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

[sjmonson]

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