article on Linux kernel changes for power savings

[Tom Metro]

This topic has come up on this list a few time in the past few months. 
This article discusses recent changes in the Linux kernel intended to 
reduce power usage, and also mentions the PowerTOP tool used for 
monitoring CPU power consumption.

  -Tom


http://www.linux-mag.com/id/4037/

   Recently, the Linux kernel has made huge steps forward in power
   conservation. This article focuses on the so-called "tickless idle"
   feature being integrated into the 2.6.21 and 2.6.23 kernel versions.
   ...
   Until 2.6.21, the Linux kernel programmed the PIT chip of the PC (or
   equivalent on other architectures) to generate interrupts at a
   regular interval of either 250 Hz (A four millisecond interval) or
   1000 Hz (a one millisecond interval).
   ...
   This timer tick approach has a certain elegance in its simplicity
   and has served Linux well since the early 1990s. However, in the
   light of the C-state hardware capabilities, a regular one
   millisecond or four millisecond interrupt has the effect of waking
   the CPU frequently from the deep sleep states (or even preventing
   the CPU from ever entering the deepest sleep states), which
   obviously makes the system consume more power than necessary.
   ...
   Enter the tickless idle feature. In the last year or so, Thomas
   Gleixner and others have worked on a feature that’s called "tickless
   idle" or just "tickless." It is the removal of the regular timer
   tick when the system is idle. This feature has become part of the
   2.6.21 kernel for the i386 architecture, and 2.6.23 will likely have
   the x86_64 version as well.
   ...
   By not having a regular timer tick when the processor is idle, it is
   theoretically possible to have really long periods of idle as long
   as there are no future timers planned. Unfortunately, in practice in
   a current Linux distribution, both the kernel and userspace
   applications set so many timers that it is not uncommon to have 500
   or more of such events per second. The system can go to a one
   millisecond sleep time to an approximately two millisecond sleep
   time. This would obviously provide only minimal power gain over the
   kernel with the regular tick.
   ...


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