WOM-Code Solutions for Low Latency and High Endurance in Phase Change Memory

Abstract

This paper describes a write-once-memory-code phase change memory (WOM-code PCM) architecture for next-generation non-volatile memory applications. Specifically, we address the long latency of the write operation in PCM—attributed to PCM SET—by proposing a novel PCM memory architecture that integrates the $\langle 2^2\rangle ^2/3$ WOM-code at the memory organization and memory controller levels. To further improve the write latency of WOM-code PCM, we propose a PCM-refresh approach that uses idle cycles to preemptively set PCM rows to the initial WOM-code state. Finally, to balance write latency improvements against WOM-code PCM overhead, we propose a WOM-code cached PCM (WCPCM) architecture that uses WOM-code PCM as the cache alongside conventional PCM main memory. Since WOM-code techniques inherently impact PCM endurance by increasing the number of bit-writes in comparison to unencoded PCM, we incorporate additional transitions from the $\langle 2^2\rangle ^2/3$ WOM-code transition graph to realize endurance-WOM-code (e-WOM-code) architectures. Transitions between the e-WOM-code states on writes to memory are integrated into an incremental coding for endurance (ICE) approach that exploits redundancies in the conventional WOM-code to reduce the number of bit-writes over unencoded PCM. Simulation results show that the proposed e-WOM-code PCM architecture is able to reduce memory write (read) latency by 19.8 percent (14.7 percent) and the number of bit-writes over unencoded PCM without (with) data-comparison write (DCW), a read-modify-write process that only updates changed cells, by 83.0 percent (22.1 percent) on average across general-purpose (SPEC CPU2006), embedded (MiBench), and high-performance (SPLASH-2) benchmarks. Further, e-WOM-code PCM with PCM-refresh can reduce memory write (read) latency by 51.5 percent (44.1 percent) and the number of bit-writes over unencoded PCM without DCW by 76.5 percent on average across the benchmarks; there is, however, an increase of 19 percent in the number of bit-writes over unencoded PCM with DCW. Finally, for just 4.7 percent memory overhead, the e-WOM-code cached PCM (e-WCPCM) architecture reduces memory write (read) latency by 47.5 percent (41.6 percent) and the number of bit-writes over unencoded PCM without DCW by 68.1 percent on average across the benchmarks; again, there is a 49 percent increase in the number of bit-writes over unencoded PCM with DCW.