Milestone improvements to power management in Windows 7

Brad Bird takes a closer look at the power management features in Windows 7 and explains why many early users are seeing big improvements, particularly with laptop battery life.

One of the most talked about features in Microsoft's new OS is power management improvement. While desktop users who want to be green will appreciate Windows 7 power management features, the main attraction will be for notebook users who use their computers while on battery.

Mobile users in the Windows 7 forums have reported superior battery life after a simple operating system upgrade. So what settings get managed exactly?

A main power consumer on any laptop computer is the screen. Lowering the default brightness and dimming your display while the system is idle are common ways to increase your battery life, along with timers to turn off the display or put the computer to sleep.

Figure A

Click to enlarge.
Basic power management settings
What distinguishes Windows 7 power management, however, is the collections of settings that are available for creating a power management plan. Windows 7 permits much more granular control over individual devices in the new power management plans. Figures B and C show you the advanced settings that can be customized in power management.

Figure B

Figure C

In addition to the available settings, improvements to the Windows 7 operating system itself make the processor work less and consume less power; for example, there are fewer background tasks running. Also, reduced power consumption in DVD playback is also handy if the DVD-ROM is used to watch movies.

Other architecture-level improvements which improve power management are:

  • Elimination of TCP DPC timer on every system timer interrupt Deferred Procedure Call or DPC timers are system timers designed to schedule lower priority tasks to make use of the processor at later scheduled times. These may prevent the system from entering into a low power state. The specific improvement sighted here is this use on the TCP or network stack.
  • Reducing the frequency of USB driver maintenance timers This is similar to DPC in that these timers request a status or create I/O activity to the USB ports, also creating a state where the system cannot bring itself into low power state.
  • Intelligent Timer Tick Distribution (ITTD) This feature extends CPU sleep times by not waking the CPU unnecessarily.
  • Timer Coalescing This is system control on application processes whereby they are grouped to be performed in quick succession to allow more time between requests, effectively increasing system idle time.

The primary focus of these features is on allowing the operating system to bring the system into a low power state sooner and more often.

Personally, I can leave my laptop on battery for almost an entire day now if the power management kicks in. Or, if I leave my laptop on the kitchen table overnight and then come to use it in the morning, when it wakes, I have plenty of power to spare before plugging it in. With Vista, I needed to leave my laptop plugged in overnight.

Have you noticed longer battery life with Windows 7? Share your experiences.