Issue 32348: Optimize asyncio.Future schedule/add/remove callback (original) (raw)

Key observation: 99.9% of the time Futures and Tasks have only one callback.

Currently we have a list of callbacks in each Future/Task. We can avoid list object allocation if we add a field to store the first callback. This way we'll only need to ever allocate the list if a Future/Task has more than one done callbacks.

asyncio with the proposed patch applied shows that 3-6% better performance of an asyncio echo server implemented using asyncio.streams. This benchmark involves quite a bit of other Python code, so this improvement is actually quite significant.

The patch consists of:

1. first callback / callbacks list refactoring.

2. a free list implementation for Future.await objects (which are created on almost all awaits in asyncio).

3. a big cleanup of the code, ensuring that Future/Task are always initialized properly.