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- Power management question
- Clayton Sutton
September 25, 2006, 4:57 pm
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I once read in Windows IT Pro. about a way to turn on/off CPU power
management at the command line, now I can't find that info. and I need it.
Anyone know what I am talking about? I also read about an Intel utility
called "Intel Processor Identification Utility" that will let you test what
speed your CPU was running at.
Bottom line is I want to be able to keep my laptop CPU running at 3.8Ghz
even on battery. And I want my monitor to run at 100% brightness even on
I also want to be able to throttle back my servers after hours (by using a
script) to save on the power bill
Can anyone point me to the command line?
- Clayton Sutton
September 25, 2006, 10:16 pm
Re: Power management question - I found it!
From the January 2006 Edition
of Windows IT Pro
December 27, 2005
Windows Power Tools
Windows IT Pro
I've written extensively about Windows XP Service Pack 2's (SP2's) useful
new features, but one feature that I didn't notice until recently is
Powercfg, a command-line tool that lets you control your system's
power-management settings. Powercfg has roughly the same functionality as
Control Panel's Power Options applet, but-as is often the case with
command-line tools-it offers some functionality that you don't get from
Control Panel, letting you take power management a bit further. (Powercfg
also ships with Windows Server 2003 in its original release, and a
less-powerful version appeared in XP SP1.)
Powercfg has many options, so instead of trying to cover every aspect of the
tool in this short article, I'll instead offer a couple of cool Command-Line
Interface (CLI)-only suggestions: how to control whether Windows dims your
laptop's LCD panel and how Windows reduces your CPU's speed to save power.
Laptops have handily outsold desktops for the past 2 years. One of the ways
laptops are distinct from desktops is that power management is necessarily
central to their design. Although power management saves energy and
accommodates portability, its effects can be perplexing. Recently, a friend
asked me why his new laptop would dim its screen at seemingly random
moments. He wondered how he could make it stop. Laptop screens can be hard
enough to read at full brightness.
Many laptops have function keys that let you control brightness, but none
let you configure the screen to remain bright all the time. And Control
Panel offers no help on the subject. The solution is to simply open a
command prompt and type
powercfg /g off /option videodim
The /g switch is a global option that accepts four parameters in addition to
videodim. One, wakeonring, is another setting you won't find in Control
Panel; as its name suggests, wakeonring enables or disables Windows' ability
to wake your computer from standby or hibernation when some piece of
hardware is affected by some outside signal (e.g., its modem sensing an
incoming call). You can adjust the three other global settings- batteryicon
(whether to show the battery icon on the task bar), multibattery (whether to
show more than one battery icon, assuming you have more than one battery),
and resumepassword (whether to require a password when resuming from standby
or hibernation)-from Control Panel.
Have you ever run a quick CPU speed benchmark, only to find that your 1.5GHz
mobile Pentium is running at 700MHz? If so, you're seeing the effects of
power management. Windows ships with several prebuilt power-management
schemes. These schemes are simply collections of settings for when to turn
off your monitor and hard disk, when to put the system in standby mode, when
to hibernate the system, and so on. Again, a glance at Control Panel reveals
these. But what Control Panel doesn't show you is a fifth setting-the
processor throttle setting-that lets Windows slow down your processor to
save power. The four processor throttle parameters are none, constant,
degrade, and adaptive.
None. The none parameter tells the computer not to slow the processor at
all, even if it's just burning up cycles and battery power doing nothing.
Constant. The constant option tells the processor to run at the slowest
speed possible. That speed can be slow indeed: In the case of the system I'm
currently using, the 1.7GHz processor dropped to 600MHz in constant mode.
This slowest clock rate-or in power-management-speak, lowest performance
state-seems to be defined by the processor's manufacturer.
Degrade. Taking constant a step further, the degrade parameter runs the
system at the slowest acceptable clock rate (the same as in constant mode),
and to slow things down even more and save more power, employs a feature
called stop clock throttling (or linear performance reduction). The stop
clock throttling diagrams I've seen suggest that it stops the processor's
clock signal altogether, for short periods of time. As you might know, a
processor observes a minimum clock rate-in the case of my mobile Pentium,
600MHz. Running the processor at a slower speed reduces its power usage by
even more, but that means running it outside the manufacturer's acceptable
range of clock speeds. However, by stopping the clock regularly, rather than
just slowing it down, the processor still thinks it's getting its minimally
acceptable input clock speed, even though it's actually running more slowly
than that minimum clock speed. The logical result is even lower power
Adaptive. The adaptive option senses how much CPU power the system needs,
then runs the CPU as slowly as possible to run whatever processes the system
is running. The adaptive parameter seems to be the most flexible
processor-throttling setting, but exactly how your processor uses the
setting will vary with your processor type. Each chip has its own
So, the four processor-speed values are-in order of decreasing speed and
increasing battery life-none, adaptive, constant, and degrade. To control
your processor's power-management values, use the command
where name of power scheme is portable/ laptop, max battery, home/office
desk, or whatever-power-management profile you're using, and setting is
none, adaptive, constant, or degrade. That command configures the power
setting when your system is plugged into the wall; you would change
processor-throttle-ac to processor-throttle-dc to configure the power
setting for when you're running on battery. For example, to tell my system's
home/office desk power-management profile to use adaptive CPU throttling,
I'd type the following two lines:
powercfg /x "home/office desk"
powercfg /x "home/office desk"
If you have a Pentium processor and want to experiment with your power
settings' effect on your system's actual clock speed, check out Intel's
Processor Identification Utility
(http://www.intel.com/support/processors/tools/piu ). This tool tells you how
fast the chip should run and how fast it's actually running. If you have an
AMD processor, check AMD's Web site for several CPU-family-specific
utilities that show clock speeds. AMD's helpful PowerNow! Dashboard program
1276_964,00.html) shows you the speed of your laptop's processor at any
My second laptop is Turion-based, and when the power is set to adaptive
mode, I'm amazed by how the CPU speed changes from second to second. Perhaps
the most interesting part is that, were it not for the AMD utility, I would
never have known that the processor's speed fluctuated so often. In other
words, it's remarkable how often we still run on sub-GHz systems and aren't
inconvenienced at all.
Wait, There's More
There's a lot more to Powercfg, but I was just happy that it let me shut off
LCD screen dimming and helped me more fully understand and control how my
system can slow down to save power. My next project will be to build a batch
file that I can use to set a system's power settings from the command
line-functionality that will save me a lot of trouble when it comes time to
build a new server or workstation.
Mark Minasi (http://www.minasi.com/gethelp ) is a senior contributing editor
for Windows IT Pro, an MCSE, and the author of Mastering Windows Server 2003
(Sybex). He writes and speaks around the world about Windows networking.