PACTree: A High Performance Persistent Range Index Using PAC Guidelines

Non-Volatile Memory (NVM), which provides relatively fast and byte-addressable persistence, is now commercially available. However, we cannot equate a real NVM with a slow DRAM, as it is much more complicated than we expect. In this work, we revisit and analyze both NVM and NVM-specific persistent memory indexes. We find that there is still a lot of room for improvement if we consider NVM hardware, its software stack, persistent index design, and concurrency control. Based on our analysis, we propose Packed Asynchronous Concurrency (PAC) guidelines for designing high-performance persistent index structures. The key idea behind the guidelines is to 1) access NVM hardware in a packed manner to minimize its bandwidth utilization and 2) exploit asynchronous concurrency control to decouple the long NVM latency from the critical path of the index. We develop PACTree, a high-performance persistent range index following the PAC guidelines. PACTree is a hybrid index that employs a trie index for its internal nodes and B+-tree-like leaf nodes. The trie index structure packs partial keys in internal nodes. Moreover, we decouple the trie index and B+-tree-like leaf nodes. The decoupling allows us to prevent blocking concurrent accesses by updating internal nodes asynchronously. Our evaluation shows that PACTree outperforms state-of-the-art persistent range indexes by 7x in performance and 20x in 99.99 percentile tail latency.