- 1 Hardware Issues
- 2 Operating System Issues
- 3 Degraded Operation
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5.9.3 Degraded Operation
So far we have looked at ways the operating system can reduce energy usage by various kinds of devices. But there is another approach as well: tell the programs to use less energy, even if this means providing a poorer user experience (better a poorer experience than no experience when the battery dies and the lights go out). Typically, this information is passed on when the battery charge is below some threshold. It is then up to the programs to decide between degrading performance to lengthen battery life or to maintain performance and risk running out of energy.
One of the questions that comes up here is how can a program degrade its performance to save energy? This question has been studied by Flinn and Satya-narayanan (1999). They provided four examples of how degraded performance can save energy. We will now look at these.
In this study, information is presented to the user in various forms. When no degradation is present, the best possible information is presented. When degradation is present, the fidelity (accuracy) of the information presented to the user is worse than what it could have been. We will see examples of this shortly.
In order to measure energy usage, Flinn and Satyanarayanan devised a software tool called PowerScope. What it does is provide a power usage profile of a program. To use it, a computer must be hooked up to an external power supply through a software-controlled digital multimeter. Using the multimeter, software can read out the number of milliamperes coming in from the power supply and thus determine the instantaneous power being consumed by the computer. What PowerScope does is periodically sample the program counter and the power usage and write these data to a file. After the program has terminated the file is analyzed to give the energy usage of each procedure. These measurements formed the basis of their observations. Hardware energy saving measures were also used and formed the baseline against which the degraded performance was measured.
The first program measured was a video player. In undegraded mode, it plays 30 frames/sec in full resolution and in color. One form of degradation is to abandon the color information and display the video in black and white. Another form of degradation is to reduce the frame rate, which leads to flicker and gives the movie a jerky quality. Still another form of degradation is to reduce the number of pixels in both directions, either by lowering the spatial resolution or making the displayed image smaller. Measures of this type saved about 30% of the energy.
The second program was a speech recognizer. It sampled the microphone to construct a waveform. This waveform could either be analyzed on the laptop computer or sent over a radio link for analysis on a fixed computer. Doing this saves CPU energy but uses energy for the radio. Degradation was accomplished by using a smaller vocabulary and a simpler acoustic model. The win here was about 35% The next example was a map viewer that fetched the map over the radio link. Degradation consisted of either cropping the map to smaller dimensions or telling the remote server to omit smaller roads, thus requiring fewer bits to be transmitted. Again here a gain of about 35% was achieved.
The fourth experiment was with transmission of JPEG images to a Web browser. The JPEG standard allows various algorithms, trading image quality against file size. Here the gain averaged only 9%. Still, all in all, the experiments showed that by accepting some quality degradation, the user can run longer on a given battery.