tl;dr: Python 3.12 should be built with no-omit-frame-pointer if
upstream recommends it.
Hello,
You might be aware of a Fedora change proposal [0] (discussed on
fedora-devel [1] and FESCo [2]) are discussing turning on C compiler
flags that help with performance *measurement*, but might hurt
performance itself: `-fno-omit-frame-pointer` and
`-mno-omit-leaf-frame-pointer`.
Apparently there are some benchmarks that make Python look extra slow
when the flags are turned on -- which I don't quite understand.
Meanwhile, on the upstream side, Python 3.12 (due next year, main Python
for Fedora 39 [3]) has support for `perf`. Upstream plans to recommend
compiling with these flags when measuring performance [4], and AFAIK,
the plan is to recommend *always* compiling with them.
The idea is that possible speedups from "allowing anyone to
profile/optimize their workflow" are worth the initial slowdown.
I'm not much of a performance expert myself, but I do get drawn into the
relevant discussions on the CPython side.
As far as I can see, performance geeks are enthusiastic for `perf`
support, and I'd like to get them to (continue to) use Fedora builds.
If CPython upstream does recommend these flags (or makes them default),
I'm considering to turn the no-omit options on for Python 3.12 even if
Fedora as a whole doesn't.
Note that even a 2% slowdown will likely won back by general performance
improvements – the Faster CPython team is targeting a 20% average
speedup for pure-Python code in 3.12, on top of the ~25% for 3.11. And
the people responsible for this speedup have a say in the upstream
recommendations.
Technically there are three separate places where the flags can be set,
I think we should turn them on everywhere:
- CPython itself & its standard library
- The debug build (/usr/bin/python3-debug)
- Default for libraries in Fedora (RPM macros)
- Befault for libraries built by users (sysconfig settings)
[0]: https://fedoraproject.org/wiki/Changes/fno-omit-frame-pointer
[1]:
https://lists.fedoraproject.org/archives/list/de...@lists.fedoraproject.org/thread/OOJDAKTJB5WGMOZRXTUX7FTPFBF3H7WE/
[2]: https://pagure.io/fesco/issue/2817
[3]: https://fedoraproject.org/wiki/Changes/Python3.12
[4]:
https://docs.python.org/3.12/howto/perf_profiling.html#how-to-obtain-the-best-results
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