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 one-over-the-tick-rate seconds. This is how the kernel keeps track of both wall time and system uptime. Wall timethe actual time of dayis of most importance to user-space applications. The kernel keeps track of it simply because the kernel controls the timer interrupt. A family of system calls provide the date and time of day to user-space. The system uptimethe relative time since the system bootedis 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 readingsnow and thenis a simple measure of this relativity.

The timer interrupt is very important to the management of the operating system. A large number of kernel functions live and die by the passing of time. Some of the work executed periodically by the timer interrupt includes

• Updating the system uptime

• Updating the time of day

• On an SMP system, ensuring that the scheduler runqueues are balanced, and if not, balancing them (as discussed in Chapter 4, "Process Scheduling")

• Checking whether the current process has exhausted its timeslice and, if so, causing a reschedule (also discussed in Chapter 4)

• Running any dynamic timers that have expired

• Updating resource usage and processor time statistics

Some of this work occurs on every timer interruptthat is, the work is carried out with the frequency of the tick rate. Other functions execute periodically, but only every n timer interrupts. That is, these functions occur at some fraction of the tick rate. The section "The Timer Interrupt Handler" looks at the timer interrupt handler itself.