Recently, the dramatic enhancement in performance of laptops and mobile devices have led to the explosive increase in their sales, which have outstripped that of desktop PCs. With the explosive growth in sales, power management of those devices is gaining ever increasing importance, as they always lack from limited battery life. In addition, thermal management has also become more im-portant, since it is difficult to aid from hardware cooling solutions, due to the limited area. To efficiently alleviate these problems, we propose three schemes: 1) Display power management scheme, 2) CPU power management scheme, and 3) Thermal management scheme.
Firstly, we tackle the essential weakness of the existing DPM (display power management) schemes that turns on or off the display based on user presence detection; regardless of the user presence, the display only needs to be turned on when the user is gazing at the display. Based on this proposition, we propose the user-aware display power management that uses a camera to determine if the user is actually gazing at the display. Our scheme captures pictures and detects user’s frontal face from the obtained pictures. When a frontal face is detected, our scheme determines that the user is gazing at the display. In our evaluation results, we show that our proposed scheme turns off the display for 52% of the total time, by detecting the user intent. Our scheme reduces average system-wide energy by 5%, compared to the most widely used timeout-based DPM, with ac-ceptable amount (less than once an hour) of user irritation.
Secondly, we investigate the CPU power management, which has become essential in recent mobile devices, where multi-core processors are adopted to handle CPU-intensive applications. Although there have been many studies to reduce the power consumption of mobile devices, there have not been sufficient efforts to consider the characteristics of the mobile applications. In this thesis, we introduce the limitations of the conventional power management schemes used in the mobile devices. Then, we propose an application-aware power man-agement scheme for multi-core based mobile devices that categorizes most exist-ing applications to either performance-sensitive application or QoS-sensitive ap-plication. We evaluate our scheme compared to conventional schemes, hotplug and ondemand, based on real measurements. In case of performance sensitive applications, our scheme saves energy (by average of 12% and 13% compared to hotplug and ondemand, respectively) and enhances the performance (by av-erage of 4.3% and 16% compared to hotplug and ondemand, respectively) at the same time for most applications. In case of QoS sensitive applications, our scheme saves 9.8% energy compared to hotplug and consumes similar energy compared to ondemand without a noticeable QoS degradation.
Lastly, we look into the thermal problems of recent mobile devices, which are caused by the adaptation of high performance processors. As the higher per-formance inevitably leads to power density increase, they eventually results in thermal problems. In order to alleviate the thermal problems, recent mobile de-vices scales down the voltage and frequency, when there are thermal emergen-cies. Unfortunately, this operation leads to an excessive number of voltage and frequency scaling, which not only worsen the performance but also the power efficiency. In this thesis, we describe how the conventional thermal management worsen power efficiency. To overcome the weakness of the conventional ther-mal management, we propose a temperature-aware DVFS (dynamic voltage and frequency scaling) scheme for Android-based mobile devices to optimize power or performance depending on the user-selectable option. We evaluate our scheme in the off-the-shelf mobile device. Our evaluation results show that by using the power optimizing option, our scheme saves energy consumption by 12.7%, on average, without any degradation in performance. By using the per-formance optimizing option, our scheme enhances the performance by 6.3%, on average, by using the performance optimizing scheme, still reducing the energy consumption by 6.7%.
There are no doubts that power management issues will become more severe as the industry continues introducing higher performance laptops and mobile devices, while there is no remarkable improvement in the battery. Furthermore, thermal management issues will also become more severe as the power density of the processors continues to increase with the advance in process technology. In this thesis, we propose applicable power and thermal management schemes that efficiently address the power and thermal problems.