GH-124567: Reduce overhead of cycle GC.

[markshannon]

This PR makes the following changes to cycle GC, to reduce the overhead observed in #124567:

• Does not form the transitive closure of the young generation before collecting it. Avoids scanning too much of the heap that is likely reachable.
• Performs an amount of work proportional to the young collection in the incremental collection, avoiding n**2 performance on large heaps (this was a regression w.r.t. 3.12)
• At the start of each full scan, mark all objects reachable from sys.modules, so they are not scanned this scan.
• Computes the delta in heap size during incremental collection, and accelerates incremental collection if the heap is growing
• Adds a _testinternalcapi.get_heap_size() function for more precise testing of the GC

Before merging, I need to:

• Get performance numbers and stats for this version
• Likewise for an alternative version that does form the transitive closure of the young generation, but only does so after marking reachable objects.
  I still need benchmarks and stats for this before merging.

• Issue: Incremental GC causes a significant slowdown for Sphinx #124567

[Privat33r-dev]

As a quick preliminary check, I would suggest running doc tests, since they indicated the issue at first and caused further findings (see #118891 for details).

Currently, it appears to me that the PR achieves the opposite of its purpose.

Doctest on main branch takes ~12-15 minutes
With PR it takes >40m (still running): https://github.com/Privat33r-dev/cpython/actions/runs/11082906901/job/30796516669?pr=4
Main (before PR): https://github.com/Privat33r-dev/cpython/actions/runs/11083066610/job/30796883045?pr=5

[AA-Turner]

The test at https://github.com/Privat33r-dev/cpython/actions/runs/11082906901/job/30796516669?pr=4 was killed by the 60 minute timeout, less than half-way through the build (source files not fully read). This PR isn't running documentation tests as no files in Doc/ have changed and there's no NEWS entry currently.

A

[Privat33r-dev]

This PR isn't running documentation tests as no files in Doc/ have changed.

That's how I triggered it in my repo. I suggest to add/remove space in Docs in this PR as well, so we can see the end result of the changes.

I probably should've added further context for clarity. I used this PR as a base to make a PR in my repository, with 1 more commit from me (space change in docs), so Doctest will start automatically. It took more than 1 hour and stopped halfway through, because the job had 1 hour limit set.
Privat33r-dev#4

At the same time, I made a control Doctest in my repo on main branch + minor doc change. It took 13 m:
Privat33r-dev#5

[rruuaanng]

Binary operations, where two operands require spaces, may be more standard.
(gcstate->heap_size - gcstate->prior_heap_size) * 3

[markshannon]

Closing this, as we don't need an urgent fix (3.13 goes back to generational collection)

Longer term, I expect reduced reference counting operations to make faster-cpython/ideas#693 viable.

The pre-scan to find live objects is definitely worth further investigation, though.