No Silver Bullet - Garbage Collection for Java in Embedded Systems

Flexibility Required

Being able to characterize the dynamic memory allocation behavior of an application helps in choosing a garbage collection algorithm. For example, if it appears that the embedded application is known to build no cyclic data structures, then a derived reference counting algorithm might be appropriate. If allocated objects are known to be fairly similar in sizes, then a non compacting algorithm might be used.

Instrumentation of memory allocations behavior, including peak size, object sizes and life spans statistics will help a developer to select the correct garbage collection algorithm. Such a monitoring can be performed during development and the test phases by using a special version of the runtime environment. The next step is to assist the developer by allowing them to plug and tune the garbage collector of their choice.

These issues are being seriously examined at Cygnus Solutions, which is developing a Java runtime and Java extensions for the GCC compiler [see side bar "The Cygnus GNUPro compiler for the Java language"]. Being in control of the tools that generate the code gives us opportunities to ease the work of a garbage collector in many ways. We can precisely perform any relevant static allocation of objects and implement write barriers, even though the implementation of this may be challenging if we want to support pluggable collectors. An efficient write barrier needs to interact very closely with the collector in use, being aware of either its internal data structures or the object marking in use. They usually feature between ten to thirty instructions and can't really afford the versatility of using function calls. One solution might be to specify the write barrier as a user-editable piece of code, which can be inserted as necessary by the compiler during the instruction generation phase. Developing our own compiler also gives us a chance to analyze the way finalizers are written, thus detecting actions that resuscitate objects being thrown away. This could be done by analyzing the use of this as an expression right-hand side or method parameter.