multiprocessing.connection.wait high CPU load with empty object_list

[danielhrisca]

Bug report

Bug description:

 from multiprocessing.conenction import wait
 import os
 from time import time
 
 print(os.getpid())
 start = t
 
 while True:

    for pipe in wait([], 1):
        pass
    print(time()-t)

multiprocessing.connection.wait causes 100% CPU load if the object list is empty; otherwise the CPU load very low

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Linked PRs

• gh-145587: fix busy loop in multiprocessing.connection.wait on Windows #145597

[danielhrisca]

I've also tested with Python 3.12.3 on Linux and there the CPU load is very low with an empty object_list

[YvesDup]

Could you provide a runnable and complete example, please ?

[danielhrisca]

I've updated the example. The CPU load is high even without the print statement

If I run it on Linux I see that the wait call takes in the order of ms

35.23168587684631
36.233556509017944
37.2396240234375
38.24300479888916
39.2457799911499
40.2487576007843
41.33868384361267
42.34193706512451
43.34582734107971
44.3491108417511

The same example when running on Windows produces this output

59.44313287734985
59.44317555427551
59.443217039108276
59.44330191612244
59.44336795806885
59.44341683387756
59.44345951080322
59.44350242614746
59.443572759628296
59.443615674972534
59.4436571598053

[Shrey-N]

Hiya @danielhrisca I have verified this issue with a script on Windows.

The issue is in Lib/multiprocessing/connection.py,
While the Unix implementation uses selectors.select, the Windows version attempts to build a handle list. If that list is empty, it returns an empty result immediately without ever hitting a blocking call.

I am submitting a PR in a minute that adds a check for an empty object_list to ensure the function properly respects the timeout before returning.

Reproduction Script:-

from multiprocessing.connection import wait
import time

start = time.time()
# This should take approx 1 second, but returns instantly on Windows
wait([], 1.0) 
print(f"Elapsed: {time.time() - start:.4f}s")

Hope the PR, and explanation helps :)