Abstract

The Real-Time Speci cation for Java [3] provides a framework for building real-time systems. We implemented the memory management portion of this speci cation and wrote sample applications to test it. We discuss the structure of our implementation and problems encountered during implementation and application development. The primary implementation problem arises from a design in which some threads are vulnerable to garbage collection pauses because they can allocate memory in the garbage-collected heap, while other threads must execute independently of the garbage collector to provide acceptable response times even in the face of garbage collection pauses. The Real-Time Speci cation for Java allows the dierent kinds of threads share the same resource. Using blocking locks to manage access to these shared resources can lead to unacceptable interactions in which one thread indirectly and unacceptably forces another thread to observe garbage collection pauses. We solved this potential problem by using non-blocking, optimistic synchronization primitives throughout the implementation. A second implementation problem arises from the need to manage the memory associated with the memory management data structures. The lack of a recycling memory area which can be accessed from any execution point led to the development of a new memory area not mentioned in the speci cation. Because we found it dicult, if not impossible, to correctly use the Real-Time Java extensions without automated assistance, we developed an extensive debugging facility.

Citations