6.2. The Linux Timekeeping Architecture

Linux must carry on several time-related activities. For instance, the kernel periodically:

• Updates the time elapsed since system startup.
• Updates the time and date.
• Determines, for every CPU, how long the current process has been running, and preempts it if it has exceeded the time allocated to it. The allocation of time slots (also called "quanta") is discussed in Chapter 7.
• Updates resource usage statistics.
• Checks whether the interval of time associated with each software timer (see the later section "Software Timers and Delay Functions") has elapsed.

Linux's timekeeping architecture is the set of kernel data structures and functions related to the flow of time. Actually, 80 x 86-based multiprocessor machines have a timekeeping architecture that is slightly different from the timekeeping architecture of uniprocessor machines:

• In a uniprocessor system, all time-keeping activities are triggered by interrupts raised by the global timer (either the Programmable Interval Timer or the High Precision Event Timer).
• In a multiprocessor system, all general activities (such as handling of software timers) are triggered by the interrupts raised by the global timer, while CPU-specific activities (such as monitoring the execution time of the currently running process) are triggered by the interrupts raised by the local APIC timer.

Unfortunately, the distinction between the two cases is somewhat blurred. For instance, some early SMP systems based on Intel 80486 processors didn't have local APICs. Even nowadays, there are SMP motherboards so buggy that local timer interrupts are not usable at all. In these cases, the SMP kernel must resort to the UP timekeeping architecture. On the other hand, recent uniprocessor systems feature one local APIC, so the UP kernel often makes use of the SMP timekeeping architecture. However, to simplify our description, we won't discuss these hybrid cases and will stick to the two "pure" timekeeping architectures.

Linux's timekeeping architecture depends also on the availability of the Time Stamp Counter (TSC), of the ACPI Power Management Timer, and of the High Precision Event Timer (HPET). The kernel uses two basic timekeeping functions: one to keep the current time up-to-date and another to count the number of nanoseconds that have elapsed within the current second. There are different ways to get the last value. Some methods are more precise and are available if the CPU has a Time Stamp Counter or a HPET; a less-precise method is used in the opposite case (see the later section "The time( ) and gettimeofday( ) System Calls").