Multi-Interval DomLock: Toward Improving Concurrency in Hierarchies

Locking has been a predominant technique depended upon for achieving thread synchronization and ensuring correctness in multi-threaded applications. It has been established that the concurrent applications working with hierarchical data witness significant benefits due to multi-granularity locking (MGL) techniques compared to either fine- or coarse-grained locking. The de facto MGL technique used in hierarchical databases is intention locks, which uses a traversal-based protocol for hierarchical locking. A recent MGL implementation, dominator-based locking (DomLock), exploits interval numbering to balance the locking cost and concurrency and outperforms intention locks for non-tree-structured hierarchies. We observe, however, that depending upon the hierarchy structure and the interval numbering, DomLock pessimistically declares subhierarchies to be locked when in reality they are not. This increases the waiting time of locks and, in turn, reduces concurrency. To address this issue, we present Multi-Interval DomLock (MID), a new technique to improve the degree of concurrency of interval-based hierarchical locking. By adding additional intervals for each node, MID helps in reducing the unnecessary lock rejections due to false-positive lock status of sub-hierarchies. Unleashing the hidden opportunities to exploit more concurrency allows the parallel threads to finish their operations quickly, leading to notable performance improvement. We also show that with sufficient number of intervals, MID can avoid all the lock rejections due to false-positive lock status of nodes. MID is general and can be applied to any arbitrary hierarchy of trees, Directed Acyclic Graphs (DAGs), and cycles. It also works with dynamic hierarchies wherein the hierarchical structure undergoes updates. We illustrate the effectiveness of MID using STMBench7 and, with extensive experimental evaluation, show that it leads to significant throughput improvement (up to 141%, average 106%) over DomLock.