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uvloop may sleep less than `n` for asyncio.sleep(n)

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
backend

Research direction

Start by running the provided reproduction with the default asyncio loop and uvloop, then trace how uvloop handles asyncio.sleep timers. Compare the measured duration against the requested delay and add a regression test for the minimum sleep time. Done means asyncio.sleep(n) does not return before n has elapsed.

Written by the indexing model from the issue text.

Description

Bug Description

When calling asyncio.sleep, uvloop sometimes sleeps for fractionally less then the requested sleep time. E.g. await asyncio.sleep(0.1) -> Sleeps for 0.0991.

Expected behavior

Should always sleep for at least the specified time.

Environment

  • Operating System: Fedora 43
  • Python Version: Python 3.13.12
  • uvloop Version: 0.22.1

Steps to Reproduce

Run the following script and observe output:

import asyncio
import statistics
import time

async def main():
    sleep_times = []
    for _ in range(100):
        start_time = time.perf_counter()
        await asyncio.sleep(0.1)
        end_time = time.perf_counter()
        sleep_times.append(end_time - start_time)
    print(f"Min sleep time: {min(sleep_times):.4f} seconds")
    print(f"Max sleep time: {max(sleep_times):.4f} seconds")
    print(f"Average sleep time: {statistics.mean(sleep_times):.4f} seconds")

if __name__ == "__main__":
    print("With default asyncio event loop:")
    asyncio.run(main())
    print("With uvloop event loop:")
    import uvloop
    asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
    asyncio.run(main())

Output:

With default asyncio event loop:
Min sleep time: 0.1001 seconds
Max sleep time: 0.1008 seconds
Average sleep time: 0.1002 seconds
With uvloop event loop:
Min sleep time: 0.0991 seconds
Max sleep time: 0.1011 seconds
Average sleep time: 0.1001 seconds
Dominant language
Cython
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