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Instance dictionaries should be created eagerly #88984

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markshannon opened this issue Aug 4, 2021 · 3 comments
Closed

Instance dictionaries should be created eagerly #88984

markshannon opened this issue Aug 4, 2021 · 3 comments
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@markshannon
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3.11 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) performance Performance or resource usage

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@markshannon
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BPO 44821
Nosy @terryjreedy, @markshannon
PRs
  • bpo-44821: Eagerly assign __dict__ for new objects. #27589
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = 'https://github.com/markshannon'
closed_at = <Date 2021-08-09.14:57:19.689>
created_at = <Date 2021-08-04.07:57:16.295>
labels = ['interpreter-core', '3.11', 'performance']
title = 'Instance dictionaries should be created eagerly'
updated_at = <Date 2021-08-09.14:57:19.689>
user = 'https://github.com/markshannon'

    bugs.python.org fields:

    activity = <Date 2021-08-09.14:57:19.689>
actor = 'Mark.Shannon'
assignee = 'Mark.Shannon'
closed = True
closed_date = <Date 2021-08-09.14:57:19.689>
closer = 'Mark.Shannon'
components = ['Interpreter Core']
creation = <Date 2021-08-04.07:57:16.295>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 44821
keywords = ['patch']
message_count = 3.0
messages = ['398864', '398908', '399148']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'Mark.Shannon']
pr_nums = ['27589']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue44821'
versions = ['Python 3.11']
    @markshannon
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    Currently, instance dictionaries (dict attribute) are created lazily when the first attribute is set.

    This is bad for performance for a number of reasons:

    1. It causes additional checks on every attribute access.
    2. It causes allocation of the object and its dict to be temporarily separated, most likely leading to increased physical separation and worse cache behavior.
    3. It has a large impact on specialization, as the first SET_ATTR for an object has to behave differently.

    Creating a __dict__ lazily does not save a significant amount of memory.
    If an object has a __dict__ slot, then it will end up with a dictionary before it dies in almost all cases.

    Many objects, e.g. ints, floats, don't have a dictionary. They are unaffected.

    Plain python objects that have no instance attributes are extremely rare, unless they have __slots__, in which case they don't have a dictionary anyway.

    The remaining case is subclasses of builtin types that do not add extra attributes, but these are rare, and the overhead of an empty dictionary is only 64 bytes (on a 64 bit machine).

    @markshannon markshannon added the 3.11 only security fixes label Aug 4, 2021
    @markshannon markshannon self-assigned this Aug 4, 2021
    @markshannon markshannon added interpreter-core (Objects, Python, Grammar, and Parser dirs) performance Performance or resource usage labels Aug 4, 2021
    @markshannon
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    New changeset cee67fa by Mark Shannon in branch 'main':
    bpo-44821: Eagerly assign __dict__ for new objects. (GH-27589)
    cee67fa

    @terryjreedy
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    Close this?

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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