Dalea: A Persistent Multi-Level Extendible Hashing with Improved Tail Performance

Persistent memory (PM) promises byte-addressability, large capacity, and durability. Main memory systems, such as key-value stores and in-memory databases, benefit from such features of PM. Due to the great popularity of hashing index in main memory systems, a number of research efforts are made to provide high average performance persistent hashing. However, suboptimal tail performance in terms of tail throughput and tail latency is still observed for existing persistent hashing. In this paper, we analyze major sources of suboptimal tail performance from key design issues of persistent hashing. We identify the global hash structure and concurrency control as remaining explorable design spaces for improving tail performance. We propose Directory-sharing Multi-level Extendible Hashing (Dalea) for PM. Dalea designs ancestor link-based extendible hashing as well as fine-grained transient lock to address the two main sources (rehashing and locking) affecting tail performance. The evaluation results show that, compared with state-of-the-art persistent hashing Dash, Dalea achieves increased tail throughput by 4.1x and reduced tail latency by 5.4x. Moreover, in order to provide design guidelines for improving tail performance, we adopt Dalea as a testbed to identify different impacts of four factors on tail performance, including fine-grained rehashing, transient locking, memory pre-allocation, and fingerprinting.