Windows Vista Team Blog : Power Management

Windows Vista Power Management

JimAll — Fri, 08 Dec 2006 22:50:00 GMT

The hardware that makes up today’s laptops has gotten quite efficient and we have worked hard to improve Windows' ability to manage that hardware. One of the biggest improvements in the mobile PC experience for Windows since the early 1990 is longer battery life. The trick here is to make the system as efficient as possible while still maintaining a great user experience. At the same time, the increased cost of electricity has become a key factor for enterprise customers who are under continued pressure to manage TCO (total cost of ownership). As a result of all of these factors, we decided to make some extensive changes in the power management sub-system for Windows Vista.

The Windows Vista power management goals were simple: 1) making turning a computer "off"/"on" as reliable, simple, and fast as turning off/on a TV, and 2) maximizing the active usage if on battery and reducing the energy consumption if on A/C power. The first goal involved not only improving the predictability of the behavior when a PC was switched "off", but more importantly changing the model for what "off" and "on" means. Everyone knows that turning a TV off doesn’t really turn it off. It is still available to receive the remote control signal, etc. so that it can come back on quickly. We wanted to emulate this for Windows Vista machines.

To the degree possible, "off" equals "sleep" in Windows Vista, where the system state is saved in RAM. This creates the best balance of user experience for speed of resuming and lowest usage of power. However, if the PC is running on batteries even that minimal power usage could drain the batteries eventually. Remember the top goal here is to make sure that we can enable a fast on experience (like your cell phone) and a fast off experience, while still making sure that you don't lose your work when a Windows PC is turned off. To do this, we created a new approach that we call "hybrid sleep state" that is the best of the sleep and hibernate modes (which existed separately in Windows XP). In this hybrid mode, the state is stored both in RAM and on disk, so nothing can be lost if power goes out. Then the system is suspended into the low powered sleep state for a period of time (like your cell phone). Normally, when the user returns and wakes the machine, system state is just restored from RAM, and resume responsiveness to the user is fast. However, if for some reason power is lost (for example, if the PC is unplugged to move it -- like you might move your TV), the system can still resume from the hibernate image previously saved to disk with all context and data intact.

While Hybrid Sleep can be used on a laptop computer, it isn’t as applicable to laptops for a couple of reasons. First, a desktop is vulnerable to power loss, while a laptop can of course run on its internal battery. A laptop can also detect or even wake up from standby when the battery is low so Windows can save everything to the disk before the battery is completely drained -- and remember, in the sleep state all the battery needs to do is to refresh the RAM, which takes very little power. Also, mobile users want a grab-n-go usage model, so taking extra time after the laptop lid is closed spinning the disk to write out a potentially large hibernate file could be a problem. Lastly, after a laptop has been in sleep for a while, the system will wake up and immediately go into the hibernate state. This state uses absolutely no power, so even the minimal battery drain used in standby is removed. If the system is in this power state when the user wants to use the system, then the system state is restored from disk.

In either case (suspend to RAM or disk), though, the user conceptually just thinks of "on"/"off." In the case that the machine hasn't been used for a long time, then the only difference to the user is that it just takes slightly longer to resume back to where they were. As I mentioned, if the system is A/C-powered, then by default we leave the system in the sleep state (never going into hibernate) so that the machine can be virtually instantly available, but still have substantial power savings compared to past systems. The bottom line is that because there is no reboot, the PC can respond much more quickly.

In order to ensure that "off" worked effectively with this new model, we needed to address some unpredictability that occurred with sleep in Windows XP. Instead of waiting for each application, service and device to agree before going into power saving mode, in Windows Vista we changed the approach so that we give the application, services and devices a notice of the impending suspend and then wait a maximum of 2 seconds for them to finish up any work and put themselves into a state they can continue from when the system wakes up. With Windows Vista there won’t be any more sinking feelings when the airplane is at 10,000 feet and you reached into your laptop bag to find the laptop all cozy and warm because it didn’t go into power saving mode when you were running for the plane -- caused because some device, service, or application wasn't well behaved. One of the interesting things about the new power management system is that Windows Vista is able to come back from the sleep state so quickly that the first time you open the lid on a suspended Windows Vista laptop you might wonder if it was on the whole time -- trust, me it wasn’t.

