A Scalable Wear Leveling Technique for Phase Change Memory

Abstract

Phase Change Memory (PCM), one of the recently proposed non-volatile memory technologies, has been suffering from low write endurance. For example, a single layer PCM cell could only be written approximately 10 8 times. This limits the lifetime of a PCM-based memory to a few days rather than years when memory intensive applications are running. Wear leveling techniques have been proposed to improve the write endurance of a PCM. Among those techniques, the region based start-gap (RBSG) scheme is widely cited as achieving the highest lifetime. Based on our experiments, RBSG can achieve 97% of the ideal lifetime but only for relatively small memory sizes (e.g. 8GB-32GB). As the memory size goes up, RBSG becomes less effective and its expected percentage of the ideal lifetime reduces to less than 57% for a 2TB PCM. In this paper, we propose a table-based wear leveling scheme called block grouping to enhance the write endurance of a PCM with a negligible overhead. Our research results show that with a proper configuration and adoption of partial writes (writing back only 64B subblocks instead of a whole row to the PCM arrays) and internal row shift (shifting the subblocks in a row periodically so no subblock in a row will be written repeatedly), the proposed block grouping scheme could achieve 95% of the ideal lifetime on average for the Rodinia, NPB, and SPEC benchmarks with less than 1.74% performance overhead and up to 0.18% hardware overhead. Moreover, our scheme is scalable and achieves the same percentage of ideal lifetime for PCM of size from 8GB to 2TB. We also show that the proposed scheme can better tolerate memory write attacks than WoLFRAM (Wear Leveling and Fault Tolerance for Resistive Memories) and RBSG for a PCM of size 32GB or higher. Finally, we integrate an error-correcting pointer technique into our proposed block grouping scheme to make the PCM fault tolerant against hard errors.