FastTrack: Foreground App-Aware I/O Management for Improving User Experience of Android Smartphones

The quality of user experience on a smartphone is directly affected by how fast a foreground app reacts to user inputs. Although existing Android smartphones properly differentiate a foreground app from background apps for most system activities, one major exception is the I/O service where I/O-priority inversions between a foreground app and background apps are commonly observed. In this paper, we investigate the I/O-priority inversion problem on Android smartphones. From our empirical study with real Android smartphones, we observed that the existing techniques for mitigating I/O-priority inversions are not applicable for smartphones where frequently inverted I/O priorities should be quickly corrected to avoid any user-perceived extra delay. We also identified that most noticeable I/O-priority inversions occur in the page cache and a flash storage device. Based on the analysis results, we propose a foreground app-aware I/O management scheme, called FastTrack, that accelerates foreground /O requests by 1) preempting background I/O requests in the entire I/O stacks including the storage device and 2) preventing foreground app's data from being flushed from the page cache. Our experimental results using a prototype FastTrack implementation on four smartphones show that a foreground app can achieve the equivalent level of user-perceived responsiveness regardless of the number of background apps. Over the existing Android I/O implementation, FastTrack can reduce the average user response time by 94% when six I/O-intensive apps run as background apps.