Rolf Neugebauer

Xen and the art of virtualization

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.

Energy is just another resource: energy accounting and energy pricing in the Nemesis OS

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.

A case for virtual channel processors

Modern desktop and server computer systems use multiple processors: general purpose CPU(s), graphic processor (GPU), network processors (NP) on Network Interface Cards (NICs), RAID controllers, and signal processors on sound cards and modems. Some of these processors traditionally have been special purpose processors but there is a trend towards replacing some of these with embedded general purpose processors. At the same time main CPUs become more powerful; desktop CPUs start featuring Simultaneous Multi-Threading (SMT); and Symmetric Multi-Processing (SMP) systems are widely used in server systems. However, the structure of operating systems has not really changed to reflect these trends --- different types of processors evolve at different timescales (largely driven by market forces) requiring significant changes to operating systems kernels to reflect the appropriate tradeoffs.In this position paper we propose to re-vitalise the old idea of channel processors by encapsulating operating system I/O subsystems in Virtual Channel Processors (VCPs). VCPs perform I/O operations on behalf of an OS. They provide similar development, performance, and fault isolation as dedicated (embedded) I/O processors do while offering the flexibility to split functionality between the main processor(s) and dedicated processors without affecting the rest of the OS. If part of a VCP is executed on the main processor, we propose to make use of virtual machine technology and SMT/SMP features to isolate its performance from that of the rest of the system and to protect the system from faults within the VCP.

Congestion prices as feedback signals: an approach to QoS management

Recently there has been a renewed interest in the application of economic models to the management of computational resources. Most of this interest is focused on pricing models for the Internet; in particular, on congestion or shadow prices, that address the phenomenon of what economists call external costs --- users are exposed to the costs they impose on other users when causing congestion of a resource.This paper describes how congestion prices can be applied to resource management in operating systems. Shadow prices are interpreted as feedback signals to applications which can adjust their resource requirements according to an application-specific strategy. This leads to a decentralised approach of resource management where applications are enabled and encouraged to perform resource and quality tradeoffs themselves. We have implemented a simulation environment and a number of strategies to evaluate the usefulness of congestion prices as a feedback signal and demonstrate that this approach can offer different service levels to different tasks. We also discuss how the simulation results can be applied in a real operating system and how this work can be extended to form a generic resource management framework.

Understanding PCIe performance for end host networking

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. This paper focuses on the performance implication of PCIe, the de-facto I/O interconnect in contemporary servers, when interacting with the host architecture and device drivers. We present a theoretical model for PCIe and pcie-bench, an open-source suite, that allows developers to gain an accurate and deep understanding of the PCIe substrate. Using pcie-bench, we characterize the PCIe subsystem in modern servers. We highlight surprising differences in PCIe implementations, evaluate the undesirable impact of PCIe features such as IOMMUs, and show the practical limits for common network cards operating at 40Gb/s and beyond. Furthermore, through pcie-bench we gained insights which guided software and future hardware architectures for both commercial and research oriented network cards and DMA engines.