Chihuahua: A Concurrent, Moving, Garbage Collector using Transactional Memory

Hardware Transactional Memory (HTM) offers a powerful new parallel synchronization mechanism, but one whose performance properties are different from techniques that it competes with, such as locks and atomic instructions. Because of HTM’s differing characteristics, when algorithms based on earlier synchronization mechanisms are adapted to use HTM instead, the performance may be disappointing, sometimes even appearing not to significantly outperform software transactional memory. In this paper, however, we show that HTM, and specifically its strong atomicity property, allows approaches to synchronization that would not otherwise be possible, allowing simpler synchronization algorithms than would otherwise be possible that nevertheless have promising performance. Specifically, we present a new garbage collector named Chihuahua that is designed specifically for HTM, in which garbage collector threads execute transactionally but the mutator does not. In contrast to other work which has adapted existing parallel collectors to make them transactional, Chihuahua is a transactional adaptation of a serial collector (taken from MMTk in the Jikes RVM). Although Chihuahua is a proof of concept rather than an industrial-strength, production garbage collector, we believe it highlights opportunities in the design space of garbage collectors and other parallel algorithms that are available in HTM but not available in competing techniques.