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## Summary This PR consolidates the concurrency limits used throughout `uv` and exposes two limits, `UV_CONCURRENT_DOWNLOADS` and `UV_CONCURRENT_BUILDS`, as environment variables. Currently, `uv` has a number of concurrent streams that it buffers using relatively arbitrary limits for backpressure. However, many of these limits are conflated. We run a relatively small number of tasks overall and should start most things as soon as possible. What we really want to limit are three separate operations: - File I/O. This is managed by tokio's blocking pool and we should not really have to worry about it. - Network I/O. - Python build processes. Because the current limits span a broad range of tasks, it's possible that a limit meant for network I/O is occupied by tasks performing builds, reading from the file system, or even waiting on a `OnceMap`. We also don't limit build processes that end up being required to perform a download. While this may not pose a performance problem because our limits are relatively high, it does mean that the limits do not do what we want, making it tricky to expose them to users (https://github.com/astral-sh/uv/issues/1205, https://github.com/astral-sh/uv/issues/3311). After this change, the limits on network I/O and build processes are centralized and managed by semaphores. All other tasks are unbuffered (note that these tasks are still bounded, so backpressure should not be a problem). |
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