A Hierarchical CLH Queue Lock

Abstract

Modern multiprocessor architectures such as CC-NUMA machines or CMPs have nonuniform communication architectures that render programs sensitive to memory access locality. A recent paper by Radović and Hagersten shows that performance gains can be obtained by developing general-purpose mutual-exclusion locks that encourage threads with high mutual memory locality to acquire the lock consecutively, thus reducing the overall cost due to cache misses. Radović and Hagersten present the first such hierarchical locks. Unfortunately, their locks are backoff locks, which are known to incur higher cache miss rates than queue-based locks, suffer from various fundamental fairness issues, and are hard to tune so as to maximize locality of lock accesses.

Extending queue-locking algorithms to be hierarchical requires that requests from threads with high mutual memory locality be consecutive in the queue. Until now, it was not clear that one could design such locks because collecting requests locally and moving them into a global queue seemingly requires a level of coordination whose cost would defeat the very purpose of hierarchical locking.

This paper presents a hierarchical version of the Craig, Landin, and Hagersten CLH queue lock, which we call the HCLH queue lock. In this algorithm, threads build implicit local queues of waiting threads, splicing them into a global queue at the cost of only a single CAS operation.

In a set of microbenchmarks run on a large scale multiprocessor machine and a state-of-the-art multi-threaded multi-core chip, the HLCH algorithm exhibits better performance and significantly better fairness than the hierarchical backoff locks of Radović and Hagersten.

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