Evaluation of memory system extensions

A traditional memory system for a uniprocessor consists of one or two levels of cache, a main memory and a backing store. One can extend such a memory sys tem by adding inexpensive but slower memories into the memory hierarchy. This paper uses an experimental approach to evaluate two methods of extending a memory system: direct and caching. The direct method adds the slower memory into the memory hierarchy by putting it at the same level as the main memory, allowing the CPU to access the slower memories directly; whereas the caching method puts the slower memory between the main memory and the backing store, using the main memory as a cache for the slower memory. We have implemented both approaches and our experiments indicate that applications with very large data structures can benefit significantly using an extended memory system, and that the direct approach outperforms the caching approach in memory-bound applications.