Slow startup caused by watch exclusion when --reload-excludes flag points to a very large folder

[smac89]

Hello, I'm here to report an issue I encountered with a framework (litestar) which makes use of uvicorn. You can follow the full discussion here. I will just give a tl;dr; below

Description

When starting the development uvicorn server, the initial startup time takes about 2 minutes. I initially thought this was due to running the project via WSL, and therefore the filesystem virtualization may be the root cause, but I was wrong.

Setup

After much testing, it was discovered that the issue was more isolated, and was related to how uvicorn handles excluded files (--reload-exclude flag). In this case, it was instructed to exclude all files inside a virtual environment folder created by poetry.

Problem analysis

It seems to me that when uvicorn is instructed to reload on change, and a list of excluded files are passed to it, it starts doing some computationally intensive work during those moments, which causes all other operations to hang. I say this because I ran the program through strace and for the last 70-90% of those 2 minutes, no system calls are being made and nothing is printing to the console, which means it's either doing nothing or trying to do "everything". I didn't check system resources to know what the case might be, but I'm willing to bet it was simply crunching away on some algorithm.

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Anyways, that's all I have for that. I'll do my best to respond to clarifications. Have a nice day 👋🏼

[novain-shibu]

I think I found the cause of this. In resolve_reload_patterns (uvicorn/config.py), after the include/exclude globs are expanded, nested directories are removed by comparing every pair of directories:

for j in range(len(directories)):
    for k in range(j + 1, len(directories)):
        if directories[j] in directories[k].parents: ...

That's O(N²) Path.parents checks. The exclude from the linked Litestar thread, .venv/**/*, makes the glob return every directory inside the virtualenv, so N is in the thousands. It's pure CPU work, which matches the silent part you saw in strace, and it explains why removing the exclude made startup instant.

With uvicorn's own function on a venv-like tree (same results before/after):

directories	before	after
500	5.3 s	0.09 s
1000	21 s	0.19 s
2000	120 s	0.39 s

Sorting by depth and checking each directory's parents against a set of already-kept directories gives the same result in one pass. I'll open a PR with this and a test.