23 Nov 2008 mjg59   » (Master)

Making sure we do power management the right way

I saw a posting about PowerDevil, the new KDE power management interface. It's somewhat disheartening - for the most part, it falls into the same traditional errors made in power management (ie, letting you change cpufreq settings, using "presentation" as a power management setting rather than getting applications to actually do the right thing). But my point isn't to bitch about PowerDevil. My concern is more about why we're still failing to get the message across about certain power management myths. Implementing power management incorrectly leads to wasted power, dead polar bears and wet carpets. It's important that we get this right.

  • To a first approximation, the Powersave governor will only save you power if you're playing 3D games. The performance governor will basically never give you extra performance. Don't use them. Use ondemand instead. Do not make it easy for your users to choose them. They will get it wrong, because it is difficult to explain why this result is true.
  • Thermal management is not the job of a power manager. Using power management to implement thermal management will result in your computer taking up more power overall, reducing battery life. Implement things in the right places.
  • When people say "Presentation mode", what they mean is "Disable screensaver and automatic suspend to RAM". This is not a power management policy. It is an application behaviour policy. The sensible behaviour is for applications to request that these things be disabled when they switch to full-screen presentation mode. The foolish behaviour is to request that your users select "Presentation mode" in their desktop and then start their presentation in their application and then finish their application and then remember to disable "Presentation mode".
  • It takes no more energy to scan out a framebuffer drawn with the 3D engine than with the 2D engine. If you're spending a significant proportion of the time on the GPU when in your desktop environment, you're already doing something wrong.
One of the problems is probably that most desktop programmers don't know how hardware behaves. How can we change that?

Syndicated 2008-11-23 14:45:30 from Matthew Garrett

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