Something old, something new: memory management today and looking forwards

I will give this talk in two parts. First I will present the Immix garbage collector, which I developed with Kathryn McKinley, and which is now used as the default collector in a number of virtual machines. I will then discuss where I think memory management is headed, supported by unpublished results from our current research. An abstract of our Immix work follows:

Programmers are increasingly choosing managed languages for modern applications, which tend to allocate many short-to-medium lived small objects. The garbage collector therefore directly determines program performance by making a classic space-time tradeoff that seeks to provide space efficiency, fast reclamation, and mutator performance. The three canonical tracing garbage collectors: semi-space, mark-sweep, and mark-compact each sacrifice one objective. This paper describes a collector family, called mark-region, and introduces opportunistic defragmentation, which mixes copying and marking in a single pass. Combining both, we implement Immix, a novel high performance garbage collector that achieves all three performance objectives. The key insight is to allocate and reclaim memory in contiguous regions, at a coarse block grain when possible and otherwise in groups of finer grain lines. We show that immix outperforms existing canonical algorithms, improving total application performance by 7 to 25% on average across 20 benchmarks. As the mature space in a generational collector, immix matches or beats a highly tuned generational collector, e.g. it improves SPEC jbb by 5%. These innovations and the identification of a new family of collectors open new opportunities for garbage collector design.