We identify a new capability for mobile computing that mimics the opening and closing of a laptop, but avoids physical transport of hardware. Through rapid and easy personalization and depersonalization of anonymous hardware, a user is able to suspend work at one machine and to resume it at another. Our key insight is that this capability can be achieved by layering virtual machine technology on a distributed file system. We report on an initial implementation and describe our plans for improving efficiency, portability, and security.

We demonstrate that a collaborative relationship between the operating system and applications can be used to meet user-specified goals for battery duration. We first describe a novel profilingbased approach for accurately measuring application and system energy consumption. We then show how applications can dynamically modify their behavior to conserve energy. We extend the Linux operating system to yield battery lifetimes of user-specified duration. By monitoring energy supply and demand and by maintaining a history of application energy use, the approach can dynamically balance energy conservation and application quality. Our evaluation shows that this approach can meet goals that extend battery life by as much as 30%.

the ability to adapt at run time, taking advantage of local computing devices, and coping with dynamically changing resources. Three specific technical challenges in satisfying this requirement are to (1) select an appropriate set of applications or services to carry out a user's task, (2) allocate (possibly scarce) resources among those applications, and (3) reconfigure the applications or resource assignments if the situation changes. In this paper we show how to provide a shared infrastructure that automates configuration decisions given a specification of the user's task. The heart of the approach is an analytical model and an efficient algorithm that can be used at run time to make near-optimal (re)configuration decisions. We validate this approach both analytically and by applying it to a representative scenario.

Organizations use Web caches to avoid transferring the same data twice over the same path. Numerous studies have shown that forward proxy caches, in practice, incur miss rates of at least 50%. Traditional Web caches rely on the reuse of responses for given URLs. Previous analyses of real-world traces have revealed a complex relationship between URLs and reply payloads, and have shown that this complexity frequently causes redundant transfers to caches. For example, redundant transfers may result if a payload is aliased (accessed via different URLs), or if a resource rotates (alternates between different values) , or if HTTP's cache revalidation mechanisms are not fully exploited. We implement and evaluate a technique known in the literature as Duplicate Transfer Detection (DTD), with which a Web cache can use digests to detect and potentially eliminate all redundant payload transfers. We show how HTTP can support DTD with few or no protocol changes, and how a DTD-enabled proxy cache can interoperate with unmodified existing origin servers and browsers, thereby permitting incremental deployment. We present both simulated and experimental results that quantify the benefits of DTD.

In distributed systems, transcoding techniques have been used to customize multimedia objects, utilizing trade-offs between the quality and sizes of these objects to provide differentiated services to clients. Our research uses transcoding techniques in wireless systems to customize video streams to the requirements of users, while minimizing the energy costs. We introduce an approach to dynamically determine which transcoders to execute and where to execute them (e.g., client or server). The goal is to select appropriate transcoders (a) to provide clients with the quality of service they desire while (b) minimizing the energy consumption of the end-hosts in accordance with application-specific global energy management directives. This paper investigates sample transcoder functions for video streaming on handheld devices and introduces a mechanism for selecting the most appropriate transcoders and transcoder parameters.

Our previous MMCN03 paper reported a cross-layer adaptation framework, GRACE-1, that coordinates the adaptation of CPU frequency/voltage, CPU scheduling, and application quality. GRACE-1 assumes that all application processes (or threads) are independent from each other and adapt individually. This assumption, however, is invalid for multithreaded applications that include dependent and cooperative processes. To support the joint performance requirements of such dependent processes, this paper extends GRACE-1 with a process group management mechanism. The enhanced framework, called GRACE-grp, introduces a new OS abstraction, group control block, to provide the OS-level recognition and support of process groups. Through a group control block, dependent processes can explicitly set up a group and specify their dependency in the OS kernel. Consequently, GRACE-grp schedules and adapts them in a synchronized and consistent manner, thereby delivering joint performance guarantees. We have implemented and evaluated the GRACE-grp framework. Our experimental results show that compared to GRACE-1, GRACE-grp provides better support for the joint quality of dependent processes and reduces CPU energy consumption by 6.2% to 8.7% for each process group.

This paper describes the use of programming language constructs to support run-time software adaptation. A prototype language, Adaptive Java, contains primitives that permit programs to modify their own operation in a principled manner. In case studies, Adaptive Java is being used to support adaptation for different crosscutting concerns associated with heterogeneous mobile computing and critical infrastructure protection. Examples are described in which Adaptive Java components support dynamic quality-ofservice on wireless networks, run-time energy management for handheld computers, and self-auditing of potential security threats in distributed environments.

This paper investigates the use of programming language constructs to realize adaptive behavior in support of collaboration among users of wearable and handheld computers. A prototype language, Adaptive Java, contains primitives that permit programs to modify their own operation in a principled manner. In a case study, Adaptive Java was used to construct MetaSocket components, whose composition and behavior can be adapted to changing conditions during execution. MetaSockets were then integrated into Pavilion, a web-based collaboration framework, and experiments were conducted on a mobile computing testbed containing wearable, handheld, and laptop computer systems. Performance results demonstrate the utility of MetaSockets to improving the quality of interactive audio streams and reliable data transfers among collaborating users.

This paper presents the design, implementation, and evaluation of GRACE-OS, an energy-efficient real-time CPU scheduler for multimedia applications running on a mobile device. GRACE-OS seeks to minimize the total energy consumed by the device while meeting multimedia timing requirements. To achieve this goal, GRACE-OS integrates dynamic voltage scaling into the traditional real-time CPU scheduling: It decides at what CPU speed to execute applications in addition to when to execute what applications. GRACE-OS makes these scheduling decisions based on the probability distribution of cycle demand of multimedia applications and obtains their demand distribution via online profiling. We have implemented GRACE-OS in the Linux kernel and evaluated it on a laptop with a variable-speed CPU and typical multimedia codecs. Our experimental results show four findings: First, the demand distribution of our studied codecs is stable or changes slowly. This stability implies the feasibility to perform our proposed energy-efficient scheduling with low overhead. Second, GRACE-OS delivers soft performance guarantees to these codecs by bounding their deadline miss ratio under the application-specific performance requirements. Third, GRACE-OS reduces the total energy of the laptop by 14.4 % to 37.2 % relative to the scheduling algorithm without voltage scaling and by 2 % to 10.5 % relative to voltage scaling algorithms without considering the demand distribution. Finally, GRACE-OS saves energy by 2 % to 5 % by explicitly considering the discrete CPU speeds and the corresponding total power of the whole laptop, rather than assuming continuous speeds and cubic speed-power relationship.

Multimedia-enabled mobile devices, such as camera phones, need to support multimedia se-mantics with high quality of service (QoS) requirements under limited system resources such as CPU time and battery energy. On the other hand, these mobile devices also provide new oppor-tunity for QoS provisioning and energy saving due to the adaptive hardware and software com-ponents. Researchers have therefore introduced adaptation into various system layers, ranging from hardware to applications. Previous adaptation work often adapts only some layers or only at coarse time granularity such as application entry or exit. We believe that to fully reap the ben-efits of adaptation, it is necessary to take a cross-layer adaptation approach, in which all system layers are adaptive and cooperate with each other in response to system changes at different time granularity. This thesis presents a novel operating system, called GRACE-OS, to support such cross-layer adaptation in the operating system by coordinating the adaptation in different layers and enforcing the coordinated decisions via energy-aware real-time scheduling. This thesis makes four major contributions. First, we propose a hierarchical adaptation framework to coordinate adaptation in