What are C-states?

C-states are power states for each core of a CPU. C0 is awake; deeper naps save more power but take longer to wake from.

1. Naps of different depths
A CPU's cores nap many times a second, even while you use the computer. C1 stops a core's clock; deeper naps like C6 cut its power.
2. Light nap or deep nap
The operating system guesses how long a core will stay idle and asks for a nap that fits. The chip can then make the nap lighter or deeper.
3. Residency: time in each state
Residency is the share of time a core spends in each state. On an idle computer, a core might wake only for small jobs and nap deeply most of the time.
4. Deepest sleep needs everyone asleep
The whole chip, called the package, sleeps deepest only when every core and most other parts are idle at once. It is like a house that goes dark only when everyone is asleep.
5. One wake-up source drains the battery
If one device, driver (device software) or timer keeps waking the chip, it rarely reaches deep sleep, or leaves it too soon. The battery then drains faster.

Words to know

C-state: A core's power state. C0 is awake; higher numbers are deeper naps.
Core: One of several workers inside a CPU. Each can nap on its own.
Residency: The share of time spent in one state.
Package: Here, the whole processor. The word also means the case a chip sits in.
Driver: Software that lets the operating system use a device.
Timer: An alarm that wakes the chip at a set time.

Idle tests measure how long the cores and the whole chip spend in each C-state. If deep naps are rare, testers look for what keeps waking the chip.

A laptop sits idle, but its battery drains fast. What might be wrong?Think first, then tap to see the answer.

Something, such as a device, driver or timer, keeps waking the chip, so it spends little time in deep sleep.