The os.sched_yield() implementation in posixmodule.c doesn't release the GIL

[FilipDepta]

I've reviewed the main branch and the GIL isn't released: https://github.com/python/cpython/blob/main/Modules/posixmodule.c#L7060

The 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.

[eryksun]

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

1. The man page for sched_yield() in Linux says the behavior is "unspecified" if the process isn't using a deterministic realtime scheduler such as SCHED_FIFO (first in, first out) or SCHED_RR (round robin). I'm guessing it's not common for Python applications to use realtime scheduling. ↩

[FilipDepta]

[...] using a deterministic realtime scheduler such as SCHED_FIFO (first in, first out) or SCHED_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_FIFO with two threads I sometimes call os.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?