Temporal isolation among virtual machines

Temporal isolation or performance isolation among virtual machine (VMs) refers to the capability of isolating the temporal behavior (or limiting the temporal interferences) of multiple VMs among each other, despite them running on the same physical host and sharing a set of physical resources such as processors, memory, and disks.

One of the key advantages of using virtualization in server consolidation, is the possibility to seamlessly "pack" multiple under-utilized systems into a single physical host, thus achieving a better overall utilization of the available hardware resources. In fact, an entire Operating System (OS), along with the applications running within, can be run in a virtual machine (VM). However, when multiple VMs concurrently run on the same physical host, they share the available physical resources, including CPU(s), network adapter(s), disk(s) and memory. This adds a level of unpredictability in the performance that may be exhibited by each individual VM, as compared to what is expected. For example, a VM with a temporary compute-intensive peak might disturb the other running VMs, causing a significant and undesirable temporary drop in their performance. In a world of computing that is shifting towards cloud computing paradigms where resources (computing, storage, networking) may be remotely rented in virtualized form under precise service-level agreements, it would be highly desirable that the performance of the virtualized resources be as stable and predictable as possible.

Multiple techniques may be used to face with the aforementioned problem. They aim to achieve some degree of temporal isolation across the concurrently running VMs, at the various critical levels of scheduling: CPU scheduling, network scheduling and disk scheduling.

For the CPU, it is possible to use proper scheduling techniques at the hypervisor level to contain the amount of computing each VM may impose on a shared physical CPU or core. For example, on the Xen hypervisor, the BVT, Credit-based and S-EDF schedulers have been proposed for controlling how the computing power is distributed among competing VMs. To get stable performance in virtualized applications, it is necessary to use scheduler configurations that are not work-conserving. Also, on the KVM hypervisor, some have proposed using EDF-based scheduling strategies to maintain stable and predictable performance of virtualized applications. Finally, with a multi-core or multi-processor physical host, it is possible to deploy each VM on a separate processor or core to temporally isolate the performance of various VMs.

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