Clocks

Engineering code often contains calls like these:

wait_event_timeout(..., 1000);
sleep(1);
select(fd + 1, &rfds, NULL, NULL, &tv);

They all look like “wait for a while.” But waiting inside an operating system does not mean the CPU sits still and counts time.

A timeout usually passes through several steps: the program submits a wait request, the kernel places the current thread on a wait queue, a timer records the latest wakeup time, the scheduler gives the CPU to another thread, hardware clocks or timer interrupts advance time, and the thread is woken when the condition is satisfied or the timeout expires.