Reducing Event Latency and Power Consumption in Mobile Devices by Using a Kernel-Level Display Server

Abstract

Mobile devices differ from desktop computers in that they have a limited power source, a battery, and they tend to spend more CPU time on the graphical user interface (GUI). These two facts force us to consider different software approaches in the mobile device kernel that can conserve battery life and reduce latency, which is the duration of time between the inception of an event and the reaction to the event. One area to consider is a software package called the display server. The display server is middleware that handles all GUI activities between an application and the operating system, such as event handling and drawing to the screen. In both desktop and mobile devices, the display server is located in the application layer. However, the kernel layer contains most of the information needed for handling events and drawing graphics, which forces the application-level display server to make a series of system calls in order to coordinate events and to draw graphics. These calls interrupt the CPU which can increase both latency and power consumption, and also require the kernel to maintain event queues that duplicate event queues in the display server. A further drawback of placing the display server in the application layer is that the display server contains most of the information required to efficiently schedule the application and this information is not communicated to existing kernels, meaning that GUI-oriented applications are scheduled less efficiently than they might be, which further increases power consumption. We propose moving the display server to the kernel layer, so that it has direct access to many of the event queues and hardware rendering systems without having to interrupt the CPU. This adjustment has allowed us to implement two power saving strategies, discussed in other papers, that streamline the event system and improve the scheduler. The combination of these two techniques reduces power consumption by an average of 30 percent and latency by an average of 17 ms. Even without the implementation of these power saving techniques, the KDS increases battery life by 4.35 percent or on average about 10 extra minutes for a typical mobile phone or 30 extra minutes for a typical tablet computer. It also reduces latency by 1.1 milliseconds.