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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.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.
