Hard Real-Time Garbage-Collection in the Jamaica Virtual Machine

Java S automatic memory management is the main reason that prevents Java from being used in hard realtime environments. We present the garbage collection mechanism that is used by the Jamaica Virtual Machine, an implementation of the Java Virtual Machine Specification. This mechanism differs significantly from existing implementations in the way threads are implemented, root references are found and in the object layout that is used. The implementation provides hard real-time guarantees while it allows unrestricted use of the Java language. Even dynamic allocation of normal garbage collected Java objects is possible with hard real-time guarantees. memory accesses, reference assignments, modifications of the root set and allocations are known. For such a system to be useful, this worst case performance must be small, else the worst case performance of a bigger task consisting of several of such operations quickly becomes unacceptable. The Jamaica Virtual Machine project aims at providing such hard real-time behaviour. In this paper, we present the garbage collection mechanism that is used by Jamaica to achieve this goal. 2. Required Real-Time Guarantees There are several guarantees that have to be given by a Java implementation so that it can be used for hard real-time programming: