Power management/Guide

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This is a guide to setup power management features of a laptop. Application to non-laptos should be done carefully.

Two key terms

Users should distinguish laptop_mode, a kernel feature and laptop-mode-tools, a package.

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:

KERNEL Minimum kernel setup for Power Management
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

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. [1]

Thermal sysfs driver

KERNEL Generic 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 drivers --->
        <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

Additionally, you can enable Running Average Power Limit – RAPL technology. Both PowerTOP and turbostat utilities are aware of RAPL. For RAPL enablement use:

KERNEL Generic powercap sysfs driver
Device Drivers --->
  [*] Generic powercap sysfs driver --->
    <M>   Intel RAPL Support

To make use of Intel's Linux thermal daemon you first need to emerge sys-power/thermald and enable it via:

root #rc-config add thermald


root #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.

Using TLP


TLP is similar to laptop-mode-tools but aims to work out of the box with safe, modern defaults.

To install it:

root #emerge --ask sys-power/tlp

Then simply enable it on OpenRC systems like so:

root #rc-update add tlp default
root #rc-service tlp start

Or for systemd machines:

root #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:

root #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...
acpi 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).
apm 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.
bluetooth 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.
scsi 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: acpi and apm. So what is the deal?

  • The apm USE enables support for Advanced Power Management , an older (before year 2000) standard for power management features within a system.
  • The acpi USE enables support for Advanced Configuration and Power Interface , the successor of APM. All modern laptops support ACPI.

Depending on the system, either acpi or 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:

root #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:

FILE /etc/laptop-mode/conf.d/cpufreq.conf

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.

With OpenRC:

root #rc-service laptop_mode start
root #rc-update add laptop_mode default

With systemd:

root #systemctl enable laptop-mode.service --now

How laptop-mode-tools works

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 LM_, LM_BATT_, BATT_LM_, and 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.

CODE Colliding settings
## Valid set
## Invalid set
# The following includes AC and BATT, but BATT is already defined

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:

  • performance always picks the highest frequency;
  • powersave always picks the lowest frequency;
  • userspace does not pick anything, but lets the user decide (or any process that the user is running that will decide for the user);
  • ondemand will scale the CPU frequency up to the highest frequency when load is available;
  • conservative will 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.

See also

External resources

  • 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).