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1 Introduction

Limiting the energy consumption in mobile/embedded systems such as laptops, personal digital assistants (PDAs) and cellular phones is becoming increasingly important as they become widely used and accepted. With this large user community and a highly competitive market comes the inevitable demand for integrating more features and increased performance into small devices, which, in turn, comes at a cost of increased power dissipation. Products compete based on form factors (smaller and lighter is better) as well as additional features and improved user experience, provided by fast processors, copious memory, resource-demanding software, and power-hungry hardware, thus making energy a precious resource. With hardware continuously improving in performance and price, vendors are able to build systems with higher-performance and higher-power components trying to meet users' ever-increasing demands and compete for customers. However, this results in systems that are over-provisioned with components that provide more capacity, more throughput, and more processing power than needed for the typical workload, and as a result, it is becoming more difficult to maintain long battery life in these devices. To make the situation worse, battery technology is improving at a much slower pace than hardware technology, making the gap between energy supply and demand increasingly larger. To deal with this emerging energy crisis, power management is becoming a more critical task than ever before.

Current hardware technologies allow various system components (e.g., microprocessor, memory, hard disk) to operate at different power levels and corresponding performance levels. Previous research has shown that by judiciously exploiting these power levels, depending on the workload, it is possible to have energy-limited systems built from high-performance and high-peak-power components with a minimal impact on the battery life of the device. The trick is to manage these power levels for each component intelligently based on the actual workload. For idle/normal workloads, some hardware components can be put at lower power levels or even be turned off. On the other hand, during peak workloads, the relevant hardware components are powered up to optimize for performance and provide responsive, high-quality service to users. Workloads on mobile systems such as laptops or PDAs are typically interactive, e.g., text editing, emailing, web surfing, or presenting PowerPoint slides, and due to the slow response time of human users, there are ample opportunities [13] to conserve energy by reducing power levels in various system components without any user-perceived performance degradation. During short intervals of high workload, e.g., switching slides, or recomputing a spreadsheet, the relevant system components can be briefly brought back to the higher-performance/power levels, effectively giving the user the benefits of both low-power and high-performance in a single system. However, due to non-negligible transitioning delays of some hardware components, performance/energy may suffer if not handled properly.

A large body of previous research concentrates on reducing the power dissipation of microprocessors due to their high peak-power. Using existing techniques, a Mobile Pentium 4 processor dissipates only 1-2 W on average when running typical office applications despite having a high 30 W peak-power [17]. From a software perspective, further effort to reduce power in microprocessors is likely to yield only a diminishing marginal return. On the other hand, there has been relatively little work done on reducing power used by the memory. As applications are becoming more data-centric, more power is needed to sustain a higher-capacity/performance memory system. Unlike microprocessors, a fairly substantial amount of power is continuously dissipated by the memory in the background independent of the current workload. Therefore, the energy consumed by the memory is usually as much as, and sometimes more than, that of the microprocessor in a system. Implementing Power-Aware Virtual Memory (PAVM) allows us to significantly reduce power dissipated by the memory. In the rest of the paper, we describe our experiences in designing and implementing PAVM in a working system.

Our contributions in this paper are summarized as follows.

The rest of the paper is organized as follows. Section 2 provides some background information on various memory technologies. Section 3 describes our initial design of PAVM, while Section 4 describes the limitations of this prototype design and the necessary modifications needed to handle the complexity of memory management and task interactions in a real, working implementation. Section 5 presents detailed experimentation results. In Section 6, we discuss the related work. Finally, additional remarks about PAVM and conclusion are given in Sections 7 and 8, respectively.

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