IO Core Manager for Virtual Environments

Abstract

Para-virtualization is the leading approach in IO device virtualization. It allows the hypervisor to interpose on and inspect a virtual machine's I/O traffic at run-time. Examples of such interfaces are KVM's virtio [6] and VMWare's VMXNET [7]. Current implementations of virtual I/O in the hypervisor have been shown to have performance and scalability limitations [2, 3, 5]. The overhead incurred during interposition arises from two main sources: VM exits and thread scheduling. VM exits are caused when the virtual machine requires some intervention of the hypervisor in order to continue execution. VM exits are required to perform I/O tasks since the VM does not have direct access to I/O hardware [1]. The second source of overhead is the hypervisor's thread scheduler, which is not aware of the type of work being performed by a particular thread. When executing I/O threads have work (i.e. I/O traffic to process), the scheduler schedules the thread without regard to the latency or throughput requirements of the virtual device. In workloads with a small amount of latency-sensitive traffic, the thread context switches can become prohibitively costly. The limitations can be somewhat mitigated by using the side-core [4] approach, which divides the system cores into two distinct sets: one for running VM guests, and the other dedicated to virtual I/O processing. However, the number of cores that should be assigned to each set is dependent on the constantly changing workload. For optimum performance, the resources must be allocated according to measurements taken at runtime. We present IOcm which is able to tune the system automatically for using the side-core approach. IOcm provides a better foundation for building practical systems using the side-core approach by improving its usability. IOcm includes mechanisms that expose statistics and controls that allow for better management of the system. We show that IOcm is able to provide comparable performance to a side-core system tuned by an oracle.