Python startup performance regression

[vstinner]

BPO	28637
Nosy	@rhettinger, @terryjreedy, @gpshead, @vsajip, @vstinner, @tiran, @ned-deily, @ethanfurman, @serhiy-storchaka, @wm75
Files	site-not-use-re.patch no_use_re_in_site_py.patch site-not-use-re.patch

^{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 = None
closed_at = <Date 2016-11-14.11:55:02.680>
created_at = <Date 2016-11-07.23:08:21.740>
labels = ['3.7', 'release-blocker', 'performance']
title = 'Python startup performance regression'
updated_at = <Date 2017-02-17.15:37:46.818>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-02-17.15:37:46.818>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2016-11-14.11:55:02.680>
closer = 'vstinner'
components = []
creation = <Date 2016-11-07.23:08:21.740>
creator = 'vstinner'
dependencies = []
files = ['45389', '45390', '45391']
hgrepos = []
issue_num = 28637
keywords = ['patch']
message_count = 38.0
messages = ['280256', '280258', '280259', '280260', '280261', '280262', '280266', '280267', '280268', '280269', '280271', '280272', '280273', '280274', '280296', '280301', '280302', '280304', '280305', '280306', '280312', '280328', '280346', '280353', '280362', '280370', '280378', '280380', '280383', '280409', '280552', '280558', '280559', '280629', '280636', '280753', '280758', '280760']
nosy_count = 11.0
nosy_names = ['rhettinger', 'terry.reedy', 'gregory.p.smith', 'vinay.sajip', 'vstinner', 'christian.heimes', 'ned.deily', 'ethan.furman', 'python-dev', 'serhiy.storchaka', 'wolma']
pr_nums = []
priority = 'release blocker'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue28637'
versions = ['Python 3.6', 'Python 3.7']

[vstinner]

The changeset 223731925d06 of the issue issue bpo-28082 "use IntFlag for re constants" made Python startup 34% slower:

• rev 5637c9b4dd4c: Median +- std dev: 19.5 ms +- 0.0 ms
• rev 223731925d06: Median +- std dev: 26.2 ms +- 0.0 ms

The change adds "import enum" in Lib/re.py, so the enum module is imported, whereas it wasn't before. It seems like importing enum takes 6.7 ms.

I propose to revert the change 223731925d06 in Python 3.6 and reopen the issue bpo-28082 to try to find another solution for the re module which doesn't impact Python startup performance.

[gvanrossum]

Reverting is reasonable, though it may break some code that started using this in 3.6b1. Such is the life of beta testers. The benefit of using Enum for re constants doesn't seem worth such a big increase in startup time.

[ethanfurman]

Ouch, that's a lot slower!

Victor, can you add the commands you are using for timing so I can work on making enum imports faster?

[vstinner]

Ethan Furman added the comment:

Victor, can you add the commands you are using for timing so I can work on making enum imports faster?

Results of the python_startup benchmark:
https://speed.python.org/timeline/#/?exe=4&ben=python_startup&env=1&revs=50&equid=off&quarts=on&extr=on

This website uses the following benchmark suite:
https://github.com/python/performance

You can run the benchmark using:
python3 -m performance run -b python_startup

[vstinner]

Oh, I understood why I had issues to reproduce the startup performance slowdown. When Python is run from the source code using ./python, the re module is not imported. When Python is installed, the re is not imported by default neither:

haypo@speed-python$ prefix/bin/python3 -c 'import sys; print("re" in sys.modules)'
False

BUT when Python is started from a virtual environment (created by the "venv" module), the re module is important by default.

haypo@speed-python$ venv/bin/python3 -c 'import sys; print("re" in sys.modules)'
True

If the site module is not imported, the re module is not imported:

haypo@speed-python$ venv/bin/python3 -S -c 'import sys; print("re" in sys.modules)'
False

The /home/haypo/benchmarks/prefix/lib/python3.6/site.py file is generated by the venv module and contains:

def venv(...):
    ...
    if candidate_confs:
        import re
        config_line = re.compile(CONFIG_LINE)
        ...

[python-dev]

New changeset d903a243c281 by Victor Stinner in branch '3.6':
Issue bpo-28637: Revert issue bpo-28082, don't import enum in re
https://hg.python.org/cpython/rev/d903a243c281

[ned-deily]

With being this close to release and with two weeks to go until the final beta, it seems a bit premature to immediately revert this without exploring all the alternatives especially since it seems the effects are limited to venvs. In any case, we do need to have a final answer well before the b4 cutoff (2016-11-21). Possible options:

1. leave the re contents reverted (current status)
2. find a way to mitigate the performance impact of importing re and enum (perhaps making them builtins in Setup.dist?)
3. avoid the use of re in site.py venv
4. others?

[vstinner]

Possible options:

3. avoid the use of re in site.py venv

That would be a tiny but nice enhancement.

re is a commonly used module. I would prefer to not make its import time slower. Python 3 startup is already something like 3x slower than Python 2. Please don't make it even slower :-( We worked on optimizing Python 3 startup time.

2. find a way to mitigate the performance impact of importing re and enum (perhaps making them builtins in Setup.dist?)

I reverted the change, because Guido and Ethan seem to be in favor of a revert, and I don't expect any simple solution for this issue.

I'm quite sure that importing enum.py is slow. Optimizing enum.py import time is a large task.

Another option to explore is to delay the creation/instanciation of the re flags, something like lazy module import... but at the module attribute level :-)

[vstinner]

I remove the release blocker flag since the change was reverted.

[ned-deily]

Please leave this as a "release blocker" until we have had a chance to explore all the options for 3.6.0b4. We may still very well decide that reverting is the least bad option but I don't want to rule out other options without some further investigation. We have time and the reverting could be a fairly significant change.