Energy Aware Persistence: Reducing Energy Overheads of Memory-based Persistence in NVMs

Next generation byte addressable nonvolatile memories (NVMs) such as PCM, Memristor, and 3D X-Point are attractive solutions for mobile and other end-user devices, as they offer memory scalability as well as fast persistent storage. However, NVM's limitations of slow writes and high write energy are magnified for applications that require atomic, consistent, isolated and durable (ACID) persistence. For maintaining ACID persistence guarantees, applications not only need to do extra writes to NVM but also need to execute a significant number of additional CPU instructions for performing NVM writes in a transactional manner. Our analysis shows that maintaining persistence with ACID guarantees increases CPU energy up to 7.3x and NVM energy up to 5.1x compared to a baseline with no ACID guarantees. For computing platforms such as mobile devices, where energy consumption is a critical factor, it is important that the energy cost of persistence is reduced.

To address the energy overheads of persistence with ACID guarantees, we develop novel energy-aware persistence (EAP) principles that identify data durability (logging) as the dominant factor in energy increase. Next, for low energy states, we formulate energy efficient durability techniques that include a mechanism to switch between performance and energy efficient logging modes, support for NVM group commit, and a memory management method that reduces energy by trading capacity via less frequent garbage collection. For critical energy states, we propose a relaxed durability mechanism -- ACI-RD -- that relaxes data logging without affecting the correctness of an application. Finally, we evaluate EAP's principles with real applications and benchmarks. Our experimental results demonstrate up to 2x reduction in CPU and 2.4x reduction in NVM energy usage compared to the traditional ACID persistence.