Power Management Enhancements in Windows 7

Windows XP delivered improvements in the area of power management, such as support for hibernation and standby and the ability of Windows to automatically power down monitors and hard drives to reduce energy consumption. Windows Vista built upon these advances with a number of improved power management capabilities, including:

• Improved support for ACPI 2.0. In addition, Windows Vista also provides support for selected features of ACPI 3.0.
• Standard (non-ACPI) HALs are no longer supported in Windows Vista; only the ACPI Programmable Interrupt Controller (PIC) HAL and ACPI Advanced Programmable Interrupt Controller (APIC) HAL are supported in Windows Vista.
• Simplified power plan model, including three default power plans:
  • Balanced Automatically balances system performance with energy consumption- for example, by speeding up the processor when performing CPU-intensive activities, such as playing a 3D game, and slowing down the processor when performing activities that require little CPU, such as editing a document in Microsoft Office Word. Balanced is the default power plan in Windows Vista.
  • Power Saver Saves power at the expense of maximum performance. On mobile systems, this helps to maximize battery life. On desktop and server platforms, it works to reduce energy consumption.
  • High Performance Maximizes system performance at the expense of power savings.
• Mobile users can easily switch between power plans using the enhanced battery meter in the notification area or the battery tile in Windows Mobility Center. Desktop and server users can switch power plans in Power Options in Control Panel.
• OEMs can customize the default power plans or create their own and install them as the system default. In addition, users can easily create their own custom power plans and manage them.
• A new Sleep mode called Hybrid Sleep is available. It combines the benefits of standby and hibernation. When the system transitions to Hybrid Sleep, a hibernation file is generated, and the system transitions to sleep (ACPI S3 state). Sleep and resume time are improved because in most cases, the system is resuming from memory (ACPI S3). In the event of a power failure, system state is still preserved and the computer will resume from the hibernation file.
• Sleep reliability improvements. Windows Vista does not query applications or services before transitioning to any of the Sleep states. This is a departure from how Windows XP behaves. Applications may no longer prevent the system from going to sleep when a user clicks the sleep button or closes the lid on a mobile PC. This helps prevent the system from accidentally remaining powered on when in a laptop bag or other closed environment.
• Resume performance improvements. Waking from Sleep mode is much faster, and improved power transition diagnostics help ensure consistent and predictable power transitions.
• Support for managing power settings using Group Policy. This allows businesses to easily configure the policies for powering off monitors and computers after a period of inactivity, saving money in utility expenses.
• Extensible power settings. Third-party drivers and applications can add new power settings to the system, and custom power settings can be managed in the same manner as system power settings.

Windows 7 builds upon the foundation of Windows Vista with additional improvements in this area. For instance, numerous changes were made in Windows 7 to reduce power usage while the system is idle, which is the key issue for ensuring maximum battery life for mobile computers because periodic background activity can significantly increase the power consumption of a system. Very frequent events can greatly affect processor and chipset power usage, and long-running infrequent events can prevent the system from idling to sleep to conserve battery power. The changes made in Windows 7 to reduce idle activity and extend idle periods include:

• Windows 7 is now more aggressive about placing the system in Sleep mode when idle by responding to user input and application availability requests only.
• You can now configure how much idle time must elapse before Windows automatically transitions the computer to sleep. This energy efficiency feature is also configurable by Group Policy; see the section titled "Configuring Power Management Settings Using Group Policy" later in this tutorial for more information.
• You can now configure Windows so that only user input and not application and driver activity can prevent Windows from automatically transitioning to Sleep mode. This energy efficiency feature is also configurable by Group Policy; see the section titled "Configuring Power Management Settings Using Group Policy" later in this tutorial for more information.
• A new /requests option for the Powercfg.exe command now lets you enumerate application and driver requests that prevent the computer from automatically turning off the display or entering Sleep mode.
• A new /requestsoverride option for the Powercfg.exe command now lets you override either individual availability requests or all availability requests.
• A new feature called Intelligent Timer Tick Distribution (ITTD) enables timer interrupts to be handled by a single processor on a multiprocessor system so that cores and processors can stay in Sleep states longer.
• A new feature called Timer Coalescing expires multiple distinct software timers at the same time to increase the average processor idle period.
• Open files in the client-side cache (offline files) will no longer prevent the system from sleeping. This energy efficiency feature is also configurable by Group Policy; see the section titled "Configuring Power Management Settings Using Group Policy" later in this tutorial for more information.
• The Transmission Control Protocol (TCP) Distributed Program Call (DPC) timer is eliminated on every system timer interrupt.
• The frequency of USB driver maintenance timers is reduced.

Windows 7 also includes a number of new power policies that administrators can use to manage power on client computers. The policies for the Default (Balanced) power plan are summarized in Table below.