MALRU: Miss-penalty Aware LRU-based Cache Replacement for Hybrid Memory Systems

Current DRAM based memory systems face the scalability challenges in terms of storage density, power, and cost. Hybrid memory architecture composed of emerging Non-Volatile Memory (NVM) and DRAM is a promising approach to large-capacity and energy-efficient main memory. However, hybrid memory systems pose a new challenge to on-chip cache management due to the asymmetrical penalty of memory access to DRAM and NVM in case of cache misses. Cache hit rate is no longer an effective metric for evaluating memory access performance in hybrid memory systems. Current cache replacement policies that aim to improve cache hit rate are not efficient either. In this paper, we take into account the asymmetry of cache miss penalty on DRAM and NVM, and advocate a more general metric, Average Memory Access Time (AMAT), to evaluate the performance of hybrid memories. We propose a miss penalty-aware LRU-based (MALRU) cache replacement policy for hybrid memory systems. MALRU is aware of the source (DRAM or NVM) of missing blocks and prevents high-latency NVM blocks as well as low-latency DRAM blocks with good temporal locality from being evicted. Experimental results show that MALRU improves system performance against LRU and the state-of-the-art HAP policy by up to 20.4% and 11.7% (11.1% and 5.7% on average), respectively.