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 👋🏼