Adaptive Optimization of the Sun Java Real-Time System Garbage Collector

Garbage collection (GC) is one of the largest sources of unpredictability in JavaTM applications, and a real-time virtual machine must use garbage collection algorithms that minimize delays to real-time threads and at the same time maximize the overall application’s throughput. In order to achieve the optimal tradeoff between these conflicting objectives, the GC cycle (which needs to take place periodically in order to free the memory no longer used by the application) needs to be triggered at the optimal time: if it is triggered too soon then the application’s throughput will decrease unnecessarily, while if it is triggered too late then the application can run out of free memory and block real-time threads unnecessarily. Starting with Sun Java Real-Time System 2.0 (Java RTS), a new real-time garbage collector (RTGC) is available. One of the key RTGC parameters is the StartupMemoryThreshold, which determines how low the free memory in the system can fall before a garbage collection is triggered. This paper presents a framework for dynamically adapting the StartupMemoryThreshold for achieving the optimal balance between the application’s throughput and pause time, which was integrated into the beta release of Java RTS 2.2. An experimental evaluation of this framework using the SPECjbb2005 benchmark confirmed its effectiveness. This framework can be used in conjunction with any concurrent or a time-based incremental garbage collector.