[saagarjha]

You can expect the Python developers to look very closely at any benchmark that significantly benefits from adding random padding to the object header. Performance isn’t just trying a bunch of random things and picking whatever works the best, it’s critical to understand why so you know that the improvement is not a fluke. Especially since it is very easy to introduce bias and significantly perturb the results if you don’t understand what’s going on.

[munch117]

We're not talking about random changes. We're talking about paying attention to the measured performance of changes made for other reasons.

Just like in this article. The author measured, wondered, investigated, experimented, and finally, after a lot of hard work, made the C/Rust programs faster. You wouldn't call that luck, would you? If there had been a similar performance regression in CPython, then a benchmark could have picked up on it, and the CPython developers would then have done the same.

[saagarjha]

You can look at the history of PyObject yourself: https://github.com/python/cpython/commits/main/Include/objec.... None of these changes were done because of weird CPU errata that meant that making the header bigger was a performance win. That isn't to say that the developers wouldn't be interested in such effects, or be able to detect them, but the fact that the object header happens to be large enough to avoid the performance bug isn't because of careful testing but because that's what they ended up for other reasons, far before Zen 3 was ever released. If it so happened that Python was affected because the offset needed to avoid a penalty was 0x50 or something then I am sure they would take it up with AMD rather than being content to increase the size of their header for no reason.

[munch117]

What you don't see in the logs are the experiments and branches that weren't pursued further because they didn't perform well enough.

Also: If you're going to prove that changes informed by performance measurements are absent from the commit logs, then you'll need to look in the logs for all the relevant places, which means also looking at I/O and bytes and allocator code.

[saagarjha]

Given that the performance is only affected by the size of that object header, the file I linked is all you'd need to see changes in. Look, the Python project is not picking their object sizes because it performs well on a quirk of Zen 3. End of story. I did performance work professionally in the past and now recreationally and this specific instance is 100% luck. This is not because I don't think the runtime people aren't smart or anything but this would be an insane thing to do on purpose.