i²WAP: Improving non-volatile cache lifetime by reducing inter- and intra-set write variations

Modern computers require large on-chip caches, but the scalability of traditional SRAM and eDRAM caches is constrained by leakage and cell density. Emerging non-volatile memory (NVM) is a promising alternative to build large on-chip caches. However, limited write endurance is a common problem for non-volatile memory technologies. In addition, today's cache management might result in unbalanced write traffic to cache blocks causing heavily-written cache blocks to fail much earlier than others. Unfortunately, existing wear-leveling techniques for NVM-based main memories cannot be simply applied to NVM-based on-chip caches because cache writes have intra-set variations as well as inter-set variations. To solve this problem, we propose i 2 WAP, a new cache management policy that can reduce both inter- and intra-set write variations. i 2 WAP has two features: (1) Swap-Shift, an enhancement based on previous main memory wear-leveling to reduce cache inter-set write variations; (2) Probabilistic Set Line Flush, a novel technique to reduce cache intra-set write variations. Implementing i 2 WAP only needs two global counters and two global registers. By adopting i 2 WAP, we can improve the lifetime of on-chip non-volatile caches by 75% on average and up to 224%.