kernelklok

Kernelklok is a term used in computing to refer to the kernel’s internal timekeeping system. It describes the part of an operating system kernel that tracks time, provides timestamps, and drives timers and scheduling decisions. The kernelklok maintains both the wall clock (system time) and a monotonic clock that never regresses, the latter being important for reliable time intervals and correct sequencing of events.

The kernelklok operates by combining a hardware time source with software timers. A hardware clock, such as a real-time clock or a high-resolution timer, provides a base signal, which the kernel translates into a steadily advancing time value. To support precise timing and efficient scheduling, most kernels implement a timekeeping subsystem that can switch between low-power tick-based modes and tickless operation. The subsystem exposes various clocks to user space, such as real time and monotonic time, and it handles time adjustments prompted by external references like NTP. When time is adjusted, the kernel may step or slew the clock while attempting to preserve monotonicity for ongoing operations.

Different kernels implement the kernelklok with varying mechanisms. Linux uses a timekeeping framework built around clocksources, clockevents, and high-resolution timers (hrtimers). Windows maintains its own kernel timekeeping using performance counters and related facilities. BSD systems rely on timecounter mechanisms and related components. Across platforms, the kernelklok must balance precision, efficiency, and correctness, particularly in virtualized environments, during suspend/resume cycles, and when handling leap seconds and timezone representations.

See also: timekeeping, clocksource, hrtimer, monotonic clock, system time, time synchronization.