New submission from Mark Shannon <m...@hotpy.org>:
class C:
def __init__(self, cond):
if cond:
self.a = 1
self.b = 2
c1 = C(True)
c2 = C(False)
In Python 3.5, the dictionary keys are shared
---------------------------------------------
>>> sys.getsizeof(c2)
56
>>> sys.getsizeof(c1.__dict__)
96
>>> sys.getsizeof(c2.__dict__)
96
In Python 3.9, the keys are not shared
--------------------------------------
>>> sys.getsizeof(c2)
48
>>> sys.getsizeof(c1.__dict__)
272
>>> sys.getsizeof(c2.__dict__)
232
This represents an increase of memory use for c1 of 110%. (48+272)/(56+96) ==
2.1
With compact object layout (https://github.com/faster-cpython/ideas/issues/69),
any failure to share keys will result in a tripling of memory use per object.
It is not an uncommon pattern for some attributes to initialized lazily,
which causes also prevents key-sharing.
Not only does it increase memory use, it prevents optimizations that attempt to
specialize attribute access for instances of a class, as different instances
will have different layouts.
The purpose of compact dicts was to save memory, but in this case we are using
a lot more memory by preventing PEP 412 from working.
----------
keywords: 3.9regression
messages: 398475
nosy: Mark.Shannon
priority: normal
severity: normal
status: open
title: Regression in memory use of instances due to dictionary ordering
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue44772>
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