Description

FilipDepta

opened on Aug 18, 2022

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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.

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1. FilipDepta

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Aug 18, 2022
2. eryksun

   added

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   3.11only security fixesonly security fixes

   3.10 (EOL)end of lifeend of life

   3.12only security fixesonly security fixes

   on Aug 18, 2022
3. eryksun commented on Aug 18, 2022

   

   eryksun

   on Aug 18, 2022

   Contributor

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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

   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. ↩
4. FilipDepta commented on Aug 19, 2022

   

   FilipDepta

   on Aug 19, 2022

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   [...] 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?
5. corona10

   added 2 commits that reference this issue on Oct 6, 2022

   pythongh-96078: os.sched_yield release the GIL while calling sched_yi…

   5f150a8

   pythongh-96078: os.sched_yield release the GIL while calling sched_yi…

   40c5725
6. benjaminp

   closed this as completedin b9d2e81on Oct 6, 2022
7. carljm

   added a commit that references this issue on Oct 6, 2022

   Merge branch 'main' into dictwatch

   This issue will close once commit e5d2169 is merged into the 'main' branch.

   e5d2169
8. miss-islington

   added 4 commits that reference this issue on Oct 7, 2022

   fixes pythongh-96078: os.sched_yield release the GIL while calling sc…

   This issue will close once commit 920e025 is merged into the 'master' branch.

   920e025

   fixes pythongh-96078: os.sched_yield release the GIL while calling sc…

   This issue will close once commit 4a38944 is merged into the 'master' branch.

   4a38944

   fixes gh-96078: os.sched_yield release the GIL while calling sched_yi…

   This issue will close once commit e39b511 is merged into the 'main' branch.

   e39b511

   fixes gh-96078: os.sched_yield release the GIL while calling sched_yi…

   This issue will close once commit 11945f2 is merged into the 'main' branch.

   11945f2
9. carljm

   added a commit that references this issue on Oct 8, 2022

   Merge branch 'main' into typewatch

   This issue will close once commit 415ed49 is merged into the 'main' branch.

   415ed49
10. mpage

    added a commit that references this issue on Oct 11, 2022

    fixes pythongh-96078: os.sched_yield release the GIL while calling sc…

    This issue will close once commit 0c970bd is merged into the 'main' branch.

    0c970bd
11. serhiy-storchaka

    added a commit that references this issue on Aug 9, 2026

    fixes pythongh-96078: os.sched_yield release the GIL while calling sc…

    This issue will close once commit b9d2e81 is merged into the 'main' branch.

    b9d2e81

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