Ref.counting is slow, causes too many extra memory requests. Also, when the program is multithreaded, access to cache lines recently modified by another CPU cor

Ref.counting is slow, causes too many extra memory requests. Also, when the program is multithreaded, access to cache lines recently modified by another CPU core is expensive, even more so that a cache miss. For these reasons, most garbage collection systems are using mark and sweep, or something similar.