Get What You Pay For: Providing Performance Isolation in Virtualized Execution Environments∗

Virtualization allows multiple systems encapsulated in so-called domains to share completely isolated from each other a single physical machine. Several companies are already taking advantage of virtualization technology in order to sell a certain amount of CPU speed and I/O capacity in terms of latency and throughput on demand to their customers. Providers of such a service face the challenge of isolating the performance of domains from each other. Independent of the load generated by the domains running on the system each domain has to get what its customer is paying for, not more and not less. Our study is based on the open-source virtual machine monitor Xen [1]. In the system architecture of Xen, the hy-pervisor is the lowest layer. On top of the hypervisor run several domains, which encapsulate complete operating sys-tem instances. The main tasks of the hypervisor are domain scheduling, I/O handling, and managing memory. Some of the domains running on the hypervisor are special domains which belong to the trusted code base, e.g. a driver domain where device drivers run. The driver domain is similar to a network router; domains performing I/O send their requests from their so-called frontend driver through an event chan-nel to the backend driver, located in the driver domain. The backend driver multiplexes all requests onto the actual I/O device. Several studies in the past dealt with the performance iso-lation problem in Xen. In [4] the authors evaluated the ef-fects of different schedulers in Xen on the I/O performance of domains running several applications with different I/O requirements. I/O operations are typically not considered by the scheduler. However, indirectly the CPU scheduler influ-ences the I/O performance of the applications running within a domain. The authors presented several system modifica-tion which, in many cases, provide better I/O isolation than the baseline system. Xen patched with these modifications is our baseline system