Event Loop¶
Uvicorn provides two event loop implementations that you can choose from using the --loop option:
uvicorn main:app --loop <auto|asyncio|uvloop>
By default, Uvicorn uses --loop auto, which automatically selects:
- uvloop - If uvloop is installed, Uvicorn will use it for maximum performance
- asyncio - If uvloop is not available, Uvicorn falls back to Python's built-in asyncio event loop
Since uvloop is not compatible with Windows or PyPy, it is not available on these platforms.
On Windows, the asyncio implementation uses ProactorEventLoop if running with multiple workers,
otherwise it uses the standard SelectorEventLoop for better performance.
Why does SelectorEventLoop not work with multiple processes on Windows?
If you want to know more about it, you can read the issue #cpython/122240.
Custom Event Loop¶
You can use custom event loop implementations by specifying a module path and function name using the colon notation:
uvicorn main:app --loop <module>:<function>
The function should return a callable that creates a new event loop instance.
rloop¶
rloop is an experimental AsyncIO event loop implemented in Rust on top of the mio crate. It aims to provide high performance through Rust's systems programming capabilities.
You can install it with:
pip install rloop
uv add rloop
You can run uvicorn with rloop with the following command:
uvicorn main:app --loop rloop:new_event_loop
Experimental
rloop is currently experimental and not suited for production usage. It is only available on Unix systems.
Winloop¶
Winloop is an alternative library that brings uvloop-like performance to Windows. Since uvloop is based on libuv and doesn't support Windows, Winloop provides a Windows-compatible implementation with significant performance improvements over the standard Windows event loop policies.
You can install it with:
pip install winloop
uv add winloop
You can run uvicorn with Winloop with the following command:
uvicorn main:app --loop winloop:new_event_loop