Control stack usage in large expressions

[markshannon]

BPO	43846
Nosy	@markshannon, @serhiy-storchaka, @pablogsal
PRs	bpo-43846: Use less stack for large literals and calls #25403

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

Show more details

GitHub fields:

assignee = 'https://github.com/markshannon'
closed_at = <Date 2021-04-21.09:23:59.159>
created_at = <Date 2021-04-14.12:09:31.468>
labels = ['interpreter-core', 'performance']
title = 'Control stack usage in large expressions'
updated_at = <Date 2021-04-21.09:23:59.158>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2021-04-21.09:23:59.158>
actor = 'Mark.Shannon'
assignee = 'Mark.Shannon'
closed = True
closed_date = <Date 2021-04-21.09:23:59.159>
closer = 'Mark.Shannon'
components = ['Interpreter Core']
creation = <Date 2021-04-14.12:09:31.468>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 43846
keywords = ['patch']
message_count = 4.0
messages = ['391072', '391136', '391150', '391502']
nosy_count = 3.0
nosy_names = ['Mark.Shannon', 'serhiy.storchaka', 'pablogsal']
pr_nums = ['25403']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue43846'
versions = []

[markshannon]

Large literals or function calls with many arguments can consume a lot of stack space.

This will be a problem for any future work to use a contiguous stack for data and possibly eliminate frame objects for most calls.

It is also possible (I haven't measured this) that this large stack consumption is hurting performance now, as it might leak memory by leaving giant frames in the free-list or as a zombie frame.

This fix relatively straightforward. For large literals and argument lists, build them incrementally rather than all at once.

[markshannon]

New changeset 11e0b29 by Mark Shannon in branch 'master':
bpo-43846: Use less stack for large literals and calls (GH-25403)
11e0b29

[serhiy-storchaka]

What is performance impact of this change? I expect that creating a list incrementally can hurt performance, but how much?

[markshannon]

I've not measured performance, as the sort of large literals it would impact are very rare, and the sort of calls it would change are almost non-existent.

I'd be surprised if any performance difference could be reliably detected with our current performance tools.

This change is an enabler of future optimizations, such as using a contiguous data stack. The impact of those optimizations will swamp any impact of this change, and should be easily measurable.