Careful System Hygiene is the Next Step in Power Reduction



Intel and Microsoft are working closely to target those third- and fourth-tier "gotchas" that add up to big power abusers in battery-operated devices.

With the move to 8-hour battery life in Ultrabooks and Windows 8 convertible "hybrid" thin laptops and tablets (Figure 1), "Intel and Microsoft are working closely examining power wasters in every aspect of the machine environment: software applications, CPU, PCI Express bus, peripherals, disk, memory, display, drivers, and even the Windows OS itself."

And with the laptop under attack by smartphones and tablets, it’s essential for Intel and Microsoft to get it right. Indications are they’re making great headway.

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Figure 1: Windows 8 tiles on the main interface: just don’t call it “Metro”. [Courtesy: Microsoft]

Balancing Hardware and Software Power
As presented during the Intel Developers Forum (IDF) 2012, "Optimizing Battery Life on Intel Architecture Based PCs with Windows 8"¹, Figure 1 shows the balance between hardware and software factors (Figure 2). Hardware factors dictate the extent of power consumption. The latest Tri-gate transistor 22 nm Intel Ivy Bridge Core CPUs are extremely power efficient, yet they still consume some power in all of their "on" modes, from deep sleep and standby, all the way up to idle, active and Turbo Boost modes.

On the other hand, software factors dictate when the system is active or idle. None of this is new information. The reality is that it’s the software that determines when the system enters a lower power mode and how long it can stay there. If the software is constantly waking up the CPU with a "keep alive" timer, then the deepest sleep, lowest power consumption may never be achieved. Both hardware and software must work in conjunction to reduce energy use. This is not exactly a revelation, but it’s a critical goal with next-generation all-day-battery laptops.

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Figure 2: IDF 2012 showed that it’s not just hardware that consumes energy in laptops, Ultrabooks and next-generation hybrid computers. Software plays a huge role. (Courtesy: Intel.)

Intel and Microsoft have dubbed their fine-grained power conservation efforts "Hygiene Improvements", be they in hardware or up the stack through the OS and into applications. The goal is to reduce activity duration or frequency so the CPU and other components can take advantage of the power management features that have been implemented over the past several years. Software is all too often obviating these features by preventing ICs from entering that low power state.

Timers Really Tick Me Off
While the sexy part of Windows 8 adds touch screen support and the not-really-called-Metro tiled interface, it’s what’s under the low power hood that really warrants mention. The extent of Intel’s and Microsoft’s third- and fourth-order incremental efforts are impressive, starting with the periodic system timer tick. With Windows 8, the 15.6ms periodic system timer tick was removed, now allowing the hardware to get idle and stay there for much longer periods of time. Starting in early versions of Windows, when machines were plugged in 24/7, it was acceptable to awaken the CPU every 15.6ms just to say "hi". No more.

Intel and Microsoft also looked holistically at every timer and work event in the complete system, from bus interface chips and UARTs all the way to MCUs used for battery charging. Component by component, unnecessary timers were removed (where possible) and for those that were essential, periods were extended and/or coalesced whenever possible. The result: the CPU gets awakened less frequently.

When the CPU is revived, it’s used for actual work for applications running on-screen. Minimized apps were de-prioritized and given less access to the CPU, and periodic maintenance were moved under OS control to maximize all-at-once performance. This series of many little tweaks and improvements means the CPU and its peripherals stays in a low power mode longer to save battery life (Table 1).

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Table 1: “Hygiene” recommendations for third- and fourth-order power savings in next-gen UltraBooks running Windows 8. (Courtesy: Intel.)

Tommy Can You Hear Me? Maybe Not Right Now.
The companies also took a deep look at disk and network use, removing periodic network stack timers and putting Wi-Fi D0 into a coalescent state. This reduced non-critical packet interrupts, saving cycles and power on everything from the PHY through the PCIe and memory busses, including the Core CPU.

