The single-referent collector

Abstract

Compactors that move or copy objects need to adjust pointers. In extant compactors, pointer adjustment involves inspecting every pointer in the heap and computing the target address for each pointer. At the same time, in modern Managed Runtime Environments (MREs), only a fraction of pointers in the heap changes during compaction. This is because state-of-the-art MREs do not compact the prefix of the heap that contains few dead objects, allowing gaps between live objects and tolerating small space overhead. We describe the design and implementation of the Single-Referent Collector (SRC), a new compactor that reduces the cost of pointer manipulation by avoiding inspection and adjustment of pointers that do not change. SRC exploits the fact that in modern applications, most live objects have only one incoming pointer. For such objects, SRC stores the address of the referent in the object header. Only objects that move have their referent inspected and adjusted. The remaining pointers in the heap are not processed. SRC uses an overflow table to handle objects with multiple incoming pointers. We investigate a number of standard benchmarks and open-source applications to substantiate key statistical observations that underlie the design of SRC. We implement SRC in the HotSpot JVM as part of a generational collection system and compare it empirically with the Lisp2 compactor. We find that, by decreasing the cost of pointer processing, SRC enables significant reduction in pause times and improves application throughput.