Adaptive scheduling under memory constraints on non-dedicated computational farms

Abstract

This paper presents scheduler extensions that enable better adaptation of parallel programs to the execution conditions of non-dedicated computational farms with limited memory resources. The purpose of the techniques is to prevent thrashing and co-schedule communicating threads, using two disjoint, yet cooperating extensions to the kernel scheduler. A thrashing prevention module enables memory-bound programs to adapt to memory shortage, via suspending their threads at selected points of execution. Thread suspension is used so that memory is not over-committed by parallel jobs—which are assumed to be running as guests on the nodes of the computational farm—at memory allocation points. In the event of thrashing, parallel jobs are the first to release memory and help local resident jobs make progress. Adaptation is implemented using a shared-memory interface in the /proc filesystem and upcalls from the kernel to the user space. On an orthogonal axis, co-scheduling is implemented in the kernel with a heuristic that boosts periodically the priority of communicating threads. Using experiments on a cluster of workstations, we show that when a guest parallel job competes with general-purpose interactive, I/O-intensive, or CPU and memory-intensive load on the nodes of the cluster, thrashing prevention reduces drastically the slowdown of the job at memory utilization levels of 20% or higher. The slowdown of parallel jobs is reduced by up to a factor of 7. Co-scheduling provides a limited performance improvement at memory utilization levels below 20%, but has no significant effect at higher memory utilization levels.