uvloop is grouping responses to multiple requests/messages

Start by running the example from server.py with http.py and ws.py under uvicorn's default loop, then repeat with --loop asyncio as described. Compare the reported response timing and grouping, and first reduce the example to a minimal reproducible case. Done means the uvloop behavior is reproduced and a focused regression test demonstrates the corrected response delivery.

• 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