Sunday, December 7, 2008

Laptop power consumption

I recently got a Lenovo T500 based on the new Intel 4-series chipset. It's pretty solidly built, except that the display has a dead pixel. My first action was to get it to dual-boot Ubuntu and Vista - the hardest part was getting Vista not to hog the whole disk. Maybe I'll write on that procedure sometime. Also, the laptop makes a high-pitched noise that, after 4 tech-support services, was finally diagnosed as being due to CPU power management. Apparently this happens on lots of laptops. As the CPU goes into and out of low power states (C4 - C6), it emits the noise. Lenovo's 'solution' was to disable CPU power management in BIOS. I installed powertop, a nifty utility to measure power usage (though not sure if it captures ALL power consumed or just a subset) and did the following measurements. The setup was the laptop running on battery power, with gmail running in firefox. I wasn't interacting with it.

ModePower reported by powertop
CPU power management disabled: LCD maximum brightness19.8W
CPU power management disabled: LCD minimum brightness15.2W
CPU power management enabled: LCD maximum brightness18.7W
CPU power management enabled: LCD minimum brightness14.0W
CPU power management enabled: LCD minimum brightness, RF Kill switch on13.0W

I also tried a bunch of the optimizations suggested at thinkwiki, such as disabling pcmcia and usb 1.1, and reducing disk writeback, but it didn't seem to make a difference. Of course, that could be because powertop power reporting does not include those power drains in its numbers.

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