Use fewer statics in Argument Clinic.

[ericsnowcurrently]

BPO	38141
Nosy	@larryhastings, @ericsnowcurrently, @serhiy-storchaka, @tahia-khan

^{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/ericsnowcurrently'
closed_at = None
created_at = <Date 2019-09-12.14:59:42.372>
labels = ['expert-argument-clinic', '3.9']
title = 'Use fewer statics in Argument Clinic.'
updated_at = <Date 2019-09-26.17:28:25.844>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2019-09-26.17:28:25.844>
actor = 'ta1hia'
assignee = 'eric.snow'
closed = False
closed_date = None
closer = None
components = ['Argument Clinic']
creation = <Date 2019-09-12.14:59:42.372>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 38141
keywords = []
message_count = 5.0
messages = ['352193', '352226', '352306', '352310', '352311']
nosy_count = 4.0
nosy_names = ['larry', 'eric.snow', 'serhiy.storchaka', 'ta1hia']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue38141'
versions = ['Python 3.9']

[ericsnowcurrently]

(This is a sub-task of bpo-36876, "Global C variables are a problem.".)

Currently Argument Clinic generates "_PyArg_Parser _parser" as a static variable. Dropping "static" solves the problem of thread safety (e.g. for subinterpreters not sharing the GIL).

[serhiy-storchaka]

Dropping "static" will hit performance. _PyArg_Parser is used at first place because it can cache some data between calls. If drop static, you should drop also _PyArg_Parser and return to old slow arguments parsing.

[serhiy-storchaka]

Would adding a mutex for thread-safe initialization of _Py_Identifier and _PyArg_Parser solve the problem with subinterpreters?

[ericsnowcurrently]

That might work. :)

There are two key problems under subinterpreters that do not share the GIL:

• races on refcount operations
• cache thrashing due to refcount operations (under multi-core threads)

A lock would certainly mitigate the first problem. I'm not sure how much the second would actually be a problem.

[larryhastings]

shared objects x threads = contention for notification of invalidated cache lines

If you're not running multiple threads, there's no problem. If it's only a few shared objects, it probably wouldn't be a big deal. As they say in medicine: "the dose makes the poison."

[kumaraditya303]

Duplicate of #90928