A Practical, Scalable, Relaxed Priority Queue

Priority queues are a fundamental data structure, and in highly concurrent software, scalable priority queues are an important building block. However, they have a fundamental bottleneck when extracting elements, because of the strict requirement that each extract() returns the highest priority element. In many workloads, this requirement can be relaxed, improving scalability. We introduce ZMSQ, a scalable relaxed priority queue. It is the first relaxed priority queue that supports each of the following important practical features: (i) guaranteed success of extraction when the queue is nonempty, (ii) blocking of idle consumers, (iii) memory-safety in non-garbage-collected environments, and (iv) relaxation accuracy that does not degrade as the thread count increases. In addition, our experiments show that ZMSQ is competitive with state-of-the-art prior algorithms, often significantly outperforming them.