poweraware-13 - System-Level Power Management Comparing...

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R EDUCING POWER CONSUMPTION is a chal- lenge to system designers. Portable systems, such as laptop computers and personal digital assis- tants (PDAs), draw power from batteries, so reducing power consumption extends their oper- ating times. For desktop computers or servers, high power consumption raises temperature and deteriorates performance and reliability. Soaring energy prices early last year and rising concern about the environmental impact of electronics systems further highlight the importance of low power consumption. Power reduction techniques can be classi- fied as static and dynamic. Static techniques, such as synthesis and compilation for low power, are applied at design time. In contrast, dynamic techniques use runtime behavior to reduce power when systems are serving light workloads or are idle. 1 These techniques are known as dynamic power management (DPM). 2 DPM can be achieved in different ways; for example, dynamic voltage scaling (DVS) changes supply voltage at runtime as a method of power management. Here, we use DPM specifically for shutting down unused I/O devices. We built an experimental environment on a laptop computer running Microsoft Windows. We implemented existing power- management policies and quantitatively com- pared their effects on power saving and perfor- mance degradation. A qualitative survey of power management is available in Benini et al. 2 Power management System-level power management saves power of subsystems (also called devices). Examples of devices include I/O controllers, hard disk drives, network interface cards, and displays. Shutting down hard disks and displays is the most widely adopted system-level power management on PCs. Figure 1 illustrates the concept of power management. A workload consists of multiple requests. For hard disks, requests are read or write commands; for network cards, requests are packets to send or to be received. When there are requests, the device is busy; other- wise, it is idle. Here, the device is idle between T 1 and T 4 . When the device is idle, it can be shut down to enter a low-power sleeping state. (The “Standby or Sleeping?” sidebar discusses how this work views these states.) In this illustration, the device is shut down at T 2 and woken up at T 4 , when requests arrive again. Changing power states takes time; T sd and T wu are the shutdown and wake-up delays. In the example of hard disks and displays, it takes several seconds to wake up these devices. Furthermore, waking up a sleeping device may take extra energy. In other words, changing power states has overhead. If there were no overhead, power management would be trivial: Just shut down a device whenever it is idle. Unfortunately, there is delay and/or energy overhead. Consequently, a device should sleep only if the Comparing System-Level Power Management Policies System-Level Power Management 10 System-level power management is a trade-off among several factors, as this quantitative analysis shows.
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