Speed up Deferred: isinstance() is slow

[itamarst]

Deferred callback does an instance(result, Deferred) in the hot path, which is a pretty expensive operation.

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IIRC we used to have a type(...) is check here, and changed it because we didn't have benchmarks to justify it being all that slow, at around the time DeferredList got added. I gather inheriting from Awaitable might have changed the calculus here?

[arnimarj]

I got curious and created this:

import timeit


setup = 'from twisted.internet.defer import Deferred\nx = Deferred()\nb=False'

fast = '''
if x.__class__ is Deferred or isinstance(x, Deferred):
	b = True
'''


slow = '''
if isinstance(x, Deferred):
	b = True
'''

N = 10_000_000

T = timeit.timeit(stmt=fast, number=N, setup=setup)
print(T)

T = timeit.timeit(stmt=slow, number=N, setup=setup)
print(T)

...which gave me approximately this:

0.09645763300068211
0.1443577739992179

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I got curious and created this:

The problem with this benchmark is that it goes faster only if x is a Deferred. In the case we are doing the isinstance check, it's actually much more common that x is not a Deferred.