Despite what this might look like, I don't actually enjoy starting new projects: it's a lot easier to clean up some build warnings, or add a CI, than it is to start from an empty directory.
But sometimes needs must, and I've just released version 0.1 of such a project. Below you'll find an excerpt from the README, which should answer most of the questions. Please read the README directly in the repository if you're getting to this blog post more than a couple of days after it was first published.
Feel free to file new issues in the tracker if you have ideas on possible power-saving or performance enhancements. Currently the only supported “Performance” mode supported will interact with Intel CPUs with P-State support. More hardware support is planned.
From the README:
power-profiles-daemon offers to modify system behaviour based upon user-selected power profiles. There are 3 different power profiles, a "balanced" default mode, a "power-saver" mode, as well as a "performance" mode. The first 2 of those are available on every system. The "performance" mode is only available on select systems and 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.
Note that power-profiles-daemon does not save the currently active profile across system restarts and will always start with the "balanced" profile selected.
The power-profiles-daemon project was created to help provide a solution for two separate use cases, for desktops, laptops, and other devices running a “traditional Linux desktop”.
The first one is a "Low Power" mode, that users could toggle themselves, or have the system toggle for them, with the intent to save battery. Mobile devices running iOS and Android have had a similar feature available to end-users and application developers alike.
The second use case was to allow a "Performance" mode on systems where the hardware maker would provide and design such a mode. The idea is that the Linux kernel would provide a way to access this mode which usually only exists as a configuration option in some machines' "UEFI Setup" screen.
This second use case is the reason why we didn't implement the "Low Power" mode in UPower, as was originally discussed.
As the daemon would change kernel settings, we would need to run it as root, and make its API available over D-Bus, as has been customary for more than 10 years. We would also design that API to be as easily usable to build graphical interfaces as possible.
This section will contain explanations of why this new daemon was written rather than re-using, or modifying an existing one. Each project obviously has its own goals and needs, and those comparisons are not meant as a slight on the project.
As the code bases for both those projects listed and power-profiles-daemon are ever evolving, the comments were understood to be correct when made.
thermald only works on Intel CPUs, and is very focused on allowing maximum performance based on a "maximum temperature" for the system. As such, it could be seen as complementary to power-profiles-daemon.
Both projects have similar goals, allowing for tweaks to be applied, for a variety of workloads that goes far beyond the workloads and use cases that power-profiles-daemon targets.
A fair number of the tweaks that could apply to devices running GNOME or another free desktop are either potentially destructive (eg. some of the SATA power-saving mode resulting in corrupted data), or working well enough to be put into place by default (eg. audio codec power-saving), even if we need to disable the power saving on some hardware that reacts badly to it.
Both are good projects to use for the purpose of experimenting with particular settings to see if they'd be something that can be implemented by default, or to put some fine-grained, static, policies in place on server-type workloads which are not as fluid and changing as desktop workloads can be.
It doesn't take user-intent into account, doesn't have a D-Bus interface and seems to want to work automatically by monitoring the CPU usage, which kind of goes against a user's wishes as a user might still want to conserve as much energy as possible under high-CPU usage.