Controlled Contention: Balancing Contention and Reservation in Multicore Application Scheduling

One of the benefits of multiprogramming in conventional systems is to allow effective use of resources. For example, when one application blocks for I/O, another can use the available CPU time, improving throughput and performance. In a multithreaded environment, contention for resources can lead to substantial interference between applications: an application with dependencies can suffer if a thread holding critical dependency is not scheduled in time. In the HPC community, this problem is often addressed by reservation-based schedulers such as Gang scheduling. However, such schedulers cannot reap the benefits of resource multiplexing leading to underutilization of the resources and lower overall throughput of the system. In this paper, we explore the trade off between contention and reservation in multithreaded application scheduling on multicourse systems. We show that neither approach is optimal under all conditions. We propose Controlled Contention (CC) -- a scheduling algorithm that allows controlled contention for resources, allowing the benefits of contention while supporting limited reservation to reduce interference. CC provides around 25% improvement in relative speedup over the Completely Fair Scheduler (CFS). We also show that CC can significantly benefit from application-level interference management while providing fairness that is not possible to achieve with application-level adaptation alone. The combined approach (CC with application-level adaptation)provides an average of 21% improvement in throughput, 35% improvement in relative speedup and 36% reduction in energy for the application mixes we consider.