Differences between revisions 3 and 4
⇤ ← Revision 3 as of 2010-02-21 12:02:41 →
Size: 3867
Editor: GeoffSimmons
Comment: /etc/default/cpufrequtils is no longer installed ("the defaults values are in the script now").
← Revision 4 as of 2011-03-15 22:41:29 → ⇥
Size: 4202
Editor: GeoffSimmons
Comment: Link online documentation directly, amend InterWiki, add CategoryHardware, formatting.
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= CPU frequency scaling =
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== Howto enable Cpu Frequency Scaling == The [[http://kernel.org/pub/linux/utils/kernel/cpufreq/cpufreq.html|Linux kernel CPUfreq subsystem]] provides this ability on Debian Linux systems.
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Note: In most cases, this should be enable automatically, during Debian installation. == Enabling ==
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Install the DebPkg:cpufrequtils package:{{{ Note: In most cases, this should be enabled automatically during Debian installation.

Install the DebianPkg:cpufrequtils package:{{{
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Either add "CPU Frequency Scaling Monitor" to your Gnome panel, or run the command {{{cpufreq-info}}}, which prints one bloc like this per core:
{{{
Either add "CPU Frequency Scaling Monitor" to your GNOME panel, or run the command {{{cpufreq-info}}}, which prints one block like this per core: {{{
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There are two important information:
There are two important pieces of information:
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 * The statement {{{current CPU frequency is XXXX MHz}}}, If XXXX is lower than maximum speed, it probably means that it's working.  * The statement {{{current CPU frequency is XXXX MHz}}}; if XXXX is lower than maximum speed, it probably means that it's working.
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The [cpufreq] governor decides what frequency should be used (it uses the cpufreq driver to actually switch the CPU's policy).
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The governor decides what frequency should be used (it uses the CPUFreq driver to actually switch the CPU's policy). As explained above, since Lenny, the CPUFreq modules should be loaded using DebianPkg:cpufrequtils.
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As explained above, since Lenny, the cpufreq modules should be loaded using DebPkg:cpufrequtils. In most cases, {{{ondemand}}} is the recommended governor. CPUFreq governors are actually modules (even though governors are modules, you should use {{{cpufreq-info}}} to know if the governors are loaded, and which one is active).
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'''Userspace Governors''' For more information about governors, read governors.txt (available in {{{/usr/share/doc/linux-doc-2.6.32/Documentation/cpu-freq/}}} from the package [[DebPkg:linux-doc-2.6]], you can read the latest version from [[http://kernel.org/doc/Documentation/cpu-freq/governors.txt|kernel.org]]).
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In most cases, {{{ondemand}}} is the recommended governor. Cpu-freq governors are actually modules (Even though governor are modules, you should use {{{cpufreq-info}}} to know if the governors are loaded, and which one are active). ==== Userland governors ====
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'''For more information about governors''' The userland-based governors (DebianPkg:cpufreqd, DebianPkg:cpudyn, DebianPkg:powersaved, DebianPkg:powernowd ...) are usually not needed any more.
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read governors.txt (available in {{{/usr/share/doc/linux-doc-2.6.26/Documentation/cpu-freq/}}} from the package [[DebPkg:linux-doc-2.6]], you can read the latest version from [[http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/cpu-freq/governors.txt;hb=HEAD|kernel.org]]). === Drivers ===
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==== Userspace Governors ====
The userland based governors (DebPkg:cpufreqd, DebPkg:cpudyn, DebPkg:powersaved, DebPkg:powernowd ...) are usually not needed any more.		 As explained above, the governor defines the frequency scaling policy, but it doesn't instruct the CPU to change the speed directly... it needs a driver for that. DebianPkg:cpufrequtils has an init script that should load the appropriate driver at boot time.
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=== cpufreq drivers ===
As explained above, the governor defines the frequency scaling policy, but it doesn't instructs the CPU to change the speed directly... It needs a driver for that.

DebPkg:cpufrequtils has an init script that should load the appropriate driver at boot time.

