Immix: a mark-region garbage collector with space efficiency, fast collection, and mutator performance

  • Authors:

  • Stephen M. Blackburn;Kathryn S. McKinley

  • Affiliations:

  • Australian National University, Canberra, Australia;University of Texas at Austin, Austin, TX, USA

  • Venue:
  • Proceedings of the 2008 ACM SIGPLAN conference on Programming language design and implementation
  • Year:
  • 2008

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Abstract

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 jbb2000 by 5%. These innovations and the identification of a new family of collectors open new opportunities for garbage collector design.