5.1 Where does the energy go?
Our results show that the majority of power consumption can be attributed to the GSM module and the display, including the LCD panel and touchscreen, the graphics accelerator/driver, and the backlight. In all except the GSM-intensive benchmarks, the brightness of the backlight is the most critical factor in determining power consumption. However, this is a relatively simple device from a power-management perspective, and largely depends on the user’s brightness preference. Our results confirm that aggressive backlight dimming can save a great deal of energy, and further motivates the inclusion of ambient light and proximity sensors in mobile devices to assist with selecting an appropriate brightness. Moreover, the N1 OLED results show that merely selecting a light-on-dark colour scheme can
significantly reduce energy consumption.
The GSM module consumes a great deal of both static and dynamic power. Merely maintaining a connection with the network consumes a significant fraction of total power. During a phone call, GSM consumes in excess of 800 mW average, which represents the single largest power drain in any of our benchmarks. Unfortunately, a phone-call-heavy workload presents little scope for software-level power management. Dimming the backlight during a call, as Android does, is clearly good policy, saving up to 40 % power even with the large GSM consumption.
Overall, the static contribution to system power consumption is substantial. In all of our usage scenarios, except GSM phone call, static power accounts for at least 50 % of the total. If the backlight is included, this figOLED Power (mW)
Benchmark Min. Max.
Idle 38.0 257.3
Phone call 16.7 112.9
Web 164.2 1111.7
Video 15.1 102.0
Table 10: Additional power consumed by the N1 OLED display at maximum and minimum brightness.
ure rises substantially. This leads us to the conclusion that the most effective power management approach on mobile devices is to shut down unused components and disable their power supplies (where possible). The RAM, audio and flash subsystems consistently showed the lowest power consumption. While our micro-benchmarks showed that the peak power of the SD card could be substantial ( 50 mW), in practice the utilisation is low enough such that on average, negligible power is consumed. Even video playback, one of the more data-intensive uses of mobile devices, showed SD power well under 1 % of total power. RAM has similar characteristics; micro-benchmarks showed that RAM power can exceed CPU power in certain workloads, but in practical situations, CPU power overshadows RAM by
a factor of two or more. Audio displayed a largely static power consumption in the range of 28–34 mW. Overall, RAM, audio and SD have little effect on the power consumption of the device, and therefore offer little potential for energy optimisation.
