Figure: System Timers
An operating system needs to be able to schedule an activity sometime in the future. A mechanism is needed whereby activities can be scheduled to run at some relatively precise time. Any microprocessor that wishes to support an operating system must have programmable interval timer which periodically interrupts the processor. This periodic interrupt is known as a system clock tick and it acts like a metronome, orchestrating the system's activities. Linux has a very simple view of what time it is; it measures time in clock ticks since the system booted. All system times are based on this measurement which is known as jiffies after the globally available variable of the same name.
Linux has two types of system timers, both queue routines to be called at some system time but they are slightly different in their implementations. Figure shows both mechanisms.
The first, the old timer mechanism, has a static array of 32 pointers to timer_struct data structures and a mask of active timers, timer_active . Where the timers go in the timer table is statically defined (rather like the bottom half handler table bh_base ). Entries are added into this table mostly at system initialization time. The second, newer, mechanism uses a linked list of timer_list data structures held in ascending expiry time order.
Both methods use the time in jiffies as an expiry time so that a timer that wished to run in 5s would have to convert 5s to units of jiffies and add that to the current system time to get the system time in jiffies when the timer should expire. Every system clock tick the timer bottom half handler is marked as active so that the when the scheduler next runs, the timer queues will be processed. The timer bottom half handler processes both types of system timer. For the old system timers the timer_active bit mask is check for bits that are set.
If the expiry time for an active timer has expired (expiry time is less than the current system jiffies ), its timer routine is called and its active bit is cleared. For new system timers, the entries in the linked list of timer_list data structures are checked.
Every expired timer is removed from the list and its routine is called. The new timer mechanism has the advantage of being able to pass an argument to the timer routine.