uvloop is grouping responses to multiple requests/messages

• uvloop version: 0.20.0
• Python version: 3.12.5
• Platform: Ubuntu 22.04
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?:
• Does uvloop behave differently from vanilla asyncio? How?: Yes. It groups responses, and this behavior is not observed when using asyncio.

I am opening this issue from an issue that I had previously opened in the uvicorn repository

When running a FastAPI app with uvicorn default settings, which uses uvloop, I started to notice that sometimes when handling multiple requests for the same path the response took longer than expected to arrive at the client. This issue became more noticeable when I tried using WebSockets to improve performance by returning the result in parts over the same connection instead of making multiple HTTP requests.

I made a mini FastAPI app that represents the situation of my app, a WebSocket client, and a HTTP Client.

I will try to work on a Minimal Reproducible Example.

Example Code

FastAPI App Code (server.py)

import time

from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket("/ws")
async def websocket_hadler(websocket: WebSocket):
    await websocket.accept()
    title = await websocket.receive_text()
    start = time.time()
    await websocket.send_json(
        {"id": title, "status": "start", "time": time.time() - start}
    )
    for i in range(3):
        time.sleep(2)
        await websocket.send_json(
            {
                "id": title,
                "part": i,
                "time": time.time() - start,
            }
        )
    await websocket.send_json({"id": title, "status": "end", "time": time.time() - start})
    await websocket.close()

@app.get("/slow")
async def slow_endpoint():
    start = time.time()
    time.sleep(3)
    return {"time": time.time() - start}

HTTP Client Code (http.py)

import requests
import time
from threading import Thread
from multiprocessing import Process

def req(exec_id):
   start = time.time()
   response = requests.get(f"http://localhost:8000/slow")
   end = time.time()
   print({
       "id": exec_id,
       "client": end-start,
       "server": response.json()
   })

print("Threads")
for i in range(4):
   t = Thread(target=req, args=(i,))
   t.start()

# print("Process")
# for i in range(4):
#     p = Process(target=req, args=(i,))
#     p.start()

WebSocket Client Code (ws.py)

import time
import json
from websockets.sync.client import connect

with connect("ws://localhost:8000/ws") as websocket:
   websocket.send("uvloop") # or "asyncio"
   start = time.time()
   t = {}
   while t.get("status") != "end":
       t = json.loads(websocket.recv())
       print(t)
       print("----------------------------")
       print(f"Client Time: {time.time() - start}")
       print("#############################")

Experiment Instructions

Start server with uvloop (default)

uvicorn server:app --port 8000

Run clients

python http.py

python ws.py

Start server with asyncio

uvicorn server:app --port 8000 --loop asyncio

Run clients

python http.py

python ws.py

Results

uvloop

HTTP (some times the times are OK -> 3, 6, 9 and 12)

Threads
{'id': 2, 'client': 3.0042872428894043, 'server': {'time': 3.0001935958862305}}
{'id': 0, 'client': 9.011125087738037, 'server': {'time': 3.0001726150512695}}
{'id': 1, 'client': 9.00871992111206, 'server': {'time': 3.000185012817383}}
{'id': 3, 'client': 12.006998300552368, 'server': {'time': 3.0001895427703857}}

WebSockets

{'id': 'uvloop', 'status': 'start', 'time': 9.5367431640625e-07}
----------------------------
Client Time: 6.002939939498901
#############################
{'id': 'uvloop', 'part': 0, 'time': 2.0002377033233643}
----------------------------
Client Time: 6.003247022628784
#############################
{'id': 'uvloop', 'part': 1, 'time': 4.000699281692505}
----------------------------
Client Time: 6.003509283065796
#############################
{'id': 'uvloop', 'part': 2, 'time': 6.001102924346924}
----------------------------
Client Time: 6.00377082824707
#############################
{'id': 'uvloop', 'status': 'end', 'time': 6.001464366912842}
----------------------------
Client Time: 6.004014253616333
#############################

asyncio

HTTP (the grouping behavior was not observed in any attempt)

Threads
{'id': 2, 'client': 3.0055391788482666, 'server': {'time': 3.0002896785736084}}
{'id': 0, 'client': 6.010128021240234, 'server': {'time': 3.000213384628296}}
{'id': 1, 'client': 9.011209487915039, 'server': {'time': 3.000107526779175}}
{'id': 3, 'client': 12.008730173110962, 'server': {'time': 3.0002121925354004}}

WebSockets

{'id': 'asyncio', 'status': 'start', 'time': 7.152557373046875e-07}
----------------------------
Client Time: 0.0005409717559814453
#############################
{'id': 'asyncio', 'part': 0, 'time': 2.0002756118774414}
----------------------------
Client Time: 2.0013813972473145
#############################
{'id': 'asyncio', 'part': 1, 'time': 4.0009095668792725}
----------------------------
Client Time: 4.0020751953125
#############################
{'id': 'asyncio', 'part': 2, 'time': 6.001420021057129}
----------------------------
Client Time: 6.0024802684783936
#############################
{'id': 'asyncio', 'status': 'end', 'time': 6.001809120178223}
----------------------------
Client Time: 6.003089427947998
#############################

Environment

I am using Ubuntu 22.04 and a Python 3.12.5 environment created via conda (4.10.3).
pip freeze output:

annotated-types==0.7.0
anyio==4.5.0
certifi==2024.8.30
charset-normalizer==3.3.2
click==8.1.7
dnspython==2.6.1
email_validator==2.2.0
fastapi==0.115.0
fastapi-cli==0.0.5
gevent==24.2.1
greenlet==3.1.0
h11==0.14.0
httpcore==1.0.5
httptools==0.6.1
httpx==0.27.2
idna==3.10
Jinja2==3.1.4
markdown-it-py==3.0.0
MarkupSafe==2.1.5
mdurl==0.1.2
pydantic==2.9.2
pydantic_core==2.23.4
Pygments==2.18.0
python-dotenv==1.0.1
python-multipart==0.0.9
PyYAML==6.0.2
requests==2.32.3
rich==13.8.1
setuptools==75.1.0
shellingham==1.5.4
sniffio==1.3.1
starlette==0.38.5
typer==0.12.5
typing_extensions==4.12.2
urllib3==2.2.3
uvicorn==0.30.6
uvloop==0.20.0
watchfiles==0.24.0
websocket==0.2.1
websockets==13.0.1
wheel==0.44.0
zope.event==5.0
zope.interface==7.0.3

Beginne damit, das Beispiel aus server.py mit http.py und ws.py unter uvicorns standardmäßigem Loop auszuführen, und wiederhole es anschließend wie beschrieben mit --loop asyncio. Vergleiche die gemeldete Antwortzeit und Gruppierung und reduziere das Beispiel zunächst auf einen minimalen reproduzierbaren Fall. Erledigt ist die Aufgabe, wenn das Verhalten von uvloop reproduziert ist und ein fokussierter Regressionstest die korrigierte Zustellung der Antworten nachweist.