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Old 01-09-09, 04:38 PM   #2
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Originally Posted by Daox View Post
I fired up the computer without SpeedStep and took my readings. At idle it was using 210W. So, I enabled SpeedStep and watched the power consumption go down slowly. Unfortunately, this didn't bring it down nearly as far as I had hoped. It would at most dip down to 204W. I was quite disappointed in this as I've read reviews that MUCH more power has been saved with this technology. But, it differs from processor speed to processor speed as to how far the CPU will slow itself down.
I confirm that on an old PentiumIV : slowing down doesn't change the consumption. In fact I saw the consumption depends on the CPU load. Between 0 and 100% CPU load there was around 40W. When the CPU is faster, the load decreases sooner, but...

...but what the slower speed changed is the temperature. Between high and low speeds there was around 20C at the CPU level. This is important as in my living room this means a 2C difference in summer when you come back home => air conditioning saving. For this reason I configure all the linux laptops I manage with the "ondemand" CPU clock manager : the CPU speed is maximal only if the load is big enough.

Next it was the monitor's turn. At 100% brightness which is way too bright, and the montior was drawing 44W. I dialed it down to 30% which looks way better and it dropped the consumption down to 32W. So, I got a 12W savings there.
On laptops this is the secret to make the battery to last longer. During meeting without electric plugs, I set the backlight at the minimum comfortable for my eyes in that room (often 50%) and set the screen to switch off after 2 minutes. Of course just blank the screen, don't launch a CPU hungry screen saver... Now by default I set the backlight at 50%, but increase it if my eyes are needing it.

The processors in notebooks have also a special mode (last laptop processors should have it). By default in C0 they consume at maximum. The lower is the number of interruptions (soft&hard) per second, the deeper the CPU go to sleep. Under Linux I can lower the total number of interruption at such a level the CPU stays in C3 95% of time, but this need the deactivation of some peripherals or functions such as wireless and 3D on some video cards. I also avoid some programs such as firefox with lots of tabs or acrobat reader.

With speedstep management, sleep mode and screen management the consumption of a notebook can be divided by 2.

All in all I'll be saving 18 watts or roughly 6.5%. This savings over 8.5 hours a day for approximately 37 weeks a year nets me a grand total of 39.6 kWh saved a year. Not much, but if the well over 100 computers in this office all did it we could save around 3,960 kWh a year which doesn't sound as shabby.
I changed 2 laptops (run 24x7) by 2 notebooks at home and I saved 140W in continue (+ the consumption of the LCD screen I don't remember) ie 1200kWh/year.

You are lucky when professional computers are switched on only when their users are present... I never saw such a situation.
Lets imagine if modern OS were not loaded with unuseful CPU hungry programs and visual effects. Lets imagine if modern OS were configured to switch the screen off after a while and switch off the computer after a couple hours...

Earth absorbs 1.8 t CO2/head/yr, while a French generates 6.2 t CO2/yr
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  • kg saved 06/08-08/09: 1816.9+382.9 (ecodriving / 1420mi not driven) = 2199.8
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    (2.66 kg/l diesel)
  • kg saved by 3kWc photo-voltaic solar panels : 187 kg/yr
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    (59.1 g/kWh)
Radioactive wastes saved by 3kWc photo-voltaic solar panels :
  • Long life (>100,000 years) : 2.85 g/yr (0.9 mg/kWh)
  • Short life (<300 years) : 31.7 g/yr (10.0 mg/kWh)
Based upon "official" French figures...
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