How does Ubuntu 22.04 handle power modes?

Question

As you've probably noticed, power mode option has been added to new Ubuntu . I wonder how this feature works. I mean, what happens when you put a machine into whether power saving mode, max performance or balanced mode. Does it change kernel settings? If yes, how? Does it disable some apps or services? Does it change CPU frequency?

Answer 1

Short answer:

It optimizes various system settings, such as time before it turns of the display as well as CPU settings (usually) to get the target level of performance vs. battery.

Long answer:

We can see that Gnome depends on power-profiles-daemon. There isn't much info on the Power Profiles Daemon, but let's start by looking at its README on GitLab:

... There are 3 different power profiles, a "balanced" default mode, a "power-saver" mode, as well as a "performance" mode. The "performance" mode is ... implemented by different "drivers" based on the system or systems it targets.

In addition to those 2 or 3 modes (depending on the system), "actions" can be hooked up to change the behaviour of a particular device. For example, this can be used to disable the fast-charging for some USB devices when in power-saver mode.

GNOME's Settings and shell both include interfaces to select the current mode, but they are also expected to adjust the behaviour of the desktop depending on the mode, such as turning the screen off after inaction more aggressively when in power-saver mode.

The gist is that it can do a number of things, like disable fast-charging + turn off the screen quicker. Futher down, there is some more information:

Operations on Intel-based machines

The "driver" for making the hardware act on the user-selected power profile on Intel CPU-based machines is based on the Intel P-State scaling driver. It is only used if a platform_profile driver isn't available for the system, and the CPU supports hardware-managed P-states (HWP). If HWP isn't supported, or the P-State scaling driver is set to passive mode.

If the Intel P-State scaling driver is in passive mode, either because the system doesn't support HWP, or the administator has disabled it, then the placeholder driver will be used, and there won't be a performance mode. Finally, if the Intel P-State scaling driver is used in active mode, the P-State scaling governor will be changed to powersave as it is the only P-State scaling governor that allows for the "Energy vs Performance Hints" to be taken into consideration, ie. the only P-State scaling governor that allows power-profiles-daemon to work.

I'm not going to dive into the Intel P-State driver, but more info on it is here. The general idea is that it is a way to scale performence on Intel CPUs, and it adjusts various CPU-related settings to achive either fast performence or good battery or something in between. I don't know enough about it to understand exactly what it does, though.