Priority Queues Are Not Good Concurrent Priority Schedulers

The need for priority scheduling arises in many algorithms. In these algorithms, there is a dynamic pool of lightweight, unordered tasks, and some execution orders are more efficient than others. Therefore, each task is given an application-specific priority that is a heuristic measure of its importance for early scheduling, and the runtime system schedules these tasks roughly in this order. Concurrent priority queues are not suitable for this purpose. We show that by exploiting the fact that algorithms amenable to priority scheduling are often robust to small deviations from a strict priority order, and by optimizing the scheduler for the cache hierarchy of current multicore and NUMA processors, we can implement concurrent priority schedulers that improve the end-to-end performance of complex irregular benchmarks by orders of magnitude compared to using state-of-the-art concurrent priority queues.