Unit test test_incremental_gc_handles_fast_cycle_creation sometimes fails

[nascheme]

Bug report

Bug description:

On a debug build on Linux AMD64, this fails for me:

./python -m test -uall test___all__ test_atexit test_audit test_ast test_argparse test_gc
[...]
test test_gc failed -- Traceback (most recent call last):
  File "/home/nas/src/cpython/Lib/contextlib.py", line 85, in inner
    return func(*args, **kwds)
  File "/home/nas/src/cpython/Lib/test/test_gc.py", line 1184, in test_incremental_gc_handles_fast_cycle_creation
    self.assertLess(new_objects, 27_000, f"Heap growing. Reached limit after {i} iterations")
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 27013 not less than 27000 : Heap growing. Reached limit after 6579 iterations

The exact set of tests required before test_gc seems to depend on other factors. More reliable command might be:

./python -m test -uall test___all__ test_buffer test_atexit test_audit test_bigaddrspace test_bdb test_abc test_bufio test_ast test_base64 test_annotationlib test_baseexception test__colorize test__interpreters test_asyncgen test_augassign test_bigmem test_binascii test_bz2 test_abstract_numbers test__osx_support test_c_locale_coercion test_bool test_argparse test_gc

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-133532: Run GC fast cycles test in subprocess. #133533
• [3.14] gh-133532: Run GC fast cycles test in subprocess. (gh-133533) #133716

[nascheme]

There seems to be some non-linear behavior happening. Creating a few thousand extra container objects (I used empty lists) before running the test results in the following.

Initial objects	Max new
23,576	17,199
33,452	22,407
43,452	26,418
63,452	32,592
103,452	39,606
183,452	46,998
343,452	53,235

A log curve seems to fit the data pretty closely.

[markshannon]

The larger the heap, the longer it takes to scan it, so there's more lag in collecting cycles. That's my guess.

[nascheme]

The GH-133533 change has resolved this issue.