Row Buffer Locality Aware Caching Policies for Hybrid Memories

Phase change memory (PCM) is a promising technology that can offer higher capacity than DRAM. Unfortunately, PCM's access latency and energy are higher than DRAM's and its endurance is lower. Many DRAM-PCM hybrid memory systems use DRAM as a cache to PCM, to achieve the low access latency and energy, and high endurance of DRAM, while taking advantage of PCM's large capacity. A key question is what data to cache in DRAM to best exploit the advantages of each technology while avoiding its disadvantages as much as possible. We propose a new caching policy that improveshybrid memory performance and energy efficiency. Our observation is that both DRAM and PCM banks employ row buffers that act as a cache for the most recently accessed memory row. Accesses that are row buffer hits incur similar latencies (and energy consumption) in DRAM and PCM, whereas accesses that are row buffer misses incur longer latencies (and higher energy consumption) in PCM. To exploit this, we devise a policy that avoids accessing in PCM data that frequently causes row buffermisses because such accesses are costly in terms of both latency and energy. Our policy tracks therow buffer miss counts of recently used rows in PCM, and caches in DRAM the rows that are predicted to incur frequent row buffer misses. Our proposed caching policy also takes into account the high write latencies of PCM, in addition to row buffer locality. Compared to a conventional DRAM-PCM hybridmemory system, our row buffer locality-aware caching policy improves system performance by 14% and energy efficiency by 10% on data-intensive server and cloud-type workloads. The proposed policyachieves 31% performance gain over an all-PCM memory system, and comes within 29% of the performance of an allDRAM memory system (not taking PCM's capacity benefit into account) on evaluated workloads.