Space-efficient 64-bit Java objects through selective typed virtual addressing

Memory performance is an important design issue for contemporary systems given the ever increasing memory gap. This paper proposes a space-efficient Java object model for reducing the memory consumption of 64-bit Java virtual machines. We propose selective typed virtual addressing (STVA) which uses typed virtual addressing (TVA) or implicit typing for reducing the header of 64-bit Java objects. The idea behind TVA is to encode the object´s type in the object´s virtual address. In other words, all objects of a given type are allocated in a contiguous memory segment. As such, the type information can be removed from the object´s header which reduces the number of allocated bytes per object. Whenever type information is needed for the given object, masking is applied to the object´s virtual address. Unlike previous work on implicit typing, we apply TVA to a selected number of frequently allocated and/or long-lived object types. This limits the amount of memory fragmentation. We implement STVA in the 64-bit version of the Jikes RVM on an AIX IBM platform and compare its performance against a traditional VM implementation without STVA using a multitude of Java benchmarks. We conclude that STVA reduces memory consumption by on average 15.5% (and up to 39% for some benchmarks). In terms of performance, STVA generally does not affect performance, however for some benchmarks we observe statistically significant performance speedups, up to 24%.