uvloop¶

uvloop is a libuv-based drop-in replacement for the stdlib asyncio event loop. On Linux and macOS it typically trims 20–30% off per-wakeup latency for serial I/O on top of the stdlib selector — see Performance for measured numbers.

Install¶

uv add "anyserial[uvloop]"
# or
pip install "anyserial[uvloop]"

The uvloop extra pins uvloop>=0.22.1 and is declared only for Linux and macOS installs. It is a no-op on Windows and BSD.

Activating it¶

uvloop is an asyncio loop implementation — the AnyIO backend name stays "asyncio", you just flip the uvloop option:

import anyio


async def main() -> None:
    ...


anyio.run(main, backend="asyncio", backend_options={"use_uvloop": True})

That's the whole story for async code.

For the sync wrapper, configure the process-wide portal once before the first open:

from anyserial.sync import configure_portal, SerialPort

configure_portal(backend="asyncio", backend_options={"use_uvloop": True})

with SerialPort.open("/dev/ttyUSB0") as port:
    ...

See Sync wrapper for the configure-before-first-open constraint.

When uvloop helps¶

Measured on Linux pty loops (single-byte p50 receive):

Backend	Receive p50	Receive max
asyncio + uvloop	99 µs	509 µs
asyncio (default)	126 µs	961 µs
trio	124 µs	781 µs

The median gap is ~20–30 µs; the tail gap is 2–3×. uvloop's wakeup scheduler is what moves the numbers — serial-I/O throughput itself is bound by the wire and the driver.

Use uvloop when any of these apply:

You're below ~500 µs round-trip budget on pty or loopback.
You're running many ports concurrently (the fan-out test in Performance shows sub-linear scale under uvloop).
You need predictable tail latency under an otherwise-busy loop.

Skip it when:

You're already under budget on stock asyncio.
Your application pulls in trio — pick trio directly; uvloop is asyncio-only.
You're on Windows or BSD. Use winloop on Windows via pip install "anyserial[winloop]" and the same use_uvloop=True switch; Windows coverage is experimental.

Caveats¶

uvloop sometimes regresses. It's tuned for sockets; serial fds use the same wait_readable / wait_writable plumbing but the win isn't automatic. Benchmark your workload.
Debugging. Stack traces through uvloop sometimes lose the asyncio frames you'd expect. Switch back to stock asyncio temporarily when chasing an obscure traceback.
Loop policy. uvloop installs a custom asyncio loop policy. If your codebase also manipulates the policy (asyncio.set_event_loop_policy), let uvloop go first — or let AnyIO handle it end-to-end.
Version floor. 0.22 or newer. Earlier releases had AnyIO 4.x compatibility gaps.

Checking at runtime¶

import asyncio

loop = asyncio.get_running_loop()
print(type(loop).__module__)  # 'uvloop.loop' when uvloop is active

For use inside anyserial code, prefer the backend-agnostic anyio.get_current_task / cancel-scope APIs — don't branch on loop type.