Why is the nix-compiled Python slower?

I stumbled upon the regression section on the new wiki today.

I repeated the synthetic benchmark between Ubuntu Noble (3.12.2, in Docker) and NixOS Unstable (3.12.2) and apparently we are faster now?

root@18cfe19cdf22:/# python3 -c "import timeit; print(timeit.Timer('for i in range(100): oct(i)', 'gc.enable()').repeat(5))"
[4.54713627404999, 4.525180662050843, 4.5404908519703895, 4.536089212982915, 4.561447929008864]
❯ nix-build -A python312
❯ ./result/bin/python3.12 -c "import timeit; print(timeit.Timer('for i in range(100): oct(i)', 'gc.enable()').repeat(5))"
[3.88701227796264, 3.9353582749608904, 3.904149228008464, 3.8978957710787654, 3.8842451529344544]

Additionally, I’ve also corrected some assumptions in that section, notably that LTO was disabled entirely, which is not the case. It is however only enabled on Linux x64.

LTO had initially been enabled unconditionally back in 2021-05, so I’m not sure why the post said it wasn’t enabled at all.

Feel free to fact-check me.



Awesome, thanks for checking and updating! FWIW, I did check if we had LTO enabled and from what I could tell, on the channel/derivation we were trying to use, it is disabled, see:

enableOptimizations = false;
enableLTO = false;

Also, at least internally, testing various versions of Python, I did find that 3.6 was slower than 3.8 and 3.8 was slower than 3.10. It could make sense that 3.12 is faster than 3.8 or 3.10. :slight_smile:

Regarding the original post, at least through my own testing, the performance of the nix 3.8 Python matches a non-LTO and non-PGO compilation on a Ubuntu 18.04 x64 system.

Your wiki update looks fine to me!

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That’s python3Minimal, which you more or less shouldn’t be using unless you need to.

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