SEAL: User Experience-Aware Two-Level Swap for Mobile Devices

App caching is important for mobile devices, which enables fast switching and state restoration of apps by caching all the pages in memory. Memory swapping can improve app caching capability by evicting pages to the secondary storage. However, enabling memory swapping could induce jitters in interactions, which significantly degrades the user experience. As a result, storage-based swapping is disabled by default in most mobile devices. This article proposes a novel swap framework, SEAL, a user experience-aware two-level swapping, which maximizes the benefits of memory swapping and minimizes the negative impact on user experience in interactions. Inspired by a study on the access characteristics of a set of popular apps on mobile devices, the framework adopts compressed memory as the first swap level (SL1) and secondary storage as the second swap level (SL2). To optimize user experience comprehensively, three schemes are proposed. First, a novel page identification scheme is proposed to guide the page placement between these two levels. Second, a hidden page loading (HPL) scheme is proposed to load pages from SL2 to SL1 for optimized user experience during app execution. Finally, an app-granularity swapping scheme is proposed to swap data in the unit of apps. Experiments on real devices show that app caching capability is improved by $2.43\times $ on average when enabling SEAL while minimizing the negative impact on user experience.