An Efficient Directory Entry Lookup Cache with Prefix-Awareness for Mobile Devices

Modern mobile devices, such as smartphones, maintain a directory cache (DCache) to accelerate directory and file accesses. However, the original DCache recursively walks through all the components of a path name for each directory entry (dentry) lookup operation, leading to low lookup efficiency. In this article, we first investigate intrinsic characteristics of dentry lookup operations in smartphones and make several interesting findings: 1) file path lookup operations are called frequently (up to 104 times per second) by mobile applications; 2) file path lookup operations present high temporal and spatial locality; and 3) the latency of a file path lookup is linear to the depth of the path name. Based on our findings, we further propose an efficient directory entry lookup cache architecture, named dynamic skipping cache (DS-Cache), which adopts an ASCII-based hash table to simplify the path lookup complexity. In DS-Cache, a dynamic skip lookup algorithm is proposed to skip the common prefixes of different accessing paths. A rename algorithm and a delete algorithm are designed to promise the consistence of DS-Cache and the backend file system. We also design a prefix-aware cache replacement scheme to optimize the DS-Cache hit ratio. We have implemented and deployed DS-Cache on a Google Nexus 6P smartphone. The experimental results show that we can significantly reduce the latency of invoking system calls by up to 81%, and further reduce the completion time of real-world mobile applications by up to 67%.