MorLog: Morphable Hardware Logging for Atomic Persistence in Non-Volatile Main Memory

Abstract

Byte-addressable non-volatile memory (NVM) is emerging as an alternative for main memory. Non-volatile main memory (NVMM) systems are required to support atomic persistence and deal with the high overhead of programming NVM cells. To this end, recent studies propose hardware logging and data encoding designs for NVMM systems. However, prior hardware logging designs incur either extra ordering constraints or redundant log data. Moreover, existing data encoding designs are unaware of the characteristics of log data, resulting in writing unnecessary log bits. In this paper, we propose a morphable hardware logging design (MorLog) that only logs the data necessary for recovery and dynamically selects encoding methods with least write overhead. We observe that (1) only the oldest undo and the newest redo data in each transaction are necessary for recovery, and (2) the log data for clean bits are clean. The first motivates our morphable logging mechanism. This mechanism logs both undo and redo data for the first update to the data in a transaction, and then logs only redo data. Undo data are eagerly written to NVMM to ensure atomicity, while redo data are buffered in a volatile log buffer and L1 caches to write only the newest redo data to NVMM. The second motivates our selective log data encoding mechanism. This mechanism simultaneously encodes log data with different methods, and writes the encoded log data with the least write cost to NVMM. We devise a differential log data compression method to exploit the characteristics of log data. This method directly discards clean bits from log data and compresses remained dirty bits. Our evaluation shows that MorLog improves performance by 72.5%, reduces NVMM write traffic by 41.1%, and decreases NVMM write energy by 49.9% compared with the state-of-the-art design.