Exploring power-saving modes in Linux

When a system is in the idle state and does not have work to do, it can be put in a sleep state to save power. Similar to human sleep, it cannot do anything until it is woken up by external event, for example an alarm clock.

Linux supports multiple sleep modes. The choice of sleep mode and the amount of power it can save depends on the hardware support and the time it takes to enter the mode and wake up from it. 

The supported modes are as follows:

• Suspend-to-idle (S2I): This is a light sleep mode that can be implemented purely in software and does not require any support from the hardware. The devices are put into low-power mode and time keeping is suspended to let the processor spend more time in a power-efficient idle state. A system is woken up by an interrupt from any of the peripheral devices.
• Standby: This is similar...