Reducing generational copy reserve overhead with fallback compaction

As programming languages with managed runtimes become in-creasingly popular, it is essential that virtual machines are imple-mented efficiently. The performance of the memory management subsystem can be a defining factor in the performance of the vir-tual 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 ex-periments show that performance is comparable to or better than a standard generational copying collector