LessWatts.org - Saving Power on Intel systems with Linux

Glossary

Here are some terms that might be helpful. In some cases, beyond their descriptions, their implications to power savings are described, as well.

Terms	Definition
Amp or Ampere	AM pihr, is the unit used to measure the rate of flow of an electric current. It is one of seven base units in the metric system. There is an electric current of 1 ampere when 1 unit of electric charge flows past a cross section of an electric circuit in 1 second. The unit of electric charge is called a coulomb. Thus, 1 ampere equals 1 coulomb per second. Physicists also define amperes in terms of the magnetic force produced by electric currents in parallel wires.
C state	The C state is one of the three execution states that CPUs execute code in. It provides a power savings tradeoff which depends on the length of time the CPU sleeps. The deeper the sleep, the longer it takes for the CPU to wake up, but the more power you save. The operating system selects which state you enter, based on when you anticipate the CPU will be waking up. Improvement in the Linux kernel The CPU used to wake up every millisecond to check if there was work to do. Tickless idle, a new feature in the Linux kernel allows the CPU to wake as needed, creating a greater power savings opportunity, through deeper and longer sleep. Now we need to focus on the applications Now that the kernel allows the CPU to sleep when it’s not needed, we need to make sure the applications wake the CPU only when they need to. PowerTOP This is a tool that allows you to see which applications are waking the CPU up unnecessarily. For more information, see Race to Idle.
Disk I/O	Examples include: Disks and CDs are moving, analog parts. They consume a lot of power when in use. High speed links (SATA) consume lots of power, except when in power-save mode (idle)
P state	The P state is one of the three execution states that CPUs execute code in. The P state is scalable. It allows you to determine the voltage level you wish you use, based on your usage expectations. Voltage is important here. If you set your P state to use lower voltage, you’ll use less energy, but that also limits your CPU’s capabilities. Demand-based switching You can use this feature to monitor your CPU’s activity and select the best frequency for that activity level. If you are doing something, make sure you are using the right power level. For more information, see Race to Idle.
Polling	In electronic communication, one program or device continuously checks other programs or devices to see what state they are in, usually to see whether they are still connected or if they want to communicate. Just a few of the many examples of polling: checking if the mouse moved .. once per second (gnome-screensaver) checking if the sound volume changed .. 10 times per second (mixer applet) checking if it's time to show the next minute in the clock .. once per second (clock applet)
Race to Idle	The principle is the same one cheetahs live by--run hard, and then sleep hard and long. Processors tend to be so good at saving power during idle, that often it's better to go as fast as possible so that you can then be idle longer.
T state	The T state is one of the three execution states that CPUs execute code in. Much like the emergency brake of a car, the T state is used to forcefully reduce the CPU’s execution speed. The purpose of the T state is to prevent the CPU from overheating by lower its temperature. T-states forcefully introduce idle cycles in the processor T-states do not control voltage! Idle cycles do not get into power saving C-states Avoid T-states whenever you can.
Tickless Idle	To explain this we need to go back 15 years in time; at that time a PC had only the PIT timer chip, which was good at doing a periodic interrupt. Basically all operating systems started using this periodic timer (since it was there) to do their "perform this action X milliseconds in the future" functionality. So every time the PIT created an interrupt, the OS would check the queue of "future work" for stuff to be done, and if there was something, it would do that work. If there was no work (which is the common case), it was no big deal; the hardware of the time didn't have a huge penalty for this. Now, fast forward 15 years, PCs have more options for timer chips; notably the HPET chip and the local APIC timer. These two chips can be used more easily in a flexible manner. Another thing that changed is that the current processors have a lot of power-saving features that kick in if there is no work to do; doing this kind of regular "no-work" really hurts. So this is where tickless comes in. On the surface, it's a really simple concept: instead of doing the "interrupt every 1 millisecond" thing, Linux looks at the future works and sets the hardware to fire an interrupt exactly at the right time... and conceptually it really is that simple. It just took a long time over those 15 years, as Linux grew several assumptions about the presence of the regular tick interrupt that had to be weeded out, and in addition various funky hardware and BIOS issues also had to be weeded out.
Timers (watchdog timers, WDT)	A device that performs a specific operation after a certain period of time, if something goes wrong with an electronic system and the system does not recover on its own. Timers that are spaced out over a period of time keep waking the system up. Grouping timers system-wide keeps the system in sleep mode.
Voltage	A volt is a unit of electrical measurement. The volt measures the ability of an electric field to give energy to electric charges. An electric field is the influence that an electric charge creates in the area around it. The field exerts forces on other charges in the area.
Watt	A Watt is a metric unit of power. The watt is commonly used to measure electric power, even in countries that have not adopted the metric system. The symbol for the watt is W. A watt is the product of the voltage in volts and the current in amperes or amps.