Active Memory Processor: A Hardware Garbage Collector for Real-Time Java Embedded Devices

Java possesses many advantages for embedded system development, including fast product deployment, portability, security, and a small memory footprint. As Java makes inroads into the market for embedded systems, much effort is being invested in designing real-time garbage collectors. The proposed garbage-collected memory module, a bitmap-based processor with standard DRAM cells is introduced to improve the performance and predictability of dynamic memory management functions that include allocation, reference counting, and garbage collection. As a result, memory allocation can be done in constant time and sweeping can be performed in parallel by multiple modules. Thus, constant time sweeping is also achieved regardless of heap size. This is a major departure from the software counterparts where sweeping time depends largely on the size of the heap. In addition, the proposed design also supports limited-field reference counting, which has the advantage of distributing the processing cost throughout the execution. However, this cost can be quite large and results in higher power consumption due to frequent memory accesses and the complexity of the main processor. By doing reference counting operation in a coprocessor, the processing is done outside of the main processor. Moreover, the hardware cost of the proposed design is very modest (about 8,000 gates). Our study has shown that 3-bit reference counting can eliminate the need to invoke the garbage collector in all tested applications. Moreover, it also reduces the amount of memory usage by 77 percent.