GeezerVison

The collector walks the generation once in update_refs() to copy every reference count into the GC header, and again in subtract_refs() to subtract the references between the collected objects. In a full collection every tracked object is in the list, so the second walk needs nothing from the first, and this does both in one walk. An object that is decremented before the walk reaches it still holds its _gc_prev pointer, and the walk recovers the decrements from the difference with the pointer it knows was there.

Benchmarks

CPU time spent in the collector, taken in gc.callbacks. Six runs per executable in baseline/candidate/candidate/baseline order; medians of the runs taken while the machine was quiet.

• Issue: Speed up garbage collector #159137