Reality and myth in the quest for green computing

Standby and Suspend

Apart from the use of power-saving modes during normal operation, most computers have the option of going into Standby or Suspend to RAM mode. Standby mode only turns off a few parts of the board (and probably the display and CPU as well), whereas Suspend to RAM actually freezes the system, including running programs, and saves its state into RAM so that work can resume at a later point in time as if nothing had happened.

Standby (ACPI S1 state) was quite inefficient for all three computers: The Eee PC still needed 12.9 of its 15.3 watts (idle) power, the L3800C still used 23.4 compared with 28.4 watts, and the desktop computer continued to consume 60 of 103 watts in the idle state.

Although the desktop computer did not support ACPI S3, both notebooks dropped power to only 3 watts in Suspend to RAM mode. Before entering this mode, both had to unload their sound card drivers before executing

echo 3 >/proc/acpi/sleep

to wake up correctly later. In Suspend to RAM mode, both notebooks lasted up to five times longer on battery, so this is a good choice if you need to relocate while running on battery.

Conclusion

In these tests (and through personal experience running computers), I found that saving power, and thus extending time to run on battery, is less a question of ACPI or cpufreq settings, but rather of avoiding unneeded computation, hard disk access, or desktop effects. The Compiz screensaver, with a spinning desktop cube, reflections, and lighting effects, or openGL screensavers with fog and texture can waste all attempts at saving power, even in the most sophisticated setups. Sleep states (such as Standby) are sometimes not much help either, although techniques like Suspend to Disk (which was not discussed here) and, in general, disconnecting the computer from the electrical network completely, rather than just switching it off, are the best methods of saving power.

Standby power consumption is indeed alarming if you take into account how many electrical devices – and not just computers – use standby mode today. Trying to get an old computer to use less power with software (i.e., underclocking, idle states, or frequency scaling) is almost surely a futile attempt. Sometimes just buying a smaller, less power consuming computer can help more than any promise by vendors of supported power-save modes, which typically do not really save significant power compared with the idle state. The Linux kernel as preconfigured in all modern distributions is already doing its best to avoid unnecessary work by its CPU and peripherals, so eliminating CPU-intensive "background tasks" gets the machine into the idle/power-saving state easier than a fancy setup with a manually created power configuration.

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