Slot `asyncio.Future`

[jonathandung]

Feature or enhancement

Proposal:

When many async futures are created, for example in an asyncio.Queue, having __slots__ on the class declaration on asyncio.Future, both the pure Python fallback version and the C-accelerated version, may save memory usage.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

[picnixz]

Using pympler.asizeof (I think that is the package name), can you benchmark this first? making it a slotted class may break composition or subclasses in the wild.

[picnixz]

I also do not think this is a real bottleneck in general:

• If there are lots of attributes -> not really useful IMO.
• If there are few attributes -> may cause breakage.
• Memory of futures is usually not the bottleneck. It is the result they carry.

[jonathandung]

A list of 10000 instances of _PyFuture:

Unslotted: 2731568
Slotted: 1845272

[picnixz]

A list of 10000 instances of _PyFuture:

Are they empty futures? if so? how were they benchmarked? don't just drop numbers in general, explain how they were obtained first please.

I want to know if real-world applications would benefit from this. The possibility of breaking APIs exists and this is a stop gap in general. With slots it won't be possible to alter futures directly either which may also be a stop gap for mocking.

[jonathandung]

Sorry for the lack of clarity. I instantiated to the futures and collected them into a list in a list comprehension.

I agree that the change would be breaking. However, I'm not sure how to proceed with the benchmarking due to my inexperience. (For example, what results/exceptions to set on the futures to reflect a real-world scenario? Should they be homogeneous?)

I would also like to find code defining subclasses of Future employing multiple inheritance with a parent having non-empty __slots__ in public code, since slotting Future will break that, but I don't know of a way to do so.

[picnixz]

I would also like to find code defining subclasses of Future employing multiple inheritance with a parent having non-empty slots in public code, since slotting Future will break that, but I don't know of a way to do so.

You can search github public projects using https://grep.app/.

---

Now, I don't think it's really important to use slotted classes here. We add 100 bytes per instance (going from 184 to 273) so it's not a drastic change and I don't think it's worth it. I also don't think there is anything actionable that we can do here so I am closing this issue as not planned. If you want to suggest improvements in the future, please ensure first that they are not breaking. A breaking change for performance's sake is usually rejected unless there is a really good reason.

Also, is it really common to have 10k futures living simultaneously on a memory-restricted environment? I don't think so (but I may be wrong) so I think we are chasing the wrong issue.