Around the Linux kernel, there are a great number of functions that are influenced by time. From the scheduler to the system uptime, they all require a time reference, which includes both absolute and relative time. For example, an event that needs to be scheduled for the future, represents a relative time, which, in fact, implies that there is a method used to count time.
The timer implementation can vary depending on the type of the event. The periodical implementations are defined by the system timer, which issues an interrupt at a fixed period of time. The system timer is a hardware component that issues a timer interrupt at a given frequency to update the system time and execute the necessary tasks. Another one that can be used is the real-time clock, which is a chip with a battery attached that keeps counting time long after the system was shut down. Besides the system time, there are dynamic timers available that are managed by the kernel dynamically to plan events that run...
