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Power management/HOWTO

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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. In this guide, we cover how to setup your Gentoo installation so it manages power-hungry resources in a flexible yet automated manner.

Introduction

About this document...

This document describes the setup of power management features on your laptop. 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, we will focus primarily on the 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, you will need to disable the feature from the laptop mode tools 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 allowing you to tweak other power-related settings on the system.

Linux Kernel Configuration

Minimum kernel setup

There are different kernel sources in Portage. We recommend using gentoo-sources , but if you want advanced hibernation support you might need tuxonice-sources . To enable proper power management features in the Linux kernel, enable at least the following settings:

Kernel configurationMinimum kernel setup for Power Management

General setup --->
  [*] Configure standard kernel features (expert users) --->
  
Power management and ACPI options --->
  [*] Run-time PM core functionality
  [*] ACPI (Advanced Configuration and Power Interface) Support --->
    <*> AC Adapter
    <*> Battery
    -*- Button
    -*- Video
    <*> Fan
    <*> Processor
    <*> Thermal Zone
    [*] Power Management Timer Support
  
  [*] CPU Frequency scaling --->
    [*] CPU Frequency scaling
    <*>   'performance' governor
    <*>   'powersave' governor
    <*>   'userspace' governor
    <*>   'ondemand' governor
    <*>   'conservative' governor
    <*> ACPI Processor P-States driver

Don't forget to enable the CPU frequency scaling driver for your CPU, located right after the ACPI Processor P-States driver mentioned above.

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 through emerge 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. your laptop is not too old (~ 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. your laptop is very old
bluetooth Depend on net-wireless/bluez , enabling the laptop-mode-tools to manage bluetooth settings (enabling/disabling the service based on battery availability) your laptop (and kernel) supports bluetooth
scsi Depend on sys-apps/sdparm , enabling the laptop-mode-tools to manage SCSI ( and not SATA) disk parameters. your laptop uses SCSI disks

As you can see, there are two USE flags that seem to collide: acpi and apm . So what's the deal there?

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

Depending on your system, you will either need acpi or apm set. In the remainder of this guide, we assume that your laptop is recent enough to use ACPI.

So, with the USE flags set, let's install laptop-mode-tools .

root # emerge --ask laptop-mode-tools

Configuration

Having laptop-mode-tools installed on your system does not automatically enable the power management features that you might need. 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 you want to combine laptop-mode-tools with other services like cpufreqd . In this example case, you will need to set CONTROL_CPU_FREQUENCY=0 :

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

CONTROL_CPU_FREQUENCY=0

The next few sections will help you configure laptop-mode-tools to suit your needs. When you are finished, start the laptop_mode service and make sure it is started when you boot up your system.

root # /etc/init.d/laptop_mode start
root #
rc-update add laptop_mode default

How does laptop-mode-tools work

When running the laptop_mode service, the software will check in which state your 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 (so you can 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 to 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_GOVERNOR ) isn't.

CodeColliding 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 let 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 , you can have the laptop mode tools govern the brightness of your 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 - you will need to adjust this to /sys/class/backlight/acpi_video0/brightness instead.

The values you can use are between 0 and 15, 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 willl then signal or reload the appropriate services so they can use the new configuration file.

Using cpufreqd

Installation

The cpufreqd application allows you to manage CPU frequencies in a more granular approach then what laptop-mode-tools supports. But before we dive into the installation of cpufreqd , let's 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 your laptop is not too old (~ 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 your 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 you want to use advanced events through lm_sensors
nforce2 Enable support for NForce, allowing cpufreqd to change the NForce FSB clock and video card frequency you have an NVidia graphical card based on the NForce chipset
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 you have an NVidia graphical card
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. your laptop does not support ACPI or APM

The USE flags acpi , apm and pmu overlap, so you should only have one active. If your laptop is sufficiently recent, acpi is your best bet. If not, apm offers all that is needed. When even APM isn't supported, you can try 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 you use 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.

Let's 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 2Ghz will have by this policy a minimum frequency of 800Mhz).

As you can see, the cpufreqd application can offer a more granular approach on CPU frequency scaling. But not only that, you can tweak the CPU frequency scaling based on various other metrics available. The default configuration offers a sample rule for when you watch a movie, where you want maximum performance, unless the CPU temperature is getting too high.

When you have configured cpufreqd , 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

Resources

Tools

Articles and Guides

  • 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).

Acknowledgements

We would like to thank the following authors and editors for their contributions to this guide:


  • swift