Lock-free Concurrent Level Hashing for Persistent Memory

With high memory density, non-volatility, and DRAMscale latency, persistent memory (PM) is promising to improve the storage system performance. Hashing-based index structures have been widely used in storage systems to provide fast query services. Recent research proposes crash-consistent and write-efficient hashing indexes for PM. However, existing PM hashing schemes suffer from limited scalability due to expensive lock-based concurrency control, thus making multi-core parallel programing inefficient in PM. The coarsegrained locks used in hash table resizing and queries (i.e., search/insertion/update/deletion) exacerbate the contention. Moreover, the cache line flushes and memory fences for crash consistency in the critical path increase the latency. In order to address the lock contention for concurrent hashing indexes in PM, we propose clevel hashing, a lock-free concurrent level hashing, to deliver high performance with crash consistency. In the clevel hashing, we design a multi-level structure for concurrent resizing and queries. Resizing operations are performed by background threads without blocking concurrent queries. For concurrency control, atomic primitives are leveraged to enable lock-free search/insertion/update/deletion. We further propose context-aware schemes to guarantee the correctness of interleaved queries. Using real Intel Optane DC PMM, experimental results with real-world YCSB workloads show that clevel hashing obtains up to 4.2× speedup than the state-of-the-art PM hashing index.