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Use per-thread freelists in --disable-gil builds #111968

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Use per-thread freelists in --disable-gil builds#111968
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corona10
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3.13only security fixestopic-free-threadingtype-featureA feature request or enhancement

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@colesbury
colesbury
opened on Nov 10, 2023

Feature or enhancement

CPython uses freelists for frequently allocated Python objects, like dict, list, and slice. There freelists are generally stored in the per-interpreter state, which is not thread-safe without the GIL. In --disable-gil builds, the freelists should be stored in the per-thread state (i.e., PyThreadState). I think we probably want to keep the freelists in the interpreter state for the default build. This will probably require some refactoring.

Freelists:

  • float
  • slice (slice_cache)
  • tuple
  • list
  • dict (PyDictKeysObject and PyDictObject)
  • generator (value_freelist and asend_freelist)
  • PyContext

For context, here are similar changes in the nogil-3.12 fork, but I expect the changes in CPython 3.13 to be a bit different:

Linked PRs

Activity

  1. colesbury
    added
    type-featureA feature request or enhancement
    3.13only security fixes
    topic-free-threading
    on Nov 10, 2023

  2. Yaro1 commented on Dec 10, 2023

    @Yaro1
    Yaro1
    on Dec 10, 2023

    Hi!
    I'm new here, but if you don't mind I would like to take that issue

  3. colesbury commented on Dec 10, 2023

    @colesbury
    colesbury
    on Dec 10, 2023
    ContributorAuthor

    Hi @Yaro1, thanks for volunteering. You are welcome to take this on, but I don't think it's a great first issue. There are some subtleties and it also depends on #112776.

  4. Yaro1 commented on Dec 10, 2023

    @Yaro1
    Yaro1
    on Dec 10, 2023

    Got it, thanks.
    Can I take please in that case some of the issues from #108219 ?

  5. colesbury commented on Dec 10, 2023

    @colesbury
    colesbury
    on Dec 10, 2023
    ContributorAuthor

    I don't think there are any good issues right now that are scoped out and not blocked on other work.

  6. Yaro1 commented on Dec 10, 2023

    @Yaro1
    Yaro1
    on Dec 10, 2023

    Okay, got it. Thank you

  7. corona10 commented on Dec 27, 2023

    @corona10
    corona10
    on Dec 27, 2023
    Member

    @colesbury I would like to take a look at this issue, Is there any pending issues that are related to this issue?

  8. colesbury commented on Dec 27, 2023

    @colesbury
    colesbury
    on Dec 27, 2023
    ContributorAuthor

    @corona10, thanks! I think the issues were fixed (problems with PyThreadState_Clear() being called by the wrong interpreter)

  9. corona10
    self-assigned this
    on Dec 27, 2023
  10. corona10
    added 2 commits that reference this issue on Dec 30, 2023

    pythongh-111968: Introduce _Py_freelist_state and _PyFreeListState_GE…

    6acf43d

    pythongh-111968: Introduce _Py_freelist_state and _PyFreeListState_GE…

    cd4863c

  11. corona10 commented on Jan 2, 2024

    @corona10
    corona10
    on Jan 2, 2024
    Member

    #113584 will introduce seamless management of freelists that runtimes do not need to care about the implementation detail :)

    graph TD;
    PerThread --> FreeList;
    PerInterpreter --> FreeList;
    GC -- if free-threaded --> gc_freethreading;
    GC -- if default build --> gc_gil;
    gc_freethreading --> PerThread;
    gc_gil --> PerInterpreter;
    _PyFreeListState -- if free-threaded --> PerThread;
    _PyFreeListState -- if default build --> PerInterpreter;
    Runtime -- _PyFreeListState_GET --> _PyFreeListState;
    list --> Runtime;
    tuple --> Runtime;
    slice --> Runtime;
    float --> Runtime;
    dict --> Runtime;
    generator --> Runtime;
    PyContext --> Runtime;
    Loading
  12. corona10
    added a commit that references this issue on Jan 5, 2024

    Merge remote-tracking branch 'upstream/main' into pythongh-111968

    4e84130

  13. 73 remaining items

  14. Glyphack
    added 10 commits that reference this issue on Sep 2, 2024

    pythongh-111968: Introduce _PyFreeListState and _PyFreeListState_GET …

    2cdb9b2

    pythongh-111968: Use per-thread freelists for float in free-threading (…

    7706be8

    pythongh-111968: Unify naming scheme for freelist (pythongh-113919)

    6852eb9

    pythongh-111968: Use per-thread freelists for tuple in free-threading (…

    6f94e65

    pythongh-111968: Explicit handling for finalized freelist (pythongh-1…

    698dab6

    pythongh-111968: Use per-thread slice_cache in free-threading (python…

    de68319

    pythongh-111968: Use per-thread freelists for PyContext in free-threa…

    6da2d24

    pythongh-111968: Use per-thread freelists for generator in free-threa…

    891e195

    pythongh-111968: Fix --without-freelists build (pythongh-114270)

    beb44b3

    pythongh-111968: Unify freelist naming schema to Eric's suggestion (p…

    f617cad
  15. LukasWoodtli
    added 2 commits that reference this issue on Jan 22, 2025

    pythongh-111968: Split _Py_dictkeys_freelist out of _Py_dict_freelist (…

    6c72605

    pythongh-111968: Split _Py_async_gen_asend_freelist out of _Py_async_…

    a207147
  16. kumaraditya303
    added a commit that references this issue on Jul 15, 2025

    gh-111968: remove redundant fetching of interpreter state in `dict` i…

    a8f42e6
  17. taegyunkim
    added a commit that references this issue on Aug 4, 2025

    pythongh-111968: remove redundant fetching of interpreter state in `d…

    0dcd696
  18. Agent-Hellboy
    added a commit that references this issue on Aug 19, 2025

    pythongh-111968: remove redundant fetching of interpreter state in `d…

    8b027dc
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