Data structures for task-based priority scheduling

Abstract

We present three lock-free data structures for priority task scheduling: a priority work-stealing one, a centralized one with ρ-relaxed semantics, and a hybrid one combining both concepts. With the single-source shortest path (SSSP) problem as example, we show how the different approaches affect the prioritization and provide upper bounds on the number of examined nodes. We argue that priority task scheduling allows for an intuitive and easy way to parallelize the SSSP problem, notoriously a hard task. Experimental evidence supports the good scalability of the resulting algorithm.The larger aim of this work is to understand the trade-offs between scalability and priority guarantees in task scheduling systems. We show that ρ-relaxation is a valuable technique for improving the first, while still allowing semantic constraints to be satisfied: the lock-free, hybrid k-priority data structure can scale as well as work-stealing, while still providing strong priority scheduling guarantees, which depend on the parameter k. Our theoretical results open up possibilities for even more scalable data structures by adopting a weaker form of ρ-relaxation, which still enables the semantic constraints to be respected.

A scalable architecture for ordered parallelism

“…Prior work has developed shared-memory priority queues that scale with the number of cores [2,75], but they do so by relaxing priority order. This restricts them to benchmarks that admit order violations, and loss of order means threads often execute useless work far from the critical path [33,34].…”

The SprayList: a scalable relaxed priority queue

“…al. [28]. This implementation provides a linearizable priority queue, except that it is relaxed in the sense that each thread might skip up to k of the highest priority tasks; however, no task will be skipped by every thread.…”

“…[22]. In particular, Wimmer et al [28] explored trade-offs between ordering and scalability for asynchronous priority queues. However, despite all this effort, it is currently not clear whether it is possible to design a relaxed priority queue which provides both ordering guarantees under asynchrony, and scalability under high contention for realistic workloads.…”

“…We compare our algorithm's performance to that of the quiescentlyconsistent priority queue of Lotan and Shavit [21], the state-ofthe-art SkipList-based priority queue implementation of Lindén and Jonsson [20] and the recent k-priority queue of Wimmer et al [28]. 3 Our first finding is that our data structure shows fully scalable throughput for up to 80 concurrent threads under high-contention workloads.…”

“…Another direction by Wimmer et al [28] presents lock-free priority queues which allow the user to dynamically decrease the strength of the ordering for improved performance. In essence, the data structure is distributed over a set of places, which behave as exact priority queues.…”