Our second goal was to reduce the power consumption of PCs when they are idle. In the past the few users who did shut down their PCs when not in use had to sit through a long boot process to get their PC back on. However, many enterprises and home users just leave their PCs on 7x24. While this is nice to be able to have the PC ready to respond when you sit down at it, powering a PC overnight just so it can be available to you when you sit down at 8AM is a pretty expensive approach.

In order to achieve the second objective -- reducing power usage -- we made changes to dramatically improve idle power consumption. The dream was to reduce power consumption by powering key components down more frequently and leaving them powered down for longer periods while still maintaining a great user experience. For example, while Windows XP did a great job of lowering CPU speed and voltage whenever possible, in Windows Vista we added enhanced capabilities to use power more efficiently on multi-core systems. And we have created new APIs so that driver and application developers can get notifications about the power scheme being used on the system and then adapt their behavior for the power scheme. Finally, all of the power management configurations parameters can be controlled by the IT department (on a PC-by-PC basis) using Group Policy if they want to tweak the defaults, making corporate deployment of these features easily manageable.

How much power is used and how great the experience is depends on all the equipment and software attached to your system. So, we created some great tools including the Power Event Monitoring tool to help develop and test applications, as well as some great content for developers on power management. We have also made the power management system extensible, so that instead of writing separate power management solutions, third parties including computer manufacturers can closely integrate their unique features and capabilities with Windows.

If you look at the impact in power-savings terms of the Windows Vista design, it is pretty amazing. (BTW, there is a great white paper on this on Microsoft.com, but I will net it out for you here). A typical Pentium 4 PC with a 17" LCD monitor draws about 102.6 watts of power (think about a 100 watt light bulb). That same PC and display in a sleep state draws only 5.6 watts, or 97 fewer watts. If you figure that a PC is used for active work for 10 hours a day, 5 days a week, 52 weeks per year, that is 2,600 operating hours. With 8,760 hours in a year (365*24), there are actually 6,160 potential idle hours per year. Since sleep mode uses 97 fewer watts than full power mode, the total savings is 597 kWh per year -- and by the way, the impact is obviously even greater (760 kWh) if you use a CRT monitor since they draw more power than LCDs.

The paper uses an estimate of $0.0931 per kWh from the US Department of Energy, so for a home user with one PC, the savings amounts to $55.63/year (more if the PC is used less than 10 hours a day). While that is great saving for a home user, think about an enterprise with 10,000 desktops where the potential cost savings would be $556,300/year -- and we haven’t even tried to estimate the HVAC savings. With IT budgets becoming sequentially tighter year over year, it's nice to be able to have this kind of impact. I would be remiss if I didn’t mention the fact that there is also a significant environmental impact as well. The EPA estimates that every kWh of electricity generates 1.55 pounds carbon dioxide (CO2) emissions so each PC that moves to Windows Vista generates 926 fewer pounds of carbon dioxide or about 8% of what the EPA estimates that a typical car generates in the course of a year-- so for every 12 and a half PCs that are running with Windows Vista’s new power management capabilities, it’s like having one less car on the road.

I strongly encourage you to use the defaults in Windows Vista. And no need to think much anymore about different power-saving modes and terms like hibernate, sleep, etc -- just hit the symbol for "on" and "off" and let the system do the thinking and power saving for you.

jim

How Windows Vista RTM Build Addresses Wireless Connectivity Issues

jleznek — Mon, 04 Dec 2006 22:37:00 GMT

Wireless access is a key feature in Windows Vista, and it needs to work as reliably as possible for users. In the final version of Windows Vista, the default power setting for 802.11 wireless adapters was changed to “Maximum Performance” to offer seamless wireless access experience and solve connectivity issues with certain access points. Users and OEMs can change the setting value to deliver additional power savings, if they want to further extend the battery life.