Microsoft’s Windows Notification Service (WNS) is now available for use by all applications, alleviating the need for applications to develop and include their own. Built into Windows 8, WNS is now aware of what applications need access to the display, for example, and can both wake up the screen and display subsystem accordingly while batching notification pop-ups. It’s a series of simple things, but waking the entire machine – and all the sub-systems – for low-priority messages like an email arrival isn’t the best use of precious battery power in all cases.

Win8 can also coalesce disk I/O. This not only puts disks to sleep more frequently and quickly (HDD or SDD) to reduce power, but also for longer periods as data is batched to the disk at specific intervals when possible.

New Optimizing Tools
In order to identify small ways to reduce power that add up to big gains, Intel is making available several new tools for system developers. Power Optimizer is a "whole platform" power management framework, part of which relies on the new and very interesting Battery Life Analyzer (BLA). Using Power Optimizer, Intel predicts dramatic "lid closed" power savings of greater than 20 percent by 2013 and by 20x over the next several years – savings that require intelligent software decisions in order to achieve what the hardware is capable of (Figure 3). The system-level savings affect not only Intel CPUs, but the chipset PCH and third-party devices designed to work with the framework.

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Figure 3: Intel’s Power Optimizer power management framework works with Intel Core CPUs, Platform Controller Hub chipsets, and third-party compliant devices. Collectively, massive power savings are possible beyond what the hardware alone can provide. (Courtesy: Intel.)

The way Power Optimizer works uses a device’s latency tolerance for interrupt, snoop and non-snoop DMA activities. The software takes all devices’ requirements into account and "plumbs" the system latency tolerance from the OS all the way through the device endpoints and recommends system-level decisions such as timers and warm-up in order to maximize a device’s low power time. Additionally, those pesky timers discussed above are adjusted to minimize tick events and maximize the time period to the best, next essential timer tick.

An example of several UltraBook device categories is shown in Figure 4, with the resulting aligned activities under Windows 8 shown in Figure 5. The difference in Power Off between the current generation Windows 7 and Microsoft’s Windows 8 is dramatic, resulting in substantial power savings (per Figure 3).

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Figure 4: Power Optimizer for an UltraBook aligns devices’ latency tolerance top-to-bottom while instrumenting timer events to keep devices off for as long as possible. (Courtesy: Intel.)
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Figure 5: Using Power Optimizer as shown in Figure 4, multiple software activities are aligned which allows devices to stay in low power states as long as possible. For this UltraBook, overall system savings can be up to 20 percent more than by hardware options alone. Note the removal of the traditional 15.6ms Windows timer tick. (Courtesy: Intel.)

Power-Ful Future
Intel and Microsoft haven’t worked this closely in years, but the stakes couldn’t be higher. As OSes – like Android and iOS running on battery-operated devices like smartphones and tablets – eat into Intel’s and Microsoft’s core markets, both companies recognize the need to convert current users to tomorrow’s all-day laptops like Ultrabooks or convertible hybrid tablets, clamshells and slates.

Besides what’s been mentioned above, the two companies have more tricks up their sleeves further lowering power in the still popular laptop form-factor. These include: Panel Self Refresh; Latency Tolerance Reporting (an extension of Power Optimizer); new low-power interfaces for I2C, SDIO, I2S and UART; and Link Power Management states for the existing interfaces USB, SATA and PCI Express. As well, Intel’s Battery Life Analyzer tool for the Intel Core processor family and Windows Vista/7/8 is available today to can help designers do a deep dive on their current systems and dentify ways to lower power even further. Check out http://intel.ly/S8RaeV for more information.

¹ Look for Session EBLS001 on the IDF website for more details.

 


ciufo_chris

Chris A. Ciufo is senior editor for embedded content at Extension Media, which includes the EECatalog print and digital publications and website, Embedded Intel® Solutions, and other related blogs and embedded channels. He has 29 years of embedded technology experience, and has degrees in electrical engineering, and in materials science, emphasizing solid state physics. He can be reached at cciufo@extensionmedia.com.

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