Killing Processes or Killing Flash? Escaping from the Dilemma Using Lightweight, Compression-Aware Swap for Mobile Devices

Android apps become increasingly memory-demanding as software vendors add more and more new features to their apps. In the mean time, Android users often launch multiple apps and conveniently switch back and forth among the apps. Although running multiple apps imposes a high pressure on memory management, virtual-memory swap, an essential feature to improve the degree of multitasking, is disabled in fear of premature retirement of flash-based storage devices. Instead, Android employs a termination-based, process-level memory reclaiming method. We observed that process killing is, unfortunately, not effective in memory reclaiming and is highly negative to user experience. In this study, we advocate re-thinking using swap in Android for improved user experience with managed write stress on flash storage. Based on a series of empirical analyses of swap activities, we propose an enhanced page replacement policy and a page-compressing frontswap module. The proposed page replacement policy jointly considers page activeness and compressibility to boost the compression ratio of swap writes. A sampled-based method for page compressibility prediction is introduced so that decisions on page replacement can be made without compressing every page. We also design a frontswap module that strategically organizes compressed pages in the swap space for reducing the overhead of swap I/O operations. Experimental results showed that compared with process killing, our method improved the app launching time and energy consumption by 58% and 19%, respectively; compared with the original swap, our approach reduced the swap write stress by 65%.