An Energy-Efficient Virtual Memory System with Flash Memory as the Secondary Storage

The traditional virtual memory system is designed for decades assuming a magnetic disk as the secondary storage. Recently, flash memory becomes a popular storage alternative for many portable devices with the continuing improvements on its capacity, reliability and much lower power consumption than mechanical hard drives. The NAND flash memory is organized with blocks, and each block contains a set of pages. The characteristics of flash memory are quite different from a magnetic disk. Therefore, in this paper, we revisit virtual memory system design considering limitations imposed by flash memory. In particular, we study the effects of the subpaging technique and storage cache management. In the traditional virtual memory system, a full page is written back to the secondary storage on a page fault. We found that this could result in unnecessary writes thereby wasting energy. The subpaging technique that partitions a page into subunits, and only dirty subpages are written to flash memory is beneficial to the energy efficiency. For the storage cache management, unlike traditional disk cache management, care needs to be taken to guarantee that the flash pages of a main memory page are replaced from the cache in sequence. Experimental results show that the average energy reduction of combined subpaging and caching techniques is 35.6%.