Kernel Notion of Time

Certainly, the concept of time to a computer is a bit obscure. Indeed, the kernel must work with the system’s hardware to comprehend and manage time. The hardware provides a system timer that the kernel uses to gauge the passing of time. This system timer works off of an electronic time source, such as a digital clock or the frequency of the processor. The system timer goes off (often called hitting or popping) at a preprogrammed frequency, called the tick rate. When the system timer goes off, it issues an interrupt that the kernel handles via a special interrupt handler.

Because the kernel knows the preprogrammed tick rate, it knows the time between any two successive timer interrupts. This period is called a tick and is equal to 1/(tick rate) seconds. This is how the kernel keeps track of both wall time and system uptime. Wall time—the actual time of day—is important to user-space applications. The kernel keeps track of it simply because the kernel controls the timer interrupt. A family of system calls provides the date and time of day to user-space. The system uptime—the relative time since the system booted—is useful to both kernel-space and user-space. A lot of code must be aware of the passing of time. The difference between two uptime readings—now and then—is a simple measure of this relativity.