Writing to the rc field even for read only access dirties cache lines and causes cache line ping ping pong in case of multi core access, where you also need to

Writing to the rc field even for read only access dirties cache lines and causes cache line ping ping pong in case of multi core access, where you also need to use slower, synchronised refcount updates so not to corrupt the count. Other GC strategies don't require dirtying cache lines when accessing the objects. Determinism: the time taken is not deterministic because (1) malloc/free, which ARC uses but other GCs usually not, are no deterministic - both can do arbitrary amounts of work like coalescing or defragmenting allocation arenas, or performing system calls that reconfigure process virtual memory - and (2) cascading deallocations as rc 0 objects trigger rc decrements and deallocation of other objects.