Toward Write Optimization for Skyrmion Racetrack Memory by Skyrmion Repermutation

Skyrmion racetrack memory (Sky-RM), as an emerging nonvolatile memory technology, provides both high-density data storage and low-access latency, accompanying with a novel feature of using current to shift data along a racetrack. However, it is time-comsuing and energy-hungry to write/inject new particle-like Skyrimons on Sky-RM compared with other manipulations. Thus, the goal of this work is to presents a novel strategy to optimize write performance in Sky-RM, permutation-write (PW). Particularly, PW reduces the number of Skyrmion injections when writing data, by "repermuting" existing Skyrmion particles within a racetrack. Moreover, based on the flexible concept of repermutation, this work further proposes PW+ strategy, which is an optimized PW strategy integrated with injection and shift optimizations to achieve better performance. Our evaluation results justify that PW+ can greatly reduce the amount of Skyrmion injections, which are considered highly expensive in Sky-RM, by at least 50%, compared with other state-of-the-art write-optimized strategies. Furthermore, PW+ only incurs 1.79 injections, averaged for every 64-bit-word write. We show that PW+ strategy can bring significant benefits over other state-of-the-art write-optimized strategies by about 16%-50% for every 64-bit-word write, for both performance and energy efficiency.