Power-Utility-Driven Write Management for MLC PCM

Abstract

Phase change memory (PCM) is a promising alternative to Dynamic Random Access Memory (DRAM) as main memory due to its merits of high density and low leakage power. Multi-level Cell (MLC) PCM is more attractive than Single-level Cell (SLC) PCM, because it can store multiple bits per cell to achieve higher density and lower per-bit cost. With the iterative program-verify write technique, MLC PCM writes demand at much higher power than DRAM writes, while the power supply system of MLC memory system is similar to that of DRAM, and the power capability is limited. The incompatibility of high write power and limited power budget results in the degradation of the write throughput and performance in MLC PCM. In this work, we investigate both write scheduling policy and power management to improve the MLC power utility and alleviate the negative impacts induced by high write power. We identify the power-utility-driven write scheduling as an online bin-packing problem and then derive a power-utility-driven scheduling (PUDS) policy from the First Fit algorithm to improve the write power usage. Based on the ramp-down characteristic of the SET pulse (the pulse changes the PCM to high resistance), we propose the SET Power Amortization (SPA) policy, which proactively reclaims the power tokens at the intra-SET level to promote the power utilization. Our experimental results demonstrate that the PUDS and SPA respectively achieve 24% and 27% performance improvement over the state-of-the-art power management technique, and the PUDS8SPA has an overall 31% improvement of the power utility and 50% increase of performance compared to the baseline system.