Constructing Large and Fast On-Chip Cache for Mobile Processors with Multilevel Cell STT-MRAM Technology

Modern mobile processors integrating an increasing number of cores into one single chip demand large-capacity, on-chip, last-level caches (LLCs) in order to achieve scalable performance improvements. However, adopting traditional memory technologies such as SRAM and embedded DRAM (eDRAM) leakage and scalability problems. Spin-transfer torque magnetic RAM (STT-MRAM) is a novel nonvolatile memory technology that has emerged as a promising alternative for constructing on-chip caches in high-end mobile processors. STT-MRAM has many advantages, such as short read latency, zero leakage from the memory cell, and better scalability than eDRAM and SRAM. Multilevel cell (MLC) STT-MRAM further enlarges capacity and reduces per-bit cost by storing more bits in one cell.

However, MLC STT-MRAM has long write latency which limits the effectiveness of MLC STT-MRAM-based LLCs. In this article, we address this limitation with three novel designs: line pairing (LP), line swapping (LS), and dynamic LP/LS enabler (DLE). LP forms fast cache lines by reorganizing MLC soft bits which are faster to write. LS dynamically stores frequently-written data into these fast cache lines. We then propose a dynamic LP/LS enabler (DLE) to enable LP and LS only if they help to improve the overall cache performance. Our experimental results show that the proposed designs improve system performance by 9--15% and reduce energy consumption by 14--21% for various types of mobile processors.