Tickless Idle

Introduction

Traditionally, the Linux kernel used a periodic timer for each CPU. This timer did a variety of things, such as process accounting, scheduler load balancing, and maintaining per-CPU timer events. Older Linux kernels used a timer with a frequency of 100Hz (100 timer events per second or one event every 10ms), while newer kernels use 250Hz (250 events per second or one event every 4ms) or 1000Hz (1000 events per second or one event every 1ms).

This periodic timer event is often called "the timer tick". The timer tick is simple in its design, but has a significant drawback: the timer tick happens periodically, irrespective of the processor state, whether it's idle or busy. If the processor is idle, it has to wake up from its power saving sleep state every 1, 4, or 10 milliseconds. This costs quite a bit of energy, consuming battery life in laptops and causing unnecessary power consumption in servers.

With "tickless idle", the Linux kernel has eliminated this periodic timer tick when the CPU is idle. This allows the CPU to remain in power saving states for a longer period of time, reducing the overall system power consumption.

It's important to note that the benefits of tickless idle will be lost if the CPU is frequently awakened by unnecessary timer events. There are variety of enhancements and patches over the basic tickless idle feature that provide incremental power savings. This project is a collection of all such enhancements.