Towards refresh-optimized EDRAM-based caches with a selective fine-grain round-robin refresh scheme

Recently, EDRAM cells have gained much attention as a promising alternative to construct on-chip memories. However, due to inherent characteristics of DRAM cells, they need to be refreshed periodically, causing a huge refresh energy burden. Particularly, employing EDRAM cells in large-scale last-level caches will make refresh burden much higher due to their large capacity. In this paper, we propose a selective fine-grain round-robin refresh scheme for both performance improvement and refresh energy reduction. To reduce bank conflicts between normal cache accesses and refresh operations, we employ a refresh scheme which refreshes cache lines in a bank-wise round-robin fashion. We also apply a selective refresh depending on the inclusive information in cache hierarchies. For the data which reside in both LLC and upper-level cache (i.e., L2 cache), the data access will be filtered by the upper-level cache. Based on this insight, we skip the refresh to the cache block in the EDRAM-based LLC which also exists in the upper-level caches. By doing so, we can reduce unnecessary refresh operations in EDRAM-based LLCs. According to our evaluation, our proposed scheme improves performance by 7.3% and reduces energy per instruction by 13.3% compared to the baseline all-bank refresh scheme.