cpython/Misc/NEWS.d/next/Core_and_Builtins/2024-11-21-16-13-52.gh-issue-126491.0YvL94.rst

Add a marking phase to the GC. All objects that can be transitively reached
from builtin modules or the stacks are marked as reachable before cycle
detection. This reduces the amount of work done by the GC by approximately
half.