CompEx++

Abstract

Multilevel/triple-level cell nonvolatile memories (MLC/TLC NVMs) such as phase-change memory (PCM) and resistive RAM (RRAM) are the subject of active research and development as replacement candidates for DRAM, which is limited by its high refresh power and poor scaling potential. In addition to the benefits of nonvolatility (low refresh power) and improved scalability, MLC/TLC NVMs offer high data density and memory capacity over DRAM. However, the viability of MLC/TLC NVMs is limited primarily due to the high programming energy and latency as well as the low endurance of NVM cells; these are primarily attributed to the iterative program-and-verify procedure necessary for programming the NVM cells. This article proposes compression-expansion (CompEx) coding, a low overhead scheme that synergistically integrates pattern-based compression with expansion coding to realize simultaneous energy, latency, and lifetime improvements in MLC/TLC NVMs. CompEx coding is agnostic to the choice of compression technique; in this work, we evaluate CompEx coding using both frequent pattern compression (FPC) and base-delta-immediate (BΔI) compression. CompEx coding integrates FPC/BΔI with ( k , m ) q “expansion” coding; expansion codes are a class of q -ary linear block codes that encode data using only the low energy states of a q -ary NVM cell. CompEx coding simultaneously reduces energy and latency and improves lifetime for negligible-to-no memory overhead and negligible logic overhead (≈ 10k gates, which is <0.1% per NVM module). Furthermore, we also propose CompEx++ coding, which extends CompEx coding by leveraging the variable compressibility of pattern-based compression techniques. CompEx++ coding integrates custom expansion codes to each of the compression patterns to exploit maximum energy/latency benefits of CompEx coding. Our full-system simulations using TLC RRAM show that CompEx/CompEx++ coding reduces total memory energy by 57%/61% and write latency by 23.5%/26%; these improvements translate to a 5.7%/10.6% improvement in IPC, a 11.8%/19.9% improvement in main memory bandwidth, and 1.8 × improvement in lifetime over classical binary coding using data-comparison write. CompEx/CompEx++ coding thus addresses the programming energy/latency and lifetime challenges of MLC/TLC NVMs that pose a serious technological roadblock to their adoption in high-performance computing systems.