Linux Kernel Development, Second Edition

Chapter 10. Timers and Time Management

The passing of time is very important to the kernel. A large number of kernel functions are time driven, as opposed to event driven^[1]. Some of these functions are periodic, such as balancing the scheduler runqueues or refreshing the screen. They occur on a fixed schedule, such as 100 times per second. The kernel schedules other functions, such as delayed disk I/O, at a relative time in the future. For example, the kernel might schedule work for 500 milliseconds from now. Finally, the kernel must also manage the system uptime and the current date and time.

Note the differences between relative and absolute time. Scheduling an event for five seconds in the future requires no concept of the absolute time—only ...