How does a tick function? - briefly
A tick is a hardware‑generated interrupt that fires at fixed intervals (commonly 1–10 ms) to update the system clock and invoke the scheduler. The interrupt handler increments the kernel tick counter, awakens timed tasks, and may trigger a context switch when a time slice ends.
How does a tick function? - in detail
A tick is a periodic signal generated by a hardware timer or a software scheduler that drives time‑dependent operations in a system. The timer hardware counts down from a preset value; when the counter reaches zero it asserts an interrupt line. The interrupt service routine (ISR) increments a global counter, updates system clocks, and notifies the scheduler that a time slice has elapsed.
The scheduler uses the tick count to enforce pre‑emptive multitasking. Each thread receives a quantum expressed in ticks; after the quantum expires, the scheduler selects the next runnable thread based on priority and readiness. This process repeats at a frequency defined by the tick rate, commonly 100 Hz to 1000 Hz in desktop operating systems.
Key components of the tick mechanism:
- Timer configuration – programming the divisor or reload value to achieve the desired interval.
- Interrupt handling – minimal ISR that records the tick, acknowledges the interrupt, and defers lengthy work to a deferred procedure call or tasklet.
- Clock update – conversion of tick count to wall‑clock time, handling of leap seconds and monotonic time sources.
- Scheduler interaction – evaluation of run queues, context‑switch initiation, and enforcement of real‑time constraints.
Modern kernels often employ a tickless design. Instead of a fixed periodic interrupt, the system programs the timer to fire only when the next scheduled event occurs. This reduces overhead, lowers power consumption, and improves latency for idle periods. When an event is pending, the timer is re‑armed with the appropriate interval, effectively reintroducing a tick only as needed.
In embedded environments, a tick may serve as the heartbeat for state machines, watchdog refresh, and peripheral timeouts. The tick period is chosen to balance resolution against interrupt load; a 1 ms tick provides millisecond granularity while generating 1000 interrupts per second.
Overall, the tick functions as the fundamental timing primitive that synchronizes clock updates, task scheduling, and time‑based services across hardware and software layers.