Hardware/software cooperative caching for hybrid DRAM/NVM memory architectures.

Non-Volatile Memory (NVM) has recently emerged for its nonvolatility, high density and energy efficiency. Hybrid memory systems composed of DRAM and NVM have the best of both worlds, because NVM can offer larger capacity and have near-zero standby power consumption while DRAM provides higher performance. Many studies have advocated to use DRAM as a cache to NVM. However, it is still an open problem on how to manage the DRAM cache effectively and efficiently. In this paper, we propose a novel Hardware/Software Cooperative Caching (HSCC) mechanism that organizes NVM and DRAM in a flat address space while logically supporting a cache/memory hierarchy. HSCC maintains the NVM- to-DRAM address mapping and tracks the access counts of NVM pages through a moderate extension to page tables and TLBs. It significantly simplifies the hardware design and offers several optimization opportunities for cache management in software layers. We thus propose utility-based cache filtering policies to improve the efficiency of DRAM cache. Experimental results show that HSCC improves system performance by up to 9.6X (77.2% on average) and reduces energy consumption by 34.3% on average, compared to a hardware-assisted DRAM/NVM memory system. HSCC also presents 15.4% and 14.5% performance improvement against a flat- addressable memory architecture and a Row Buffer Locality Aware (RBLA) caching policy for hybrid memories, respectively.