January 14, 2008, 2:41 PM — With rising energy costs and growing awareness of the environmental costs of
power production, companies of all sizes are increasingly interested in implementing
power management and today's PCs make it easier than ever to get work done in
a more energy-efficient manner. So why isn't everyone implementing power management?
Because understanding the available settings and standards is only half the
battle.
Organizations must also consider the role of power management in the organization
and how to implement it without inconveniencing end users. If a PC causes a
software conflict or other crash when it enters or exits a low-power state,
users and executives alike are apt to call for disabling power management altogether
rather than risk data or productivity losses. Likewise, if the cost of energy-efficient
PCs is greater than the money saved from increased efficiency, few enterprises
will buy them.
How power management works
The goal of power management is to use power efficiently -- to get the most work
done per unit of energy. Part of this depends on how much power the machine
uses to do a task. If one machine requires 100 watts to do one unit of work
and another machine uses 50 watts to do the same work, the second machine will,
all things being equal, use less power.
But, power management in the real world is more complicated. Computers don't
just go on and off -- instead, they (and their individual components) have various
levels of activity and idleness. Those levels, or "states," range
from unplugged (completely off) to "asleep" to actively working. To
be highly efficient, a computer should be able to switch as quickly as possible
between the lowest practical power setting and higher, more active settings.
And, it must make that switch without impairing the ability of the machine to
perform useful work. Power management that disrupts functionality is not useful.
Generally speaking, lower-power states have greater wake latency (they take
longer to "wake up") than higher-power states (the highest power state
being "awake and working"). Industry standards go to great lengths
to define specific power states for systems and system components. These are
mostly of use to manufacturers (for more information, read Advanced
Configuration and Power Interface Specifications).
From a practical standpoint, a few system states are typically readily available
to end users:
- Idle (S0): System is awake but no active work is being done. The default
"on" state for a computer.
- Standby (S3): A sleep state (the computer appears to be off) with low wake
latency (the computer "wakes" quickly at a keystroke or other defined
event). All system context is preserved by the hardware, with the exception
of CPU caches. Some devices may still be "on."
- Hibernate (S4): A sleep state with a higher wake latency in which all devices
are powered off and platform context is saved to the hard drive.
- Soft off (S5): A state in which no platform context is saved but some power
may still be applied to the system. Requires a full system boot to wake up.
Using Windows XP as an example of typical power management settings, you can
contrast two different power settings. The power settings can be changed by
going to Power Options in the Control Panel. One case is where the machine is
set as "always on" and has the following settings:
- Power schemes: Minimal Power Management
- Turn off monitor: Never
- Turn off hard disks: Never
- System standby: Never
- System hibernates: Never
In the second case, the machine conforms to ENERGY STAR1 default
requirements. This case uses the following settings:
- Power schemes: Minimal Power Management
- Turn off monitor: After 15 mins
- Turn off hard disks: Never
- System standby After 30 mins
- System hibernates: Never
Additionally, system components including processors, hard drives, add-in cards
(such as PCI Express cards), and displays can all have various levels of idleness
or activity. Again, in most situations, IT professionals will not apply power
management settings to components at a highly granular level. More likely, they
will set a few key variables, like when to turn off monitors after a period
of inactivity and when to turn off hard disks. Overall, while there are detailed
silicon- and code-level power management specifications, implementing power
management features at the user level is fairly straightforward.
