logging: Logger.setLevel() and logging.disable() are O(number of loggers) #160936

Bug report

Every Logger.setLevel() and logging.disable() call goes through Manager._clear_cache(), which walks the whole loggerDict and clears each logger's isEnabledFor cache, even the ones that never cached anything. So setting the level on N loggers one by one is quadratic. That shows up in apps or test suites with thousands of loggers (one per module/plugin/connection) that get configured individually.

import logging, time

for n in (1000, 2000, 4000, 8000):
    logging.root.manager.loggerDict.clear()
    loggers = [logging.getLogger(f"app.mod{i}") for i in range(n)]
    for lg in loggers:
        lg.isEnabledFor(logging.DEBUG)
    t = time.perf_counter()
    for lg in loggers:
        lg.setLevel(logging.INFO)
    print(f"{n:5d} loggers: {time.perf_counter() - t:.3f} s")

On 3.14.7 this gives about 0.23 s at 2000 loggers, 0.89 s at 4000 and 4.4 s at 8000 (each doubling is about 4x).

A fix is to have the manager remember which loggers actually have something in their cache (a WeakSet filled on the isEnabledFor cache-miss path, which already holds the lock) and only clear those. The cache-hit path doesn't change. I have a patch ready and can open a PR.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-160936: Make logging setLevel() and disable() cost O(cached loggers) #160937