Description

colesbury

opened on Oct 6, 2023

Issue body actions

Feature or enhancement

CPython's current reference counting implementation would not be thread-safe without the GIL. In --disable-gil builds, we should implement biased reference counting, which is thread-safe and has lower execution overhead compared to plain atomic reference counting. The design is described in the "Biased Reference Counting" section of PEP 703.

This will require changing the PyObject struct in --disable-gil builds. I expect the --disable-gil struct to look like:

struct _object {
  uintptr_t ob_tid;
  uint16_t _padding;
  uint8_t ob_mutex;
  uint8_t ob_gc_bits;
  uint32_t ob_ref_local;
  Py_ssize_t ob_ref_shared;
  PyTypeObject *ob_type;
};

Not all the fields will be used immediately, but it will be easier to include them as part of implementing this change.

I intend to split the implementation across (at least) two PRs to make it easier to review.

1. The first PR will add the new PyObject fields and implement the core biased reference counting operations, but not the inter-thread queue to reduce the initial complexity. (done)
2. A later PR will implement the inter-thread queue. (not yet implemented)

The inter-thread queue is needed eventually (before actually disabling the GIL), but since the --disable-gil builds still currently require the GIL, it doesn't have to be in the initial PR.

Linked PRs

• gh-110481: Implement biased reference counting #110764
• gh-110481: Fix _Py_ThreadId for non-free-threaded mode #111503
• gh-110481: Non-limited Py_INCREF/DECREF() in cpython/object.h #111505
• gh-110481: Implement _Py_DECREF_NO_DEALLOC for free-threaded build #111560
• gh-110481: fix 'unused function' warning for is_shared_refcnt_dead. #111974
• gh-110481: Fix Py_SET_REFCNT() integer overflow #112174
• gh-110481, doc: Add "immortal" term to the glossary #112180
• gh-110481: Fix typo in Py_SET_REFCNT() #112595
• gh-110481: Implement inter-thread queue for biased reference counting #114824
• gh-110481: Fix biased reference counting queue initialization. #117271

Reacted by Donghee Na, An Long, Michael Droettboom and Gregory P. Smith

Reacted by stonebig, jon-chuang, Vladimir and Amuthan Mannar

Reacted by Dima Tisnek

Activity

1. colesbury

   added

   type-featureA feature request or enhancementA feature request or enhancement

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   3.13only security fixesonly security fixes

   on Oct 6, 2023
2. colesbury

   self-assigned this

   on Oct 12, 2023
3. corona10

   added 2 commits that reference this issue on Oct 30, 2023

   gh-110481: Implement biased reference counting (gh-110764)

   6dfb8fe

   pythongh-110481: Fix build bot for free-threaded mode

   8ef44ef
4. vstinner

   added 4 commits that reference this issue on Oct 30, 2023

   pythongh-110481: Non-limited Py_INCREF/DECREF() in cpython/object.h

   3088de3

   pythongh-110481: Non-limited Py_INCREF/DECREF() in cpython/object.h

   0e9c6c2

   pythongh-110481: Move non-limited Py_INCREF() to cpython/object.h

   5b93bdd

   pythongh-110481: Move non-limited Py_INCREF() to cpython/object.h

   2d3a76e
5. corona10

   added 2 commits that reference this issue on Oct 30, 2023

   gh-110481: Fix _Py_ThreadId for non-free-threaded mode (gh-111503)

   55df2de

   pythongh-110481: Implement _Py_DECREF_NO_DEALLOC for free-threaded build

   6c2ffb8
6. FullteaR

   added 2 commits that reference this issue on Nov 3, 2023

   pythongh-110481: Implement biased reference counting (pythongh-110764)

   6a480d8

   pythongh-110481: Fix _Py_ThreadId for non-free-threaded mode (pythong…

   2efd0c7
7. corona10

   added a commit that references this issue on Nov 10, 2023

   gh-110481: fix 'unused function' warning for `is_shared_refcnt_dead`. (…

   fa84e5f
8. 29 remaining items
9. colesbury

   added 2 commits that reference this issue on Mar 27, 2024

   Merge branch 'pythongh-110481-brc-bind' into nogil-integration

   11897fc

   gh-110481: Fix biased reference counting queue initialization. (#117271)

   8dbfdb2
10. encukou commented on Mar 29, 2024

    

    encukou

    on Mar 29, 2024

    Member

    More actions

    Since #117271 was merged, test_multiprocessing tests are consistently failing on noGIL Refleaks buildbots. For example: https://buildbot.python.org/all/#/builders/1368/builds/606
11. encukou

    reopened this on Mar 29, 2024
12. colesbury commented on Mar 29, 2024

    

    colesbury

    on Mar 29, 2024

    ContributorAuthor

    More actions

    I put up #117370, which fixes the refleaks.
13. NGRsoftlab

    added 2 commits that reference this issue on Apr 2, 2024

    Squashed commit of the following:

    1b5fe4c

    Squashed commit of the following:

    a016a54
14. colesbury

    closed this as completedon Apr 10, 2024
15. diegorusso

    added a commit that references this issue on Apr 17, 2024

    pythongh-110481: Fix biased reference counting queue initialization. (p…

    dc8140b
16. Glyphack

    added 6 commits that reference this issue on Sep 2, 2024

    pythongh-110481: Implement biased reference counting (pythongh-110764)

    2295beb

    pythongh-110481: Fix _Py_ThreadId for non-free-threaded mode (pythong…

    8a0a8b8

    pythongh-110481: fix 'unused function' warning for `is_shared_refcnt_…

    29f912a

    pythongh-110481, doc: Add "immortal" term to the glossary (python#112180

    06048e1

    pythongh-110481: Fix Py_SET_REFCNT() integer overflow (python#112174)

    ac8e22b

    pythongh-110481: Fix typo in Py_SET_REFCNT() (python#112595)

    33fba34

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