Avoid stop-the-world when `__abstractmethods__` does not change the abstract flag

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Évaluation

Difficulté
4/5
Temps estimé
3-5 jours
Accessibilité débutants
48/100
Type d'issue
Bug
Clarté
Clairement spécifiée
Activité
Active
Stack technique
c, python
Domaine
backend, performance

Piste de recherche

Start by locating type_set_abstractmethods() in CPython’s C implementation and inspect how it reads and changes Py_TPFLAGS_IS_ABSTRACT around the stop-the-world section. The issue provides the proposed condition and a benchmark covering concrete, protocol, abstract, and plain classes; use those cases to assess the change. Done means unchanged abstract state avoids stopping the world while state changes preserve correct behavior.

Rédigé par le modèle d'indexation à partir du texte de l'issue.

Description

interpreter-core performance topic-free-threading type-feature
Summary

In a free-threaded CPython build, type_set_abstractmethods() stops the worldevery time ABCMeta writes __abstractmethods__, even when the write does notchange Py_TPFLAGS_IS_ABSTRACT.

ABCMeta performs this write for every newly created class. For a concrete subclass of an ABC, the value is normally an empty set, so the abstract flag is already clear. Nevertheless, the current implementation briefly stops all other Python threads.

Benchmark
import argparse, gc, statistics, sys, threading, time, typing
from abc import ABC, abstractmethod
from collections.abc import Sized

def make(kind, n):
    if kind == "concrete":           
        for _ in range(n):
            class X(Sized):
                def __len__(self): return 0
    elif kind == "protocol":          
        for _ in range(n):
            class P(typing.Protocol):
                def m(self) -> int: ...
            class C(P):
                def m(self) -> int: return 0
    elif kind == "abstract":
        for _ in range(n):
            class Y(ABC):
                @abstractmethod
                def m(self): ...
    elif kind == "plain":
        for _ in range(n):
            class Z: pass
    else:
        raise SystemExit(f"unknown kind {kind}")

def one_round(kind, k, n):
    barrier = threading.Barrier(k + 1)
    def worker():
        barrier.wait()
        make(kind, n)
    ts = [threading.Thread(target=worker) for _ in range(k)]
    for t in ts: t.start()
    barrier.wait()
    t0 = time.perf_counter()
    for t in ts: t.join()
    return time.perf_counter() - t0

def main():
    ap = argparse.ArgumentParser()
    ap.add_argument("--kind", default="concrete")
    ap.add_argument("--threads", type=int, default=1)
    ap.add_argument("--n", type=int, default=2000)
    ap.add_argument("--repeat", type=int, default=5)
    ap.add_argument("--no-gc", action="store_true")
    a = ap.parse_args()
    if a.no_gc:
        gc.disable()
    make(a.kind, 50)                  
    per = [one_round(a.kind, a.threads, a.n) / (a.n * a.threads) * 1e6
           for _ in range(a.repeat)]
    gil = getattr(sys, "_is_gil_enabled", lambda: True)()
    print(f"{a.kind},{a.threads},{a.n},{statistics.median(per):.2f},{min(per):.2f},{gil}")

if __name__ == "__main__":
    main()

Multiple threads run make_classes(2000) concurrently. The benchmark reports the median time per created class over three repetitions. It also includes three controls:

  • protocol: a typing.Protocol and a concrete subclass;
  • abstract: an ABC subclass that remains abstract;
  • plain: a class that does not use ABCMeta.
Implementation

In type_set_abstractmethods(), compare the requested abstract state with the current Py_TPFLAGS_IS_ABSTRACT flag before entering the stop-the-world section:

if (abstract != ((type->tp_flags & Py_TPFLAGS_IS_ABSTRACT) != 0)) {
    type_lock_prevent_release(&pinned);
    types_stop_world();
    if (abstract)
        type_add_flags(type, Py_TPFLAGS_IS_ABSTRACT);
    else
        type_clear_flags(type, Py_TPFLAGS_IS_ABSTRACT);
    types_start_world();
    type_lock_allow_release(&pinned);
}
Performance results

Each result below is the median of five outer rounds. Each benchmark invocation
uses three inner repetitions and creates 2000 classes per thread.

Case Threads Baseline Patched Speedup Spread, baseline / patched
concrete 1 6.90 6.46 1.07x ±24% / ±14%
concrete 2 920.77 6.06 151.94x ±15% / ±11%
concrete 4 704.21 6.18 113.95x ±3% / ±8%
concrete 8 490.60 7.18 68.33x ±6% / ±11%
concrete 16 348.28 8.45 41.22x ±6% / ±6%
protocol 1 22.62 21.65 1.04x ±7% / ±5%
protocol 2 2801.11 1038.10 2.70x ±4% / ±2%
protocol 4 2463.42 1111.75 2.22x ±1% / ±0%
protocol 8 1963.49 1028.07 1.91x ±1% / ±1%
protocol 16 1397.80 783.15 1.78x ±3% / ±4%
abstract 1 6.62 6.93 0.96x ±16% / ±12%
abstract 2 885.09 894.81 0.99x ±10% / ±10%
abstract 4 636.98 632.91 1.01x ±1% / ±3%
abstract 8 445.71 461.46 0.97x ±6% / ±5%
abstract 16 329.88 333.72 0.99x ±9% / ±3%
plain 1 3.74 3.95 0.95x ±20% / ±39%
plain 2 3.46 3.67 0.94x ±13% / ±11%
plain 4 3.91 3.98 0.98x ±12% / ±7%
plain 8 4.16 4.32 0.96x ±7% / ±7%
plain 16 4.16 4.14 1.00x ±13% / ±3%
Linked PRs
  • gh-161745
Langage dominant
Python
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Forks
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Merge moyen
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PR mergées (30 j)
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