[3.13] GH-124567: Revert the Incremental GC in 3.13

[Yhg1s]

Revert the Incremental GC in 3.13.

This reverts PRs #116206 (commit 1530932), #117120 (e28477f), #117213 (8bef34f), #117422 (ddf814d), #118313 (2ba1aed), #123268 (b1372e2) and #123395 (aca6511). Does not revert the raising of the first generation threshold from 700 to 2000, nor changes to C APIs (even private ones).

• Issue: Incremental GC means that Sphinx is significantly slower in 3.13 than in 3.12 #124567

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📚 Documentation preview 📚: https://cpython-previews--124770.org.readthedocs.build/

[Yhg1s]

This is a fairly simple revert, we'll have to address the what's new docs and news entries but I want to get this out there for review and testing in the meantime.

[AlexWaygood]

Happy to report that this cuts around 50% off the time it takes to do an HTML build of the CPython docs locally with Sphinx (build options were --with-lto=fat --enable-optimizations). On the tip of the 3.13 branch, the second script I posted in #124567 (comment) reports a Sphinx docs-build time of 66.28s for me locally; with this PR, it reports 34.38s.

[nascheme]

A little benchmarking from my desktop PC. "Sphinx" is build/html for typing.rst. The debug column is for the --with-debug build. The opt column is for optimized build without LTO or PGO. The "mypy" runs are for the mypy testsuite after disabling the gc.set_threshold() call it does (it sets a much higher threshold than default one). The mypy run creates quite a big heap but not much cyclic garbage. The Sphinx run is a smaller heap but quite a lot of cyclic garbage with smallish objects.

Based on this limited testing, it's looking pretty good.

Description	debug [sec]	opt [sec]	peak rss [MB]
Sphinx, 3.12	5.17	1.71	88.1
Sphinx, 3.13 inc	10.25	1.92	96.9
Sphinx, 3.13 revert	6.03	1.87	96.9
Mypy, 3.12		25.54	274.4
Mypy, 3.13 inc		28.28	276.2
Mypy, 3.13 revert		25.51	272.4

[AlexWaygood]

We can also see that the Doctest CI job on this PR took only 4 minutes, as opposed to e.g. 16-17 minutes on other recent 3.13 PRs such as

• https://github.com/python/cpython/actions/runs/11087447714/job/30806712091?pr=124738
• https://github.com/python/cpython/actions/runs/11092139738/job/30816812499?pr=124764
• https://github.com/python/cpython/actions/runs/11084048667/job/30798940333?pr=124727

[nascheme]

LGTM. I compared gc.c from before GH-108362 (d5ebf8b) and it looks like your reversion got the right bits.

[Yhg1s]

I believe the ABI changes are acceptable (the size of a private struct within another private struct changes, so later fields in the outer struct move around -- but as far as I can tell those structs are not exposed to users). The hypothesis failure seems unrelated (but possibly worth investigating).