In recent years, power management has become one of the differentiating features in the quest for finding the perfect laptop. Yet, the operating system must support the various power saving functionalities too. This guide covers the configuration of a Gentoo system to manage power-hungry resources in a flexible-yet-automated manner.
About this document...
This document describes the setup of power management features for laptops. Although some of the information in this guide can be applied to power management for servers, it is not the intention of this document to go that route. Please be careful when applying this on a non-laptop system.
Within this document, the primary focus will be on laptop mode tools since it offers a complete set of functionalities. However, we will also refer to other tools that might offer a more detailed approach on individual settings. In such cases, the feature from the laptop mode tools must be disabled so that both tools do not fight over the same resource control.
The laptop_mode setting is an in-kernel configuration setting that optimizes I/O, allowing disks to spin down properly (and not be woken up immediately afterwards for queued operations).
The Laptop Mode Tools is a software package (app-laptop/laptop-mode-tools) which allows the user to optimize power saving functions. It allows managing the laptop_mode setting in the Linux kernel, but has additional features that allow the tweaking of other power-related settings on the system.
Linux kernel configuration
Minimum kernel setup
To enable proper power management features in the Linux kernel, enable the following settings:
Power management and ACPI options ---> -*- Device power management core functionality [*] ACPI (Advanced Configuration and Power Interface) Support ---> <*> AC Adapter <*> Battery -*- Button -*- Video <*> Fan <*> Processor <*> Thermal Zone [*] CPU Frequency scaling ---> Default CPUFreq governor (ondemand) ---> -*- 'performance' governor <*> 'powersave' governor <*> 'userspace' governor for userspace frequency scaling -*- 'ondemand' cpufreq policy governor <*> 'conservative' cpufreq governor <*> ACPI Processor P-States driver
For more information about settings related to CPU frequency scaling see the dedicated Processor guide.
Selecting CPU frequency driver
Available CPU frequency drivers are summarized in the separate Processor guide.
Enabling additional drivers
Thermal and Powercap sysfs are additional advanced features. For Intel processors used in laptops since 2011. The Powecap sysfs can be used for AMD Zen processors since kernel 5.11. 
Thermal sysfs driver
Device Drivers ---> -*- Thermal drivers ---> (0) Emergency poweroff delay in milli-seconds [*] Expose thermal sensors as hwmon device -*- Enable writable trip points Default Thermal governor (step_wise) ---> [*] Fair-share thermal governor -*- Step_wise thermal governor -*- Bang Bang thermal governor -*- User_space thermal governor [ ] Thermal emulation mode support [*] Power allocator thermal governor Intel Thermal Frivers ---> <M> Intel PowerClamp idle injection driver <M> X86 package temperature thermal driver < > Intel SoCs DTS thermal driver <M> Intel PCH Thermal Reporting Driver ACPI INT340X thermal drivers ---> <M> ACPI INT340X thermal drivers
powercap sysfs driver
Device Drivers ---> [*] Generic powercap sysfs driver ---> <M> Intel RAPL Support
rc-config add thermald
systemctl enable thermald
With sys-power/thermald 2.0 and above, if the daemon is started with the "--adaptive" option, then it is not necessary to create a thermald.conf.xml file.
Kernel setup finalization
For a more detailed configuration description see the Processor Kernel article.
Build and install the new kernel (if necessary) and reboot.
TLP is similar to laptop-mode-tools but aims to work out of the box with safe, modern defaults.
To install it:
emerge --ask sys-power/tlp
Then simply enable it on OpenRC systems like so:
rc-update add tlp default
rc-service tlp start
Or for systemd machines:
systemctl enable --now tlp
TLP is a 'set and forget' style package, but it can be customized if desired.
Using Laptop Mode Tools
It comes to no surprise that installation of the Laptop mode tools software is easily done via:
emerge --ask app-laptop/laptop-mode-tools
However, this package takes on additional, optional settings through USE flag configuration. So let's first take a look at the supported USE flags and what they mean to the package.
|USE flag||Description||Suggested when...|
||Depend on sys-power/acpid so that changes in the system are captured and power saving features are automatically enabled/disabled.||the laptop is not too old (around year 2003 and later).|
||Depend on sys-apps/apmd so that changes in the system are captured and power saving features are automatically enabled/disabled.||the laptop is very old.|
||Depend on net-wireless/bluez , enabling laptop-mode-tools to manage bluetooth settings (enabling/disabling the service based on battery availability)||the laptop (and kernel) support bluetooth.|
||Depend on sys-apps/sdparm , enabling laptop-mode-tools to manage SCSI (and not SATA) disk parameters.||the laptop uses SCSI disks.|
Observe there are two USE flags that seem to collide:
apm. So what is the deal?
apmUSE enables support for Advanced Power Management , an older (before year 2000) standard for power management features within a system.
acpiUSE enables support for Advanced Configuration and Power Interface , the successor of APM. All modern laptops support ACPI.
Depending on the system, either
apm will need to be set. In the remainder of this guide, it is assumed the laptop is recent enough to use ACPI.
