Performance-asymmetric multi-cores consist of heterogeneous cores, which support the same ISA, but have different computing capabilities. To maximize the throughput of asymmetric multi-core systems, operating systems are responsible for scheduling threads to different types of cores. However, system virtualization poses a challenge for such asymmetric multi-cores, since virtualization hides the physical heterogeneity from guest operating systems. In this paper, we explore the design space of hypervisor schedulers for asymmetric multi-cores, which do not require asymmetry-awareness from guest operating systems. The proposed scheduler characterizes the efficiency of each virtual core, and map the virtual core to the most area-efficient physical core. In addition to the overall system throughput, we consider two important aspects of virtualizing asymmetric multi-cores: performance fairness among virtual machines and performance scalability for changing availability of fast and slow cores.
We have implemented an asymmetry-aware scheduler in the open-source Xen hypervisor. Using applications with various characteristics, we evaluate how effectively the proposed scheduler can improve system throughput without asymmetry-aware operating systems. The modified scheduler improves the performance of the Xen credit scheduler by as much as 40% on a 12-core system with four fast and eight slow cores. The results show that even the VMs scheduled to slow cores have relatively low performance degradations, and the scheduler provides scalable performance with increasing fast core counts.