Garbage Collection Hints

This paper shows that Appel-style garbage collectors often make suboptimal decisions both in terms of when and how to collect. We argue that garbage collection should be done when the amount of live bytes is low (in order to minimize the collection cost) and when the amount of dead objects is high (in order to maximize the available heap size after collection). In addition, we observe that Appel-style collectors sometimes trigger a nursery collection in cases where a full-heap collection would have been better. Based on these observations, we propose garbage collection hints (GCH) which is a profile-directed method for guiding garbage collection. Offline profiling is used to identify favorable collection points in the program code. In those favorable collection points, the VM dynamically chooses between nursery and full-heap collections based on an analytical garbage collector cost-benefit model. By doing so, GCH guides the collector in terms of when and how to collect. Experimental results using the SPECjvm98 benchmarks and two generational garbage collectors show that substantial reductions can be obtained in garbage collection time (up to 30X) and that the overall execution time can be reduced by more than 10%.