- 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
Start server with asyncio
uvicorn server:app --port 8000 --loop asyncio
Run clients
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
PYTHONASYNCIODEBUGin env?: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)
HTTP Client Code (http.py)
WebSocket Client Code (ws.py)
Experiment Instructions
Start server with uvloop (default)
Run clients
Start server with asyncio
Run clients
Results
uvloop
HTTP (some times the times are OK -> 3, 6, 9 and 12)
WebSockets
asyncio
HTTP (the grouping behavior was not observed in any attempt)
WebSockets
Environment
I am using Ubuntu 22.04 and a Python 3.12.5 environment created via conda (4.10.3).
pip freeze output: