[Python-Dev] Python startup time

Victor Stinner victor.stinner at gmail.com
Wed Jul 19 08:59:52 EDT 2017


On Twitter, Raymond Hettinger wrote:

   "The decision making process on Python-dev is an anti-pattern,
governed by anecdotal data and ambiguity over what problem is solved."


About "anecdotal data", I would like to discuss the Python startup time.

== Python 3.7 compared to 2.7 ==

First of all, on speed.python.org, we have:

* Python 2.7: 6.4 ms with site, 3.0 ms without site (-S)
* master (3.7): 14.5 ms with site, 8.4 ms without site (-S)

Python 3.7 startup time is 2.3x slower with site (default mode), or
2.8x slower without site (-S command line option).

(I will skip Python 3.4, 3.5 and 3.6 which are much worse than Python 3.7...)

So if an user complained about Python 2.7 startup time: be prepared
for a 2x - 3x more angry user when "forced" to upgrade to Python 3!

== Mercurial vs Git, Python vs C, startup time ==

Startup time matters a lot for Mercurial since Mercurial is compared
to Git. Git and Mercurial have similar features, but Git is written in
C whereas Mercurial is written in Python. Quick benchmark on the
speed.python.org server:

* hg version: 44.6 ms +- 0.2 ms
* git --version: 974 us +- 7 us

Mercurial startup time is already 45.8x slower than Git whereas tested
Mercurial runs on Python 2.7.12. Now try to sell Python 3 to Mercurial
developers, with a startup time 2x - 3x slower...

I tested Mecurial 3.7.3 and Git 2.7.4 on Ubuntu 16.04.1 using "python3
-m perf command -- ...".

== CPython core developers don't care? no, they do care ==

Christian Heimes, Naoki INADA, Serhiy Storchaka, Yury Selivanov, me
(Victor Stinner) and other core developers made multiple changes last
years to reduce the number of imports at startup, optimize impotlib,

IHMO all these core developers are well aware of the competition of
programming languages, and honesty Python startup time isn't "good".
So let's compare it to other programming languages similar to Python.

== PHP, Ruby, Perl ==

I measured the startup time of other programming languages which are
similar to Python, still on the speed.python.org server using "python3
-m perf command -- ...":

* perl -e ' ': 1.18 ms +- 0.01 ms
* php -r ' ': 8.57 ms +- 0.05 ms
* ruby -e ' ': 32.8 ms +- 0.1 ms

Wow, Perl is quite good! PHP seems as good as Python 2 (but Python 3
is worse). Ruby startup time seems less optimized than other

Tested versions:

* perl 5, version 22, subversion 1 (v5.22.1)
* PHP 7.0.18-0ubuntu0.16.04.1 (cli) ( NTS )
* ruby 2.3.1p112 (2016-04-26) [x86_64-linux-gnu]

== Quick Google search ==

I also searched for "python startup time" and "python slow startup
time" on Google and found many articles. Some examples:

"Reducing the Python startup time"
=>   "The python startup time always nagged me (17-30ms) and I just
searched again for a way to reduce it, when I found this: The
Python-Launcher caches GTK imports and forks new processes to reduce
the startup time of python GUI programs."

=> "Wow, Python startup time is worse than I thought."

"How to speed up python starting up and/or reduce file search while
loading libraries?"
=> "The first time I log to the system and start one command it takes
6 seconds just to show a few line of help. If I immediately issue the
same command again it takes 0.1s. After a couple of minutes it gets
back to 6s. (proof of short-lived cache)"

"How does one optimise the startup of a Python script/program?"
=> "I wrote a Python program that would be used very often (imagine
'cd' or 'ls') for very short runtimes, how would I make it start up as
fast as possible?"

"Python Interpreter Startup time"

"Python is very slow to start on Windows 7"
=> "Python takes 17 times longer to load on my Windows 7 machine than
Ubuntu 14.04 running on a VM"
=> "returns in 0.614s on Windows and 0.036s on Linux"

"How to make a fast command line tool in Python" (old article Python 2.5.2)
=> "(...) some techniques Bazaar uses to start quickly, such as lazy imports."


So please continue efforts for make Python startup even faster to beat
all other programming languages, and finally convince Mercurial to
upgrade ;-)


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