Power management/Guide

This is a guide to setup power management features of a laptop. Application to non-laptops 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:

Power management and ACPI options --->
 [*] Device power management core functionality Search for <code>CONFIG_PM</code> to find this item.
 [*] ACPI (Advanced Configuration and Power Interface) Support ---> Search for <code>CONFIG_ACPI</code> to find this item.
 <*> AC Adapter Search for <code>CONFIG_ACPI_AC</code> to find this item.
 <*> Battery Search for <code>CONFIG_ACPI_BATTERY</code> to find this item.
 <*> Button Search for <code>CONFIG_ACPI_BUTTON</code> to find this item.
 <*> Video Search for <code>CONFIG_ACPI_VIDEO</code> to find this item.
 <*> Fan Search for <code>CONFIG_ACPI_FAN</code> to find this item.
 <*> Processor Search for <code>CONFIG_ACPI_PROCESSOR</code> to find this item.
 <*> Thermal Zone Search for <code>CONFIG_ACPI_THERMAL</code> to find this item.
 CPU Frequency scaling --->
 [*] CPU Frequency scaling Search for <code>CONFIG_CPU_FREQ</code> to find this item.
 Default CPUFreq governor (schedutil) ---> Search for <code>CONFIG_CPU_FREQ_DEFAULT_GOV_SCHEDUTIL</code> to find this item.
 <*> 'performance' governor Search for <code>CONFIG_CPU_FREQ_GOV_PERFORMANCE</code> to find this item.
 <*> 'powersave' governor Search for <code>CONFIG_CPU_FREQ_GOV_POWERSAVE</code> to find this item.
 <*> 'userspace' governor for userspace frequency scaling Search for <code>CONFIG_CPU_FREQ_GOV_USERSPACE</code> to find this item.
 <*> 'ondemand' cpufreq policy governor Search for <code>CONFIG_CPU_FREQ_GOV_ONDEMAND</code> to find this item.
 <*> 'conservative' cpufreq governor Search for <code>CONFIG_CPU_FREQ_GOV_CONSERVATIVE</code> to find this item.
 [*] 'schedutil' cpufreq policy governor Search for <code>CONFIG_CPU_FREQ_GOV_SCHEDUTIL</code> to find this item.
 *** CPU frequency scaling drivers ***
 <*> ACPI Processor P-States driver Search for <code>CONFIG_X86_ACPI_CPU_FREQ</code> to find this item.
 *** shared options ***
 [*] Current frequency derived from HW provided feedback Search for <code>CONFIG_CPUFREQ_ARCH_CUR_FREQ</code> to find this item.

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 Powercap sysfs can be used for AMD Zen processors since kernel 5.11.^[1]

Thermal sysfs driver

Device Drivers --->
 <*> Thermal drivers ---> Search for <code>CONFIG_THERMAL</code> to find this item.
 (0) Emergency poweroff delay in milli-seconds Search for <code>CONFIG_THERMAL_EMERGENCY_POWEROFF_DELAY_MS</code> to find this item.
 [*] Expose thermal sensors as hwmon device Search for <code>CONFIG_THERMAL_HWMON</code> to find this item.
 Default Thermal governor (step_wise) ---> Search for <code>CONFIG_THERMAL_DEFAULT_GOV_STEP_WISE</code> to find this item.
 [*] Fair-share thermal governor Search for <code>CONFIG_THERMAL_GOV_FAIR_SHARE</code> to find this item.
 [*] Step_wise thermal governor Search for <code>CONFIG_THERMAL_GOV_STEP_WISE</code> to find this item.
 [*] Bang Bang thermal governor Search for <code>CONFIG_THERMAL_GOV_BANG_BANG</code> to find this item.
 [*] User_space thermal governor Search for <code>CONFIG_THERMAL_GOV_USER_SPACE</code> to find this item.
 [*] Power allocator thermal governor Search for <code>CONFIG_THERMAL_GOV_POWER_ALLOCATOR</code> to find this item.
 Intel Thermal drivers --->
 <M> Intel PowerClamp idle injection driver Search for <code>CONFIG_INTEL_POWERCLAMP</code> to find this item.
 <M> X86 package temperature thermal driver Search for <code>CONFIG_X86_PKG_TEMP_THERMAL</code> to find this item.
 < > Intel SoCs DTS thermal driver Search for <code>CONFIG_INTEL_SOC_DTS_THERMAL</code> to find this item.
 ACPI INT340X thermal drivers --->
 <M> ACPI INT340X thermal drivers Search for <code>CONFIG_INT340X_THERMAL</code> to find this item.
 <M> Intel PCH Thermal Reporting Driver Search for <code>CONFIG_INTEL_PCH_THERMAL</code> to find this item.

powercap sysfs driver

Additionally, you can enable Running Average Power Limit (RAPL) technology. Both PowerTOP and turbostat utilities are aware of RAPL. To enable RAPL, use:

Device Drivers --->
 <*> Generic powercap sysfs driver ---> Search for <code>CONFIG_POWERCAP</code> to find this item.
 <M> Intel RAPL Support via MSR Interface Search for <code>CONFIG_INTEL_RAPL</code> to find this item.

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

or

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

Installation

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

To install it:

Then simply enable it on OpenRC systems like so:

Or for systemd machines:

TLP is a 'set and forget' style package, but it can be customized if desired.

Using Laptop Mode Tools

Installation

It comes to no surprise that installation of the Laptop mode tools software is easily done via:

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.

Observe that these two USE flags seem to collide; 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:

Configuration

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:

CONTROL_CPU_FREQUENCY=0

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:

With systemd:

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.

## 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:

• 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.
• schedutil will scale the CPU frequency based on the scheduler context; this governor is generally regarded as a replacement for the older ondemand and conservative governors^[2] .

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.

Using power-profiles-daemon

power-profiles-daemon modify system behavior using profiles. There are 3 different power profiles, "balanced", "power-saver", and "performance".

Installation

To install it:

Now enable and start the service:

on systemd systems:

Use

To list profiles:

To set the profile:

See also

• Power management — describes methods to save energy for longer battery runtimes, a quieter computer, lower power bills, and an environmentally friendly impact.
• PowerTOP — a Linux utility that can monitor and display a system's electrical power usage.

External resources

• Laptop Mode Tools Homepage, includes About laptop mode.
• 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).

References

• Guide
• Laptops
• Power management