So, with the USE flags set, install laptop-mode-tools:
emerge --ask app-laptop/laptop-mode-tools
Having laptop-mode-tools installed on the system does not automatically enable the power management features that may be needed. To configure the package, first take a look at /etc/laptop-mode/laptop-mode.conf. This is the main configuration file for the package and is pretty well described (through comments).
But it is not the only configuration file to work with. The laptop-mode-tools package supports plugins (or modules) which have their own configuration file(s). These files are located in /etc/laptop-mode/conf.d and are named after the module they represent (such as intel-sata-powermgmt.conf).
Now, one of the important settings in each configuration file is if the laptop-mode-tools package should govern a particular setting or not. This is important when combining laptop-mode-tools with other power management services. In this example case,
CONTROL_CPU_FREQUENCY=0 must be set:
The next few sections will help the user configure laptop-mode-tools to suit specific needs. When finished, start the laptop_mode service and make sure it is started upon system boot.
rc-service laptop_mode start
rc-update add laptop_mode default
systemctl enable laptop-mode.service --now
How does laptop-mode-tools work
When running the laptop_mode service, the software will check in which state the system is in. The states are defined as:
- Battery, which is active when the system is running on battery power; the configuration files use the
BATT_prefix for settings related to this state;
- AC, which is active when the system is running on AC power; the configuration files use the
AC_prefix for settings related to this state;
- Laptop Mode, which is active when laptop mode is enabled; the configuration files use the
LM_prefix for settings related to this state;
- No Laptop Mode, which is active when laptop mode is disabled; the configuration files use the
NOLM_prefix for settings related to this state.
The AC/BATT_ and LM/NOLM_ prefixes can be combined to have a AC_LM_ prefix.
When the laptop_mode service is started, it will switch modes based on events that occur (and of course based on the configuration settings). For instance, the setting
ENABLE_LAPTOP_MODE_ON_BATTERY=1 will make sure that the laptop mode tools switch to laptop mode when battery power is used. If that is the case, then the settings starting with
BATT_ will be used.
To make sure settings do not collide, it is not allowed to have overlapping settings. In the next example, the first set (for
CPU_MAXFREQ) is valid, but the second one (for
CPU_MINFREQ) is not.
## Valid set BATT_CPU_MAXFREQ=fastest LM_AC_CPU_MAXFREQ=fastest NOLM_AC_CPU_MAXFREQ=fastest ## Invalid set BATT_CPU_MINFREQ=fastest LM_AC_CPU_MINFREQ=fastest # The following includes AC and BATT, but BATT is already defined NOLM_CPU_MINFREQ=fastest
Configuring CPU frequency management
The support for CPU frequency management in the laptop mode tools allows switching frequencies. It supports setting the CPU frequency governor, minimum frequency and maximum frequency. The configuration file used here is /etc/laptop-mode/conf.d/cpufreq.conf
The CPU frequency governor is a kernel-level policy that defines how the kernel will select the CPU frequency. We already selected the governors we want to use in the kernel configuration earlier. Let's recap:
performancealways picks the highest frequency;
powersavealways picks the lowest frequency;
userspacedoes not pick anything, but lets the user decide (or any process that the user is running that will decide for the user);
ondemandwill scale the CPU frequency up to the highest frequency when load is available;
conservativewill scale the CPU frequency up gradually when load is available.
When switching between AC or battery, or (no) laptop mode, the appropriate governor (as well as its minimum and maximum frequency) is selected.
Configuring display brightness
With /etc/laptop-mode/conf.d/lcd-brightness.conf, the laptop mode tools can govern the brightness of the LCD screen.
The file currently uses the /proc/acpi/video/VID/LCD/brightness file (bug 499544) to set brightness values. Recent kernels do not provide this anymore; it will need to adjust this to /sys/class/backlight/acpi_video0/brightness instead.
The possible values that can be used are between 0 and the value in /sys/class/backlight/acpi_video0/max_brightness, with 0 being the lowest brightness value.
Configuring other services
An interesting feature of laptop-mode-tools is to support reloading particular services (like the system logger) after switching its configuration file. This is handled through /etc/laptop-mode/conf.d/configuration-file-control.conf
If enabled, the laptop_mode application will switch the configuration file(s) of the mentioned services with the same file, but suffixed with -nolm-ac , -lm-ac or -batt. It will then signal or reload the appropriate services so they can use the new configuration file.
- USB Power Saving — describes the Linux's ability to power off USB devices and to let USB devices request to wake them up again.
- PowerTOP — a Linux utility that can monitor and display a system's electrical power usage.
- Power_management/Processor — describes the setup of power management for processors.
- Laptop Mode Tools Homepage, includes About laptop mode.
- PowerTOP, an interactive application helping users to find out which processes are forcing wakeups on the CPU most often.
- A ThinkWiki article on How to reduce power consumption (on Linux). This article offers an exhaustive list of measures one can take. However, it should be noted that the laptop mode tools implements the majority of these (if properly configured).
- AMD Zen1/Zen2/Zen3 PowerCap RAPL Support Queued For Linux 5.11 - Phoronix, Michael Larabel. Retrieved 10 September 2023.