A write-optimal and concurrent persistent dynamic hashing with radix tree assistance

Abstract

Non-volatile memory (NVM) is expected to coexist with DRAM as a hybrid memory to fully exploit DRAM’s low read–write latency and NVM’s high density, persistence, and low standby power. However, existing persistent hashing schemes cannot efficiently reap the hybrid-memory benefits, and suffer from a significant performance penalty due to consistency guarantee. In this paper, we present an optimized extendible hashing variant for hybrid DRAM-NVM memory, named OP-HMEH. In our design, hash buckets are persisted in NVM while the directory is placed in DRAM for faster access. We also keep a radix-tree-structured directory in NVM to instantaneously recover the directory in DRAM after system crashes. To reduce consistency guarantee overhead, OP-HMEH designs a cross-KV mechanism to reorganize items which can avoid the use of expensive persist barriers in most cases. Meanwhile, we employ low-overhead structural modification operations schemes to further improve system performance. For concurrency control, the original version uses two lock-based techniques. We then propose an optimistic concurrency strategy that exploits lightweight locking to protect segments and enables lock-free search/operations of directories by leveraging atomic instructions. On real Intel Optane DCPMM, experimental results with YCSB workloads show that our OP-HMEH outperforms the state-of-the-art NVM-based hashing structures by up to 2.18×. The optimized HMEH obtains higher insert performance than the original HMEH.