4.2 Improvement of Eciency 5 Problems and Its Solution 5.1 Specifying Groups 5.2 Unpredictable Cycles

Reference counting schemes are suitable for garbage collection on distributed systems because they do not require global information to nd garbage. However, reference counting schemes have a serious drawback: they cannot reclaim garbage which form cyclic structures. In this paper, we propose an extension of the traditional reference counting scheme, group reference counting, which alleviates the drawback. This scheme is able to reclaim circularly referenced objects which are contained in object groups. We describe how this scheme can be implemented on distributed object-oriented concurrent systems. The overhead of this scheme does not depend on the number of nodes. Thus this scheme works e ciently on distributed systems containing large number of nodes.