Differential Write-Conscious Software Design on Phase-Change Memory: An SQLite Case Study

Phase-change memory (PCM) has several benefits including low cost, non-volatility, byte-addressability, etc., and limitations such as write endurance. There have been several hardware approaches to exploit the benefits while minimizing the negative impact of limitations. Software approaches could give further improvements, when used together with hardware approaches, by taking advantage of write behavior present in the program, e.g., write behavior on dynamically allocated data, which is hardly captured by hardware approaches. This work proposes a software design methodology to reduce costly PCM writes. First, on top of existing hardware approach such as Flip-N-Write, we advocate exploiting the capability of PCM bit-level differential write in the software by judiciously reusing previously allocated memory resource. In order to avoid wear-out incurred by the reuse, we present software-based wear-leveling methods that distribute writes across PCM cells. In order to further reduce PCM writes, we propose identifying data, the loss of which does not affect the functionality of the underlying software, and then diverting write traffic for those data items to volatile memory. To evaluate the effectiveness of these methods, as a case study, we applied the proposed methods to the design of journaling in SQLite, which is an important database application commonly used in smartphones. For the experiments, we used an in-house PCM-based prototype board. Our experiments with four representative mobile applications show that the proposed design methods, which is applied on top of the hardware approach, Flip-N-Write, result in 75.2% further reduction in total bit updates in PCM, on average, without aggravating wear-out compared with the baseline of PCM-based journaling, which is based only on the hardware approach. Also, the proposed design methods result in 49.4% reduction in energy consumption and 52.3% reduction in runtime compared to a typical FIFO management of free resources.