OptiQL: Robust Optimistic Locking for Memory-Optimized Indexes

Abstract

Modern memory-optimized indexes often use optimistic locks for concurrent accesses. Read operations can proceed optimistically without taking the lock, greatly improving performance on multicore CPUs. But this is at the cost of robustness against contention where many threads contend on a small set of locks, causing excessive cacheline invalidation, interconnect traffic and eventually performance collapse. Yet existing solutions often sacrifice desired properties such as compact 8-byte lock size and fairness among lock requesters. This paper presents optimistic queuing lock (OptiQL), a new optimistic lock for database indexing to solve this problem. OptiQL extends the classic MCS lock---a fair, compact and robust mutual exclusion lock---with optimistic read capabilities for index workloads to achieve both robustness and high performance while maintaining various desirable properties. Evaluation using memory-optimized B+-trees on a 40-core, dual-socket server shows that OptiQL matches existing optimistic locks for read operations, while avoiding performance collapse under high contention.