[Core] Zero-copy asdict for InputMetadata

[Yard1]

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dataclasses.asdict will deepcopy tensors inside InputMetadata, leading to an avoidable overhead. This PR adds a new method to InputMetadata which will return a dict of fields and their values without copy.

Before:

After:

[simon-mo]

can you just do input_metadata.__dict__ to avoid the new method at all?

[Yard1]

Perhaps, though that may lead to unexpected interactions if InputMetadata has extra properties added and such.

[youkaichao]

No, because the post init adds self.attn_bias = None.

[youkaichao]

It seems the deepcopy of dataclasses.asdict is notorious, see python/cpython#88071 .

How about moving to attrs? https://www.attrs.org/en/stable/index.html it is not a builtin package, but just a third-party library.

from attrs import asdict, define
@define
class MyDataClass:
    a: int
    b: list
instance = MyDataClass(1, [2, 3])
data1 = asdict(instance, recurse=False)
assert data1['b'] is instance.b

Its asdict has a recursive parameter.

[youkaichao]

Here is the whole picture:

import attrs
@attrs.define(slots=False)
class MyDataClass:
    a: int
    b: list
    def __attrs_post_init__(self):
        self.c = None
instance = MyDataClass(1, [2, 3])
data1 = asdict(instance, recurse=False)
assert data1['b'] is instance.b # make sure it is shallow copy
assert "c" not in data1 # make sure asdict only contains annotated fields

[Yard1]

I don't think it makes sense to add a third-party library just for this one specific usecase. We can always revisit this if we find ourselves implementing this pattern more times.

[youkaichao]

Oh, attrs is quite a famous library, and it motivates the builtin dataclasses.

One reason that attrs does not become builtin, is because attrs does not want to be bound with python versions. So you can basically think attrs as an advanced replacement of dataclasses.

e.g. when you want to use new features from dataclasses, you have to upgrade python interpreter; however, with attrs, you just need to update a package.

I vote for attrs because later I plan to refactor vllm/config.py to use dataclass-like features (so that we can easily print/log them for debugging). And I might switch to attrs in all the places.

Open to any feedback :)

[Yard1]

I think it's fine if your PR makes use of it, but I don't think it's necessary to couple attrs with this particular PR :)

[youkaichao]

Then please make sure to rename asdict_zerocopy to _asdict_zerocopy, so that users know this is private and they won't call it.

[WoosukKwon]

Then please make sure to rename asdict_zerocopy to _asdict_zerocopy, so that users know this is private and they won't call it.

I think it should be asdict_zerocopy since it's used outside InputMetadata?

[WoosukKwon]

LGTM! Thanks for finding the performance bug. Didn't know the issue in dataclasses.asdict. TIL 😄

[WoosukKwon]

@Yard1 It seems the CI failed. Could you please double check?

[Yard1]

yeah let me see

[Yard1]

Should be good now!