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Observations on Power Management

Nov 25, 2008, 18:47 (1 Talkback[s])
(Other stories by Matthew Garrett)

"The hardware used to display a static image on the screen is the same regardless of whether the image was generated with the graphics card's 2D or 3D hardware. Regardless of the number of graphical effects used on the desktop, the common case is for the desktop to be static. Composited and traditional desktops will generally consume the same amount of power. Summary: Don't offer the choice of disabling compositing when on battery. It reduces functionality for no power benefit.

"A black screen takes up as much power on a TFT as a white one. A screensaver that draws a black screen saves no power and does nothing to protect the screen. Summary: If the user has not requested an animated screensaver, turn the screen off immediately rather than drawing a black screen.

"Displaying an image on screen requires copying that image out of graphics memory. Each memory access takes a certain amount of energy. Reducing the amount of memory to be accessed is a good thing from a power management perspective. Some modern graphics chipsets (including just about any recent one from Intel) support a mode where the contents of the screen are compressed in memory. The compressed copy can then be read, saving energy. The mechanism for this is generally a trivial run length encoding of the screen contents on a line by line basis. Continuous horizontal blocks of colour will compress well, while horizontal gradients will not. Summary: Try to design desktop backgrounds with vertical gradients rather than horizontal ones.

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