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GeezerVison is showing github.com — `_remote_debugging`: `get_child_pids()` iterates all processes four times · Issue #156060 · python/cpython

_remote_debugging: get_child_pids() iterates all processes four times #156060

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#156061
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_remote_debugging: get_child_pids() iterates all processes four times#156060
#156061
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OS-macextension-modulesC modules in the Modules dirC modules in the Modules dirtype-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

Description

@maurycy
maurycy
opened on Aug 19, 2026
Issue body actions

Bug report

Bug description:

Found while trying to understand the gcmon performance in #155828 (comment).

We treat proc_listpids() as a number of PIDs:

cpython/Modules/_remote_debugging/subprocess.c

Line 305 in 10a8454

int n_pids = proc_listpids(PROC_ALL_PIDS, 0, NULL, 0);

cpython/Modules/_remote_debugging/subprocess.c

Line 320 in 10a8454

int actual = proc_listpids(PROC_ALL_PIDS, 0, pid_list, buffer_size * sizeof(pid_t));

cpython/Modules/_remote_debugging/subprocess.c

Line 335 in 10a8454

for (int i = 0; i < actual; i++) {

...but it returns a number of bytes:

https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/libsyscall/wrappers/libproc/libproc.c#L62-L73

sizeof(pid_t) == 4, so it does 4x times more work than it needs to be.

This was changed in #144648.

Reproduction

2026-08-19T18:53:44.066591000+0200 maurycy@gimel /Users/maurycy/src/github.com/maurycy/cpython (main f40043e*?) % ps axlww | wc -l
     996
2026-08-19T18:53:46.142930000+0200 maurycy@gimel /Users/maurycy/src/github.com/maurycy/cpython (main f40043e*?) % sudo dtrace -q -n 'pid$target::proc_pidinfo:entry { @ = count(); }' -c "./python.exe repro.py"

             3996

for

import _remote_debugging, os
_remote_debugging.get_child_pids(os.getpid(), recursive=True)

Fix

I believe the simplest fix should be along the lines of:

int n_pids = proc_listpids(PROC_ALL_PIDS, 0, NULL, 0) / sizeof(pid_t);
...
int actual = proc_listpids(PROC_ALL_PIDS, 0, pid_list,
                            buffer_size * sizeof(pid_t)) / sizeof(pid_t);

(not exactly like that because proc_listpids() can return -1).

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

  • gh-156060: Do not scan every process four times in remote_debugging.get_child_pids() on macOS #156061
  • [3.15] gh-156060: Do not scan every process four times in remote_debugging.get_child_pids() on macOS (GH-156061) #156069

Activity

  1. maurycy
    added
    type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error
    on Aug 19, 2026
  2. aisk
    added
    extension-modulesC modules in the Modules dirC modules in the Modules dir
    OS-mac
    on Aug 19, 2026

  3. sergey-miryanov commented on Aug 19, 2026

    @sergey-miryanov
    sergey-miryanov
    on Aug 19, 2026
    Contributor
    More actions

    Good catch!

    Reacted by Maurycy Pawłowski-Wieroński
  4. pablogsal
    added a commit that references this issue on Aug 19, 2026

    gh-156060: Do not scan every process four times in `remote_debugging.…

    a2c9a3b
  5. pablogsal
    closed this as completedin #156061on Aug 19, 2026
  6. pablogsal
    added a commit that references this issue on Aug 19, 2026

    [3.15] gh-156060: Do not scan every process four times in `remote_deb…

    47724d8
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