NRHI: A Concurrent Non-Rehashing Hash Index for Persistent Memory

Persistent memory (PM) featured with data persistence, byte-addressability, and DRAM-like performance has been commercially available with the advent of Intel® Optane™ DC persistent memory. The DRAM-like performance and disk-like persistence invite shifting hashing-based index schemes, which are important building blocks of today’s internet service infrastructures to provide fast queries, from DRAM onto persistent memory. Numerous hash indexes for persistent memory have been proposed to optimize writes and crash consistency, but with poor scalability under resizing. Generally, resizing consists of allocating a new hash table and rehashing items from the old table into the new one. We argue that resizing with rehashing performed in either blocking or non-blocking way can degrade the overall performance and limit the scalability.In order to mitigate the limitation of resizing, this paper proposes a Non-Rehashing Hash Index (NRHI) scheme to perform resizing with no necessity of rehashing items. NRHI leverages a layered structure to link hash tables without moving key-value pairs across layers, thus reducing the time spent on rehashing in blocking way and alleviating slots contention occurred in non-blocking way. Furthermore, the compare-and-swap primitive is utilized to support concurrent lock-free hashing operations. Experimental results on real PM hardware show that NRHI outperforms the state-of-the-art PM hash indexes by 1.7× to 3.59×, and scales linearly with the number of threads.