asyncio: run newly scheduled callbacks eagerly before polling for I/O #160760

Feature or enhancement

Currently, BaseEventLoop._run_once() runs only the callbacks that were ready when the iteration started. Callbacks scheduled by those callbacks wait for the next iteration, which first polls the selector (select()/epoll_wait()) with a zero timeout. Code that resolves futures, wakes tasks or chains call_soon() therefore pays for one I/O poll per hop, even when there is no I/O to process.

The good example is using asyncio.Queue: await q.put(item) pushes an item into the queue and wakes up the getter on the next event loop iteration only.
The same is true with network protocols: StreamReaderProtocol.data.received() feeds data to StreamReader and wakes await reader.read() after selector.select() call. Custom code that works with asyncio protocols (or loop.sock_recv()) explicitly suffers from the same problem.
If the code uses more moving parts, e.g. protocol->message-parser->user-code, each indirection level adds its own delayed call that increases latency.

I propose letting the loop keep running newly scheduled callbacks for a short, bounded time before it polls for I/O again (“eager processing”).

If after running scheduled callbacks (asyncio.Handler instances) there are new callbacks registered for the next loop iteration, they are executed immediately.

The implementation adds time- and count-based limits for eager callback execution for preventing possible infinite callbacks chain.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/an-eager-way-to-set-result-on-asyncio-future/106161/27

Linked PRs

• gh-160760: asyncio: run newly scheduled callbacks eagerly before polling for I/O #145149