The list of cpu-freq drivers available on your system can be obtained by running:		 The list of CPUFreq drivers available on your system can be obtained by running:
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modinfo /lib/modules/$(uname -r)/kernel/arch/*/kernel/cpu/cpufreq/* | grep "^[fd]" /sbin/modinfo /lib/modules/$(uname -r)/kernel/arch/*/kernel/cpu/cpufreq/* | grep "^[fd]"
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For more information about governors, read cpu-drivers.txt (available in {{{/usr/share/doc/linux-doc-2.6.26/Documentation/cpu-freq/}}} from the package [[DebPkg:linux-doc-2.6]], you can read the latest version from [[http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/cpu-freq/cpu-drivers.txt;hb=HEAD|kernel.org]]). For more information about governors, read cpu-drivers.txt (available in {{{/usr/share/doc/linux-doc-2.6.32/Documentation/cpu-freq/}}} from the package [[DebianPkg:linux-doc-2.6]], you can read the latest version from [[http://kernel.org/doc/Documentation/cpu-freq/cpu-drivers.txt|kernel.org]]).
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=== See Also ===
 * The documentation in {{{usr/share/doc/linux-doc-2.6.26/Documentation/cpu-freq/}}}
 * DebPkg:cpufrequtils's documentation (esp. [[DebianMan:1/cpufreq-info|cpufreq-info(1)]] and [[DebianMan:1/cpufreq-set|cpufreq-set(1)]])
 * DebPkg:powertop is a tool to check what prevents the kernel from going to sleep mode (also read also [[http://www.lesswatts.org/projects/powertop/]]).		 == See Also ==

 * The documentation in {{{/usr/share/doc/linux-doc-2.6.32/Documentation/cpu-freq/}}}
 * DebianPkg:cpufrequtils's documentation (esp. [[DebianMan:1/cpufreq-info|cpufreq-info(1)]] and [[DebianMan:1/cpufreq-set|cpufreq-set(1)]])
 * DebianPkg:powertop is a tool to check what prevents the kernel from going to sleep mode (also read http://www.lesswatts.org/projects/powertop).

== External Links ==

 * http://kernel.org/pub/linux/utils/kernel/cpufreq/cpufreq.html
 * http://kernel.org/doc/Documentation/cpu-freq/governors.txt
 * http://kernel.org/doc/Documentation/cpu-freq/cpu-drivers.txt

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CategoryHardware

Translation(s): none

CPU frequency scaling

Dynamic CPU frequency scaling (also known as CPU throttling) is a technique in computer architecture where a processor is run at a less-than-maximum frequency in order to conserve power (src: Wikipedia).

The Linux kernel CPUfreq subsystem provides this ability on Debian Linux systems.

Enabling

Note: In most cases, this should be enabled automatically during Debian installation.

Install the cpufrequtils package:

aptitude install cpufrequtils

Troubleshooting

Check how CPU is configured

Either add "CPU Frequency Scaling Monitor" to your GNOME panel, or run the command cpufreq-info, which prints one block like this per core: 

analyzing CPU 0:
  driver: acpi-cpufreq
  CPUs which need to switch frequency at the same time: 0
  hardware limits: 1000 MHz - 1.83 GHz
  available frequency steps: 1.83 GHz, 1.33 GHz, 1000 MHz
  available cpufreq governors: userspace, powersave, conservative, ondemand, performance
  current policy: frequency should be within 1000 MHz and 1.83 GHz.
                  The governor "ondemand" may decide which speed to use
                  within this range.
  current CPU frequency is 1000 MHz (asserted by call to hardware).
  cpufreq stats: 1.83 GHz:10.34%, 1.33 GHz:0.62%, 1000 MHz:89.04%  (1068280)

There are two important pieces of information:

  • The statement The governor "ondemand", which means that this governor is loaded and enabled.

  • The statement current CPU frequency is XXXX MHz; if XXXX is lower than maximum speed, it probably means that it's working.

Governor ??

The governor decides what frequency should be used (it uses the CPUFreq driver to actually switch the CPU's policy). As explained above, since Lenny, the CPUFreq modules should be loaded using cpufrequtils.

In most cases, ondemand is the recommended governor. CPUFreq governors are actually modules (even though governors are modules, you should use cpufreq-info to know if the governors are loaded, and which one is active).

For more information about governors, read governors.txt (available in /usr/share/doc/linux-doc-2.6.32/Documentation/cpu-freq/ from the package linux-doc-2.6, you can read the latest version from kernel.org).

Userland governors

The userland-based governors (cpufreqd, cpudyn, powersaved, powernowd ...) are usually not needed any more.

Drivers

As explained above, the governor defines the frequency scaling policy, but it doesn't instruct the CPU to change the speed directly... it needs a driver for that. cpufrequtils has an init script that should load the appropriate driver at boot time.

The list of CPUFreq drivers available on your system can be obtained by running: 

/sbin/modinfo /lib/modules/$(uname -r)/kernel/arch/*/kernel/cpu/cpufreq/* | grep "^[fd]"

For more information about governors, read cpu-drivers.txt (available in /usr/share/doc/linux-doc-2.6.32/Documentation/cpu-freq/ from the package linux-doc-2.6, you can read the latest version from kernel.org).

See Also

CategoryHardware