In recent years, spurred on by the development and availability of programmable NICs, end hosts have increasingly become the enforcement point for core network functions such as load balancing, congestion control, and application specific network offloads. However, implementing custom designs on programmable NICs is not easy: many potential bottlenecks can impact performance.

Numerous systems have been designed which use virtualization to subdivide the ample resources of a modern computer. Some require specialized hardware, or cannot support commodity operating systems. Some target 100% binary compatibility at the expense of performance. Others sacrifice security or functionality for speed. Few offer resource isolation or performance guarantees; most provide only best-effort provisioning, risking denial of service.This paper presents Xen, an x86 virtual machine monitor which allows multiple commodity operating systems to share conventional hardware in a safe and resource managed fashion, but without sacrificing either performance or functionality. This is achieved by providing an idealized virtual machine abstraction to which operating systems such as Linux, BSD and Windows XP, can be ported with minimal effort.Our design is targeted at hosting up to 100 virtual machine instances simultaneously on a modern server. The virtualization approach taken by Xen is extremely efficient: we allow operating systems such as Linux and Windows XP to be hosted simultaneously for a negligible performance overhead --- at most a few percent compared with the unvirtualized case. We considerably outperform competing commercial and freely available solutions in a range of microbenchmarks and system-wide tests.

In this paper, we argue that, with an appropriate operating system structure, energy in mobile computers can be treated and managed as just another resource. In particular, we investigate how energy management could be added to the Nemesis OS which provides detailed and accurate resource accounting capabilities in order to provide Quality of Service (QoS) guarantees for all resources to applications. We argue that, with such an operating system, accounting of energy to individual processes can be achieved. Furthermore, we investigate how an economic model, proposed for congestion avoidance in computer network, and recently applied to CPU resource management, can be used as a dynamic, decentralised energy management system, forming a collaborative environment between operating system and applications.