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Effective Prefetch for Mark-Sweep Garbage Collection

Robin Garner, Stephen Blackburn and Daniel Frampton

The 2007 International Symposium on Memory Management (ISMM 2007)
Montreal, Canada, 21-22 October, 2007


Garbage collection is a performance-critical feature of most modern object oriented languages, and is characterized by poor locality since it must traverse the heap. In this paper we show that by combining two very simple ideas we can significantly improve the performance of the canonical mark-sweep collector, resulting in significant improvements in application performance. We make three main contributions: 1) we develop a methodology and framework for accurately and deterministically analyzing the tracing loop at the heart of the collector, 2) we offer a number of insights and improvements over conventional design choices for mark-sweep collectors, and 3) we find that a small, but counter-intuitive modification to traversal order improves locality and greatly increases opportunities for software prefetching.

We perform a thorough analysis in the context of MMTk and Jikes RVM on a wide range of benchmarks and four different architectures. Our baseline system (which includes a number of our improvements) is very competitive with highly tuned alternatives. We show a marking mechanism which offers modest but consistent improvements over conventional choices. Finally, we show that enqueuing the \emph{edges} (pointers) of the object graph rather than the \emph{nodes} (objects) significantly increases opportunities for software prefetch, despite increasing the total number of queue operations. Combining edge ordered enqueuing with software prefetching yields average performance improvements over a large suite of benchmarks of 20-30\% in garbage collection time and 4-6\% of total application performance in moderate heaps, across four architectures.

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