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The os.sched_yield() implementation in posixmodule.c doesn't release the GIL #96078
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Interestingly,
os.sched_yield()was added 11 years ago, and no one has ever mentioned this oversight. I guess the users, if any1, are satisfied that it can only yield execution to ready threads that aren't waiting to acquire the GIL. The behavior was never documented, so I'm tagging this as a bug in 3.10+.Footnotes
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The man page for
sched_yield()in Linux says the behavior is "unspecified" if the process isn't using a deterministic realtime scheduler such asSCHED_FIFO(first in, first out) orSCHED_RR(round robin). I'm guessing it's not common for Python applications to use realtime scheduling. ↩
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[...] using a deterministic realtime scheduler such as
SCHED_FIFO(first in, first out) orSCHED_RR(round robin). I'm guessing it's not common for Python applications to use realtime scheduling. ↩That is exactly what I do - using
SHED_FIFOwith two threads I sometimes callos.sched_yield()to ensure that the second thread was executed as freshly as possible.Maybe it's a good idea to add an annotation in docs for older cpythons that os.sched_yield() doesn't release the GIL?
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on Oct 11, 2022
I've reviewed the
mainbranch and the GIL isn't released: https://github.com/python/cpython/blob/main/Modules/posixmodule.c#L7060The forum thread: https://discuss.python.org/t/does-the-os-sched-yield-release-the-gil/18301
Expected: The GIL is released before sched_yield() and acquired after that.