Lifeline-based load balancing schemes for Asynchronous Many-Task runtimes in clusters

Abstract

A popular approach to program scalable irregular applications is Asynchronous Many-Task (AMT) Programming. Here, programs define tasks according to task models such as dynamic independent tasks (DIT) or nested fork-join (NFJ). We consider cluster AMTs, in which a runtime system maps the tasks to worker threads in multiple processes.

Thereby, dynamic load balancing can be achieved via cooperative work stealing, coordinated work stealing, or work sharing. A well-performing cooperative work stealing variant is the lifeline scheme. While previous implementations of this scheme are restricted to single-worker processes, a recent hybrid extension combines it with intra-process work sharing between multiple workers. The hybrid scheme, which was proposed for both DIT and NFJ, comes at the price of a higher complexity.

This paper investigates whether this complexity is indispensable for multi-worker processes by contrasting the hybrid scheme with a novel pure work stealing extension of the lifeline scheme to multiple workers. We independently implemented the extension for DIT and NFJ. In experiments based on four benchmarks, we observed the pure scheme to be on a par or even outperform the hybrid one by up to 18% for DIT and up to 5% for NFJ.

Building on this main result, we studied a modification of the pure scheme, which prefers local over global victims, and more heavily loaded over less loaded ones. The modification improves the performance of the pure scheme by up to 15%. Finally, we explored whether the lifeline scheme can profit from a change to coordinated work stealing. We developed a coordinated multi-worker implementation for DIT and observed a performance improvement over the cooperative scheme by up to 17%.