The spraylist: A scalable relaxed priority queue

D Alistarh, J Kopinsky, J Li, N Shavit�- Proceedings of the 20th ACM�…, 2015 - dl.acm.org
Proceedings of the 20th ACM SIGPLAN Symposium on Principles and Practice of�…, 2015dl.acm.org
High-performance concurrent priority queues are essential for applications such as task
scheduling and discrete event simulation. Unfortunately, even the best performing
implementations do not scale past a number of threads in the single digits. This is because
of the sequential bottleneck in accessing the elements at the head of the queue in order to
perform a DeleteMin operation. In this paper, we present the SprayList, a scalable priority
queue with relaxed ordering semantics. Starting from a non-blocking SkipList, the main�…
High-performance concurrent priority queues are essential for applications such as task scheduling and discrete event simulation. Unfortunately, even the best performing implementations do not scale past a number of threads in the single digits. This is because of the sequential bottleneck in accessing the elements at the head of the queue in order to perform a DeleteMin operation. In this paper, we present the SprayList, a scalable priority queue with relaxed ordering semantics. Starting from a non-blocking SkipList, the main innovation behind our design is that the DeleteMin operations avoid a sequential bottleneck by ``spraying'' themselves onto the head of the SkipList list in a coordinated fashion. The spraying is implemented using a carefully designed random walk, so that DeleteMin returns an element among the first O(p log^3 p) in the list, with high probability, where p is the number of threads. We prove that the running time of a DeleteMin operation is O(log^3 p), with high probability, independent of the size of the list. Our experiments show that the relaxed semantics allow the data structure to scale for high thread counts, comparable to a classic unordered SkipList. Furthermore, we observe that, for reasonably parallel workloads, the scalability benefits of relaxation considerably outweigh the additional work due to out-of-order execution.
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