astral-sh/uv

Investigate why uv holds so many file handles open

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#17512 opened on Jan 16, 2026

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Description

We got reports that uv overruns the default open files limit (ulimit) for users on Linux and macOS:

The default ulimits can be low, for example 1024 on linux, and we spawn up to a thread per core and several subprocesses. However, Cargo and other Rust tools which have very similar workloads and often very similar problems to us, don't have any problems with ulimits. A cargo maintainer told us:

From my testing for fine grain locking (unstable feature), I found that Cargo generally peaks with around ~70-80 file descriptors opened at once, and this remainly mostly static even for large projects.

So the question is, why are we running into the ulimit, while Cargo doesn't? What file descriptors is uv even holding, do we need them? Is Cargo doing something different that we can adopt to avoid this?

This requires some Unix expertise and rust parallelism knowledge, but should otherwise not require much uv specific details, this is mostly around figuring out how to analyze this this in uv and determining the differences to Cargo.

Contributor guide

Tech stack
rust
Domain
performance
Issue type
research
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Unix file descriptor basicsRust async/threading knowledgeUnderstanding of Cargo's behavior
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Start by profiling uv's file descriptor usage during common operations like `uv sync` and `uv build`. Use tools like `lsof` and `strace` to list open FDs. Compare with Cargo's behavior under similar workloads. Investigate uv's thread pool and subprocess spawning code in the repository, particularly in files like `crates/uv client/` or `crates/uv installer/`. Look at Cargo's source for how they manage file descriptors (e.g., using `openat` or other syscalls). Report findings with suggestions for reducing FD count.