RCU-HTM: Combining RCU with HTM to Implement Highly Efficient Concurrent Binary Search Trees

Abstract

In this paper we introduce RCU-HTM, a technique that combines Read-Copy-Update (RCU) with Hardware Transactional Memory (HTM) to implement highly efficient concurrent Binary Search Trees (BSTs). Similarly to RCU-based algorithms, we perform the modifications of the tree structure in private copies of the affected parts of the tree rather than in-place. This allows threads that traverse the tree to proceed without any synchronization and without being affected by concurrent modifications. The novelty of RCU-HTM lies at leveraging HTM to permit multiple updating threads to execute concurrently. After appropriately modifying the private copy, we execute an HTM transaction, which atomically validates that all the affected parts of the tree have remained unchanged since they've been read and, only if this validation is successful, installs the copy in the tree structure.We apply RCU-HTM on AVL and Red-Black balanced BSTs and compare theirperformance to state-of-the-art lock-based, non-blocking, RCU- and HTM-basedBSTs. Our experimental evaluation reveals that BSTs implemented with RCU-HTMachieve high performance, not only for read-only operations, but also for update operations. More specifically, our evaluation includes a diverse range of tree sizes and operation workloads and reveals that BSTs based on RCU-HTM outperform other alternatives by more than 18%, on average, on a multi-core server with 44 hardware threads.