What do CPUs do when idle?

[Rovillalob]

I was wondering why are CPUs always working even if it looks like its doing nothing. Lets say I start the pc in safe mode with no internet conection, what is the cpu doing if I dont do anything with it? Right now my video card is under low load (no games or other heavy programs runing) and the fans are still but the fans on the CPU are always spinning. I guess my real question is: why can the GPU do that but not the CPU?

[Electronics Wizardy]

they basically turn off, thats what the c states do, so power is very low(less than a watt per core normally)

[AnonymousGuy]

Most of the chip will turn itself off through lower frequencies and power gates and such when there's low load and the OS commands it to power down.

 

You could use a CPU fan profile that turns the fan off until a temperature threshold is reached, there's no technical reason why this isn't possible.

[The Tan Stallion]

If the CPU shuts down , imagine what is the loss of restarting the CPU and the latency in re engaging with the threads which might have a lot of context (process info) with the scheduler ?

You are looking at threads being orphan and technically loosing all the process information that is currently being executed.

That is exactly why the rest of the scheduler is executing an idle process, so that whenever it is required, it can switch back to service the process.