Pat Stemen, a program manager on Windows Kernel team, in a November 8 blog post, discuss of developing an OS to reduce power consumption.
Stemen notes, "We've seen an ever-increasing emphasis on power management in the OS from two perspectives. First, as Windows 8 comes to market, it is easy to see two-thirds of all PCs shipping as portable devices operating on batteries some or most of the time. And second, in the workplace, there is an increasing demand for desktop machines with a reduced carbon footprint as we look to save energy wherever we can."
PCs attain their longest battery life when all devices, including the processor, storage, and peripheral devices enter low-power modes. Almost every device in the modern PC has some kind of power management technology, and runtime device power management determines how we use those technologies seamlessly without impact to the user experience.
"Efficient power management of devices is performed by the driver for the device, in conjunction with the Windows kernel power manager and platform firmware. The power manager makes it easy for the drivers of these devices to implement their power management routines and coordinate any power state transitions with other devices on the platform," said Stemen.
He said, "For Windows 8, we've built a new device power framework that allows all devices to advertise their power management capabilities and integrate them with a special driver called the Power Engine Plug-in or PEP, designed for SoC systems. This allows device drivers like our USB host controller or a keyboard driver to be built once, and still deliver optimal power management on all platforms from SoC-based PCs to datacenter servers."
Stemen, notes that Windows 8 software can singlehandedly influence power consumption, by using resources including the processor, the hard disks, RAM, etc.
"The Metro style application model is designed from the beginning to be power-friendly. The power management benefit is that the model makes it easy for developers to ensure their application is running only at the right time--applications in the background are suspended such that they do not consume resources and power when not in use," Stemen said.
Windows 8 Metro apps are suspended when not in use, which means that they're not using any hardware resources, unless the user needs them to run. As such, the Redmond company managed to reduce the power cost associated with them as low as possible, when the Metro applications are not active.
"When the screen is on, we've already moved the bar significantly from a maximum idle duration of 15.6ms in Windows 7 to 35% of our durations longer than 100ms in Windows 8! With the screen off and during Connected Standby, our idle durations are even longer, currently in the tens of seconds," Stemen explained.
Task Manager showing suspended Metro style apps
DC power supplies with built-in measurement capability
Reference platform instrumented for DC power supply and measurement