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

Aperta
  • Qualcosa non va?
#161,744 0 commenti 0 reazioni 0 assegnatari Vedi su GitHub

I maintainer di solito rispondono entro 1 giorno

Nessuno ha ancora preso questa issue.

Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
c, python
Ambito
backend, performance

Direzione di ricerca

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.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

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
Lingua principale
Python
Stelle
78.2k
Fork
36.6k
Merge medio
19h 57m
PR unite (30g)
630

Preparare l'ambiente

Apri in Codespaces

Avvia il container di sviluppo del progetto nel browser, con il tuo account GitHub.

Come iniziare

  1. Leggi tutta la issue e poi la guida ai contributi del progetto.
  2. Commenta sulla issue per dire che te ne occupi tu — evita che due persone facciano lo stesso lavoro.
  3. Fai un fork del repository e lavora su un branch.
  4. Apri una pull request che faccia riferimento al numero della issue.

Altre issue di python/cpython

Tutte le issue di python/cpython

Issue simili

Altre issue su Python

Ricevi le nuove issue nella tua casella

Un breve riepilogo di issue GitHub adatte ai principianti.