false, "huntrleaks": false, "verbose2": true, "print_slow": false, "random_seed": 2495022, "use_mp": 10, "verbose3": false, "forever": false, "header": false, "failfast": false, "match_tests": null, "ignore_tests": null, "pgo": false, "timeout": null, "wait": false, "worker_args": null, "start": null, "match_filename": null, "ignore_filename": null, "use": [["all", "-largefile", "-audio", "-gui"]], "memlimit": null, "threshold": null, "nowindows": false, "list_tests": false, "list_cases": false, "pgo_extended": false, "fail_env_changed": false, "xmlpath": null, "tempdir": "/home/skip/src/python/cpython/build/test_python_1572195", "cleanup": false, "args": []}, "test_multiprocessing_spawn"]
1575489 /home/skip/src/python/cpython/python -bb -E -Wdefault -c from multiprocessing.resource_tracker import main;main(3)
1575547 /home/skip/src/python/cpython/python -bb -E -Wdefault -c from multiprocessing.forkserver import main; main(6, 7, ['__main__', 'test.test_multiprocessing_forkserver'], **{'sys_path': ['/home/skip/src/python/cpython', '/home/skip/tmp/nogilpgo_build/lib/python39.zip', '/home/skip/src/python/cpython/Lib', '/home/skip/src/python/cpython/build/lib.linux-x86_64-3.9', '/home/skip/.local/lib/python3.9/site-packages', '/home/skip/tmp/nogilpgo_build/lib/python3.9/site-packages']})
1589033 /home/skip/src/python/cpython/python -bb -E -Wdefault -c from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=4, pipe_handle=12) --multiprocessing-fork
1589034 /home/skip/src/python/cpython/python -bb -E -Wdefault -c from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=4, pipe_handle=14) --multiprocessing-fork

At the moment I can't pstack either of the last two processes. Unfortunately, I think I need to tweak ptrace.conf and reboot.

----------
messages: 405718
nosy: skip.montanaro
priority: normal
severity: normal
status: open
title: test_multiprocessing_spawn hangs sometimes

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45714>
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