DP2: reducing transaction overhead with differential and dual persistency in persistent memory

Emerging non-volatile memory (NVRAM) technologies, like phase change memory, envision persistent memory architectures. In case of power failure, operations on persistent memory should be in transactional semantics by adopting techniques such as WAL. To ensure consistency and atomicity, persist barriers are widely adopted, to prevent persistent memory controller from scheduling writes and exploiting bank-level parallelism of NVRAM devices. Besides, unified retention time for persistent writes, i.e., log and data writes, further reduces the performance of persistent memory system, while retention time for log writes does not need to be so long due to periodic truncation. In this paper, we study how NVRAM write latency affects the system throughput and propose DP2, which consists of two main techniques: differential persistency and dual persistency. Differential persistency distinguishes log writes from data writes, and enhances the NVRAM memory controller to schedule log writes across persist barriers to fully utilize bank level parallelism. Dual persistency relaxes the retention time of log writes to reduce write latency and the iterations per write, which in turn enhances lifetime of NVRAM devices. Evaluation results show that our proposed techniques improve system throughput up by 43% on average and extend lifetime up by 47%, with 104-s retention time for log writes.