Efficient STT-RAM last-level-cache architecture to replace DRAM cache

Abstract

Recent research has proposed die-stacked Last Level Cache (LLC) to overcome the Memory Wall. Lately, Spin-Transfer-Torque Random Access Memory (STT-RAM) caches have been recommended as they provide improved energy efficiency compared to DRAM caches. However, the recently proposed STT-RAM cache architecture unnecessarily dissipates energy by fetching unneeded cache lines into the row buffer. In this paper, we propose a Selective Read Policy for STT-RAM. This is policy only fetches those cache lines into the row buffer that are likely to be reused. This is reduces the number of cache line reads and thereby reduces the energy consumption. Further, we propose two key performance optimizations namely Row Buffer Tags Bypass Policy and LLC Data Cache. Both optimizations reduce the LLC access latency and therefore improve the overall performance. For evaluation, we implement our proposed architecture in the Zesto simulator and run different combinations of SPEC2006 benchmarks on an 8-core system. We show that our synergetic policies reduce the average LLC dynamic energy consumption by 72.6% and improve the system performance by 1.3% compared to the recently proposed STT-RAM LLC. Compared to the state-of-the-art DRAM LLC, our architecture reduces the LLC dynamic energy consumption by 90.6% and improves system performance by 1.4%.