Temporal isolation or performance isolation among

virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized har ...

(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.

Introduction to the problem

One of the key advantages of using

virtualization In computing, virtualization or virtualisation (sometimes abbreviated v12n, a numeronym) is the act of creating a virtual (rather than actual) version of something at the same abstraction level, including virtual computer hardware platforms, st ...

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 An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

(OS), along with the applications running within, can be run in a

(VM). However, when multiple VMs concurrently run on the same physical host, they share the available physical resources, including

CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...

(s),

network adapter A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network. E ...

(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 Cloud computing is the on-demand availability of computer system resources, especially data storage ( cloud storage) and computing power, without direct active management by the user. Large clouds often have functions distributed over m ...

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.

Possible solutions

Multiple techniques may be used to face with the aforementioned problem. They aim to achieve some degree of

temporal isolation In computer science, temporal isolation is the capability of a set of processes running on the same system to run without interferences concerning their temporal constraints among each other. Specifically, there is temporal isolation among process ...

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 strategiesFabio Checconi, Tommaso Cucinotta, Dario Faggioli, Giuseppe Lipari
Hierarchical Multiprocessor CPU Reservations for the Linux Kernel
Proceedings of the 5th International Workshop on Operating Systems Platforms for Embedded Real-Time Applications (OSPERT 2009), Dublin, Ireland, June 2009 to maintain stable and predictable performance of virtualized applications.Tommaso Cucinotta, Gaetano Anastasi, Luca Abeni

Proceedings of the 2nd IEEE International Workshop on Real-Time Service-Oriented Architecture and Applications (RTSOAA 2009), Seattle, Washington, July 2009Tommaso Cucinotta, Gaetano Anastasi, Luca Abeni

Proceedings of the 29th Real-Time System Symposium (RTSS 2008) -- Work in Progress Session, Barcelona, December 2008 Finally, with a multi-core or multi-processor

physical host Physical may refer to: *Physical examination In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally cons ...

, it is possible to deploy each VM on a separate processor or core to temporally isolate the performance of various VMs. For the network, it is possible to use traffic shaping techniques to limit the amount of traffic that each VM can impose on the host. Also, it is possible to install multiple network adapters on the same physical host, and configure the virtualization layer so that each VM may grant exclusive access to each one of them. For example, this is possible with the driver domains of the Xen hypervisor. Multi-queue network adapters exist which support multiple VMs at the hardware level, having separate packet queues associated to the different hosted VMs (by means of the IP addresses of the VMs), such as the Virtual Machine Device Queue (VMDq) devices by

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...

.Shefali Chinni, Radhakrishna Hiremane
Virtual Machine Device Queues
Intel Virtualization Technology White Paper, 2007 Finally, real-time scheduling of the CPU may also be used for enhancing temporal isolation of network traffic from multiple VMs deployed on the same CPU.Tommaso Cucinotta, Dhaval Giani, Dario Faggioli and Fabio Checconi

Proceedings of the 5th Workshop on Virtualization and High-Performance Cloud Computing (VHPC 2010), Ischia (Naples), Italy, August 2010. When using real-time scheduling for controlling the amount of CPU resources reserved for each VM, one challenging problem is properly accounting for the CPU time applicable to system-wide activities. For example, in the case of the Xen scheduler, the Dom0 and the driver domains services might be shared across multiple VMs accessing them. Similarly, in the case of the KVM hypervisor, the workload imposed on the host OS due to serving network traffic for each individual guest OS might not be easily distinguishable, because it mainly involves kernel-level device drivers and the networking infrastructure (on the host OS). Some techniques for mitigating such problems have been proposed for the Xen case.Diwaker Gupta, Lucy Cherkasova, Robert Gardner, Amin Vahdat
Enforcing Performance Isolation Across Virtual Machines in Xen
Proceedings of the 7th International Middleware Conference (Middleware 2006), Lecture Notes in Computer Science, Volume 4290/2006, pp.342-362, Melbourne, Australia, November 2006 Along the lines of

adaptive reservations Adaptation, in biology, is the process or trait by which organisms or population better match their environment Adaptation may also refer to: Arts * Adaptation (arts), a transfer of a work of art from one medium to another ** Film adaptation, a ...

, it is possible to apply feedback-control strategies to dynamically adapt the amount of resources reserved to each virtual machine to maintain stable performance for the virtualized application(s). Following the trend of adaptiveness, in those cases in which a virtualized system is not fulfilling the expected performance levels (either due to unforeseen interferences of other concurrently running VMs, or due to a bad deployment strategy that simply picked up a machine with insufficient hardware resources), it is possible to live-migrate virtual machines while they are running, so as to host them on a more capable (or less loaded) physical host.

References

{{DEFAULTSORT:Temporal Isolation Among Virtual Machines Computing terminology Computer science articles needing expert attention Real-time computing