Context

Test results from Microsoft and our customers show that some Windows Vista beta users experienced connectivity problems when connecting to public WiFi hotspots. In many cases, the root cause of the problem is access point or router hardware which is not compatible with the 802.11 power save protocol. The symptoms of the problem are either failing connections or extremely poor connection performance and throughput. Typically, these problems are experienced only when the computer is on battery power—connecting to AC power solves the issue.

By default, Windows Vista enables many platform power management features, including wireless adapter power saving modes. On all Windows Vista systems, the default power plan is Balanced, and pre-release versions of Windows Vista enabled Medium Power Savings for the 802.11 wireless adapter when the computer is on battery power. When the computer is on AC power, Maximum Performance (no power savings) is enabled for the 802.11 wireless adapter. This explains why connecting to AC power solves the connectivity issue for many users, as did changing the power plan to High Performance or changing the wireless adapter power setting to Maximum Performance in Power Options.

When power save mode is enabled for an 802.11 wireless network adapter, the adapter periodically enters a low-power state where the radio transmitter and receiver are in “sleep” mode. The wireless adapter in the computer (client adapter) indicates the “sleep” mode by setting the power save option in its packets or 802.11 frames sent to the access point. The access point receiving frames with the power save option set determines that the client adapter wishes to enter power save mode, and begins buffering packets for the client adapter while it is asleep. The client adapter’s radio periodically wakes up and communicates with the access point to retrieve the buffered packets. This scheme enables the wireless adapter to consume less power by sleeping and waking periodically, just at the right time to receive network traffic from the access point.

However, this power savings scheme for 802.11 wireless adapters depends on cooperation of the access point. The problem is that many access points do not implement or support the power save feature correctly. Some broken access points keep sending the packets to the client—even when the client adapter’s radio is asleep. The packets sent to the client radio while it is asleep are lost, which leads to the connectivity, performance and throughput issues that some Windows Vista beta users were encountering.

Wireless access is a key feature in Windows Vista, and it needs to work as reliably as possible for users. In the final version of Windows Vista, the default power setting for the 802.11 wireless adapter is “Maximum Performance”. This means, that by default, on battery power or on AC power, wireless adapters will not use power-saving modes. OEMs are able to change any power setting when they are building systems with Windows Vista, so the setting might be different on a machine released with Windows Vista. The obvious downside to the power setting change is a potential decrease in computer battery life. But, it may be difficult to diagnose the root cause of the wireless connectivity problem, so the wireless power setting was changed accordingly for the most common default case.

If you want to re-enable power savings for your 802.11 wireless adapter, you can easily do this in Windows Vista. There are two primary ways:

Change the wireless power saving setting:

Open Power Options in Control Panel
Choose Change Settings for the current power plan
Choose Change Advanced Power Settings
Expand Wireless Adapter Settings
Expand Power Saving Mode
Choose Maximum Power Saving, Medium Power Saving, Low Power Saving to enable various levels of 802.11 power save modes.

Choose the Power Saver power plan:

Click on the battery meter on the desktop and choose Power Saver.
The Power Saver plan has 802.11 power saving mode enabled for both battery and AC power.

Microsoft is committed to both seamless wireless access and extended battery life. We are actively working with industry partners to fix wireless access points so they work correctly with 802.11 power save mode.

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UPDATE

I have seen a few articles written over the past few days regarding this post. Most seem to be focusing on the sentence that said the change made to default wireless settings could result in a “potential decrease in computer battery life.” I want to clarify that even with the wireless power management feature turned off for there should be no noticeable difference in battery life than what you get with Windows XP today; the native wireless power management feature is new in Windows Vista, and is therefore an added bonus to people’s overall battery life experience when in use. (Ars Technica got it right.)

The true intent of the post was to let our beta testers know that we made connecting to wireless access points more reliable in the final version of Windows Vista. As I wrote in the original post, We are actively working with industry partners to fix wireless access points so they work correctly with 802.11 power save mode, and in the meantime we’ve optimized Windows Vista to deliver the best wireless experience possible.