what is a tickless OS?

Question

I've heard the term "Tickless OS" thrown around.

• What does it mean?
• Which OSes are tickless?
• How does it differ from a non-tickless (tickful?) OS?

Answer 1

A 'tick' in OS terms is an interval after which the OS will wake up to process any pending events.

So, if you have a 100Hz tick, then 100 times a second, the OS will wake up, check to see whether any timers have expired, execute their actions if there are, then go to sleep again. On a tick-less system, rather than waking up 100 times a second to see if anything is ready to process, the OS will look at when the next timer is due to expire and then sleep until that time.

Linux can run with or without ticks, with a compile-time switch. I don't know about any other systems.

Answer 2

This link provides some insights: Avoiding Processor Wake-Ups Saves Power

A fragment from the above link,

In order to take best advantage of the low power states offered by the latest processors, the operating system has to allow the processor to stay in those states as much as possible. A long-standing feature of the Linux* operating system has been a timer tick that supports services like helping the operating system keep internal time and monitor CPU usage by various applications and processes.

While that timer tick is useful, it also has the unfortunate side effect of waking the processor when it is in a low power state as many as 1,000 times per second. In fact, under some circumstances, the tick can prevent the processor from entering the deep power-saving states at all. Clearly, this effect can have a dramatic negative impact on power usage by the system.