Reducing generational copy reserve overhead with fallback compaction

As programming languages with managed runtimes become increasingly popular, it is essential that virtual machines are implemented efficiently. The performance of the memory management subsystem can be a defining factor in the performance of the virtual machine as a whole. We present a technique by which garbage collector performance can be improved. We describe an algorithm that combines a standard generational copying collector with a mark and compact collector. We observe that, since most objects do not survive a garbage collection, it is not necessary to reserve space to copy them all. The result is a generational copying collector that operates with a smaller copy reserve overhead than traditional Appel-style collectors. We maintain correctness in the worst case through the use of mark and compact collection. When the reduced copy reserve overflows, a compacting phase ensures that all data are accommodated. We have implemented this algorithm within the framework of Jikes RVM and MMTk. For most benchmarks examined, our experiments show that performance is comparable to or better than a standard generational copying collector.