[Python-Dev] Program runs in 12s on Python 2.7, but 5s on Python 3.5 -- why so much difference?

[Nathaniel Smith]

I'd probably start with a regular C-level profiler, like perf or
callgrind. They're not very useful for comparing two versions of code
written in Python, but here the Python code is the same (modulo
changes in the stdlib), and it's changes in the interpreter's C code
that probably make the difference.

On Tue, Jul 18, 2017 at 9:03 AM, Ben Hoyt <benhoyt at gmail.com> wrote:
> Hi folks,
>
> (Not entirely sure this is the right place for this question, but hopefully
> it's of interest to several folks.)
>
> A few days ago I posted a note in response to Victor Stinner's articles on
> his CPython contributions, noting that I wrote a program that ran in 11.7
> seconds on Python 2.7, but only takes 5.1 seconds on Python 3.5 (on my 2.5
> GHz macOS i7), more than 2x as fast. Obviously this is a Good Thing, but I'm
> curious as to why there's so much difference.
>
> The program is a pentomino puzzle solver, and it works via code generation,
> generating a ton of nested "if" statements, so I believe it's exercising the
> Python bytecode interpreter heavily. Obviously there have been some big
> optimizations to make this happen, but I'm curious what the main
> improvements are that are causing this much difference.
>
> There's a writeup about my program here, with benchmarks at the bottom:
> http://benhoyt.com/writings/python-pentomino/
>
> This is the generated Python code that's being exercised:
> https://github.com/benhoyt/python-pentomino/blob/master/generated_solve.py
>
> For reference, on Python 3.6 it runs in 4.6 seconds (same on Python 3.7
> alpha). This smallish increase from Python 3.5 to Python 3.6 was more
> expected to me due to the bytecode changing to wordcode in 3.6.
>
> I tried using cProfile on both Python versions, but that didn't say much,
> because the functions being called aren't taking the majority of the time.
> How does one benchmark at a lower level, or otherwise explain what's going
> on here?
>
> Thanks,
> Ben
>
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-- 
Nathaniel J. Smith -- https://vorpus.org