Memory issues with newer versions of Uvicorn

[Apakottur]

Hello and thank you for this great project.

We are using running a Uvicorn + Starlette server, which has been stable for years now without any issues.
We recently tried to upgrade from uvicorn[standard]==0.38.0 to uvicorn[standard]==0.40.0, but that results in our server crashing due to it running out of memory.

Here's how it looks on AWS ECS:

At 11:15 we deploy the new uvicorn version, and you can see that the memory usage never stabilizes, resulting in the server crashing.
The uvicorn update is the only change between these two deployments, so we are almsot certain that it's the root problem.

This is how we run uvicorn:

app = create_starlette_app()
uvicorn.run(
        app,
        host=HOST,
        port=PORT
        log_level="warning",
        access_log=False,
        timeout_keep_alive=120,
        ws="websockets-sansio",
        ws_ping_interval=20,
        ws_ping_timeout=20,
    )

We are using both REST API and Websockets on this server.

Has there been any recent changes that might lead to this? How can we debug this further?

[Kludex]

Can you try this: #2795 ?

[Apakottur]

Here's the results:

1. Left - stable on 0.38.0
2. Middle - Revert "fix(websockets): Send close frame on ASGI return (#2769)" #2795, memory leaking.
3. Right - Revert "explicitly start ASGI run with empty context (#2742)" #2799, no mem leak but one of the containers crashed due to a segfault, not sure if it's related:

File "/.venv/lib/python3.13/site-packages/uvicorn/main.py", line 594 in run
File "/.venv/lib/python3.13/site-packages/uvicorn/server.py", line 67 in run
File "/usr/local/lib/python3.13/asyncio/runners.py", line 195 in run
File "/usr/local/lib/python3.13/asyncio/runners.py", line 118 in run
Current thread 0x00007f37350a7b80 (most recent call first):
<no Python frame>
Thread 0x00007f373254f6c0 (most recent call first):
<no Python frame>
Thread 0x00007f36ef9d76c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36eefd66c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36ee5d56c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36edbd46c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36ecbd36c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36cbeff6c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36cb4fe6c0 (most recent call first):
Fatal Python error: Segmentation fault

File "/.venv/lib/python3.13/site-packages/uvicorn/main.py", line 594 in run
File "/.venv/lib/python3.13/site-packages/uvicorn/server.py", line 67 in run
File "/usr/local/lib/python3.13/asyncio/runners.py", line 195 in run
File "/usr/local/lib/python3.13/asyncio/runners.py", line 118 in run
Current thread 0x00007f37350a7b80 (most recent call first):
<no Python frame>
Thread 0x00007f373254f6c0 (most recent call first):
<no Python frame>
Thread 0x00007f36ef9d76c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36eefd66c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36ee5d56c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36edbd46c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36ecbd36c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36cbeff6c0 (most recent call first):
File "/usr/local/lib/python3.13/threading.py", line 1015 in _bootstrap
File "/usr/local/lib/python3.13/threading.py", line 1044 in _bootstrap_inner
File "/usr/local/lib/python3.13/threading.py", line 995 in run
File "/usr/local/lib/python3.13/concurrent/futures/thread.py", line 90 in _worker
Thread 0x00007f36cb4fe6c0 (most recent call first):
Fatal Python error: Aborted

[Kludex]

It seems the only change they did was to bump uvicorn in those deployments, and I've given two PRs reverting changes.

We need to confirm if #2742 has a leak cc @pmeier

[Kludex]

I spent some time on this, and I'm still not able to reproduce it. What is your Python version (with patch included)? Based on the evidence, it seems it's not related to websockets, but to HTTP, so the httptools protocol is being used, and I'm not sure there's a leak, but it seems that the context is being created more than once - I'm not sure if in the same connection, given that the keep alive seems to have a high number - which may indicate why we didn't have any other user reporting this.

[Apakottur]

Thanks for looking into this.
Our Python version is Python 3.13.11 (main, Feb 3 2026, 03:10:57) [GCC 12.2.0] on linux.

Is there anything we can do on our end to investigate this further?
We will just stay pinned on 0.38.0 for now.

[Kludex]

I tested on 3.13.0, and I'm on darwin, I'll try to match the exact version and check.

[pmeier]

@Kludex Happy to help out as well. I can bring a linux box if you want to test something specific.

[Kludex]

@pmeier I was trying to find out if there was a leak by specifically checking the keep alive logic in the HTTP protocol implementations (h11_impl.py and httptools_impl.py) - More specifically with a big contextvar.

Any help is welcome.