Performance and Power-Efficient Design of Dense Non-Volatile Cache in CMPs

In this paper, we present a novel cache design based on Multi-Level Cell Spin-Transfer Torque RAM (MLC STT-RAM) that can dynamically adjust the set capacity and associativity to efficiently use the full potential of MLC STT-RAM technology. We exploit the asymmetric nature of the MLC storage scheme to build cache lines featuring heterogeneous performances, that is, half of the cache lines are read-friendly, while the other half are write-friendly. Furthermore, we propose to opportunistically deactivate cache ways in underutilized sets to convert MLC to Single-Level Cell (SLC) mode, which features overall better performance and lifetime. Our ultimate goal is to build a cache architecture that combines the capacity advantages of MLC and performance/energy advantages of SLC. Our experimental evaluations show an average improvement of 43 percent in total numbers of conflict misses, 27 percent in memory access latency, 12 percent in system performance, and 26 percent in L3 access energy, with a slight degradation in lifetime (about 7 percent) compared to an SLC cache.