Efficient On-the-Fly Cycle Collection

A reference counting garbage collector cannot reclaim unreachable cyclic structures of objects. Therefore, reference counting collectors either use a backup tracing collector seldom, or employ a cycle collector to reclaim cyclic structures. Recently, the first on-the-fly cycle collector, that may run concurrently with program threads, was presented by Bacon and Rajan [3]. This demonstrated the ability to run on-the-fly reference counting without resorting to an auxiliary tracing collector. In this paper, we present an improved on-the-fly cycle collector by combining techniques developed in that paper with the sliding views collectors. The resulting collector gains two advantages. First, it improves over the efficiency of the original collector significantly, making the cycle collection solution usable in practice. Second, it eliminates the termination problem that appeared in the original algorithm. There, a rare race may delay the reclamation of an unreachable cyclic structure forever. The new cycle collector guarantees reclamation of all unreachable cyclic structures.