Power management/Guide

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.

Article status

This article has some todo items:

• Add instructions for kernel configuration of remaining CPU frequency drivers

Introduction

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.

About laptop_mode

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).

About laptop-mode-tools

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

There are different kernel sources in Portage sys-kernel. We recommend using sys-kernel/gentoo-sources, but if advanced hibernation support is desired, TuxOnIce might be needed.

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

Intel P-state

For newer Intel Core series of processors (based on the Sandy Bridge microarchitecture or newer), use the Intel P-state driver.

In this case, the userspace, ondemand, and conservative governors are unnecessary. The performance governor should be selected as the default.

KERNEL Kernel setup for Intel Sandy Bridge and newer processors

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 (performance)  --->
    -*- 'performance' governor
    <*> Intel P state control

Enabling additional drivers

For Intel processors used in laptops since 2011 you may want to enable additional advanced features - Thermal and powercap sysfs.

Thermal sysfs driver

KERNEL Generic Thermal sysfs driver

Device Drivers --->
  -*- Generic Thermal sysfs driver --->
    (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   
    <M>   Intel PowerClamp idle injection driver
    <M>   X86 package temperature thermal driver
    < >   Intel SoCs DTS thermal driver                                                                                          
          ACPI INT340X thermal drivers  --->                                                                                     
             <M> ACPI INT340X thermal drivers
    <M>   Intel PCH Thermal Reporting Driver

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

or

root #systemctl enable thermald

Note
dptfxtract from Intel may be used to auto generate a thermald.conf.xml for the system.

Kernel setup finalization

For a more detailed configuration description see the Power management/Processor#Kernel article.

Build and install the new kernel (if necessary) and reboot.

Using Laptop Mode Tools

Installation

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

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 such as cpufreqd. In this example case, CONTROL_CPU_FREQUENCY=0 must be set:

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

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:

root #rc-service laptop_mode start

root #rc-update add laptop_mode default

With systemd:

root #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 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
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.

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 cpufreqd

Warning
Package sys-power/cpufreqd is deprecated and has been removed from the Portage tree. sys-power/ncpufreqd can still be used for 2.6.x kernels.

Installation

The cpufreqd application allows the user to manage CPU frequencies in a more granular approach than what laptop-mode-tools supports. But before we dive into the installation of cpufreqd, let us first look at the USE flags it supports.

USE flag	Description	Suggested when...
acpi	Enable support for ACPI, allowing cpufreqd to be notified about specific events as well as govern power through the ACPI interface	the laptop is not very old (around year 2003 and later)
apm	Enable support for APM, allowing cpufreqd to be notified about specific events as well as govern power through the APM interface	the laptop is very old
lm_sensors	Enable support for the Linux hardware sensors (through sys-apps/lm_sensors), allowing to switch profiles based on hardware sensor results	using advanced events through lm_sensors
nforce2	Enable support for NForce, allowing cpufreqd to change the NForce FSB clock and video card frequency	an NVidia graphics card based on the NForce chipset is present
nvidia	Enable support for NVidia graphical card configuration (through the NVidia nvclock interface), allowing cpufreqd to change the video card frequency of NVidia graphical cards	an NVidia graphics card is present
pmu	Enable the Power Management Unit plug-in of cpufreqd. This allows the software to poll the Linux kernel Power Supply interface, getting more detailed information on battery charge	the laptop does not support ACPI or APM

The acpi, apm, and pmu USE overlap, so only one should be active. If the laptop is sufficiently recent, acpi is the best bet. If not, apm offers all that is needed. When even APM isn't supported, try using pmu.

With the USE flags configured, it is time to install cpufreqd.

root #emerge --ask cpufreqd

Configuration

The cpufreqd application monitors the status of the system through several plugins. Based on the feedback it receives from those plugins, it will adjust the policy used to govern the CPU frequency.

cpufreqd can be configured by editing /etc/cpufreqd.conf. It contains three different sections:

1. The [General]...[/General] section contains general configuration information.
2. The [Profile]...[/Profile] section defines the policies that the cpufreqd daemon can switch to. The section is very similar to the information used when manually setting the CPU frequency policy using cpufreq-set.
3. The [Rule]...[/Rule] section is the work-horse of the cpufreqd daemon, defining when the daemon decides to switch to a different profile.

Take a quick look at an example rule.

FILE /etc/cpufreqd.confSample cpufreqd rule

[...]
  
[Profile]
name=On Demand High
minfreq=40%
maxfreq=100%
policy=ondemand
[/Profile]
  
[Rule]
name=AC Off - High Power
ac=off
battery_interval=70-100
profile=On Demand High
[/Rule]
  
[...]

In the above example, cpufreqd will switch the system to the On Demand High profile (also shown in the above excerpt). This profile by itself uses the ondemand governor with a minimum frequency of 40% (iow, a CPU of 2 GHz will have by this policy a minimum frequency of 800 MHz).

The cpufreqd application can offer a more granular approach on CPU frequency scaling. But not only that, but the CPU frequency scaling can be tweaked based on various other metrics available. The default configuration offers a sample rule: when a movie is watched, maximum performance is desired (unless the CPU temperature is getting too high).

When cpufreqd has been configured, it is time to start it (and make sure the service is loaded automatically). Make sure that CPU frequency handling by other tools (like laptop-mode-tools) is disabled!

root #rc-update add cpufreqd default

root #/etc/init.d/cpufreqd start

See also

• USB Power Saving

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).