Power efficiency enhancements in Vista SP1

PowerMost if not all the attention surrounding Windows Vista Service Pack 1 has been around performance, reliability and compatibility, but you probably didn’t know SP1 also makes short strides in terms of Vista’s power efficiency. And by short, I really do mean short because we’re talking about improvements in the magnitude of only a couple percents. That in the context of 5 hours battery life is a mere additional 6 minutes. Having said that, some is better than none.

The first of two improvements is in the display subsystem. As you are probably aware of, the Desktop Window Manager (DWM) in Vista utilities the 3D rendering capabilities of your graphics card to draw the desktop and windows.

If you’ve ever played 3D games, you might have come across a graphics option called VSync. In a nutshell, VSync makes sure the output of the graphics card is synchronized with the refresh rate of your monitor. This prevents moving images from an effect of being teared. By design, Vista enables VSync to prevent a glitch free experience.

The side effect of VSync however is that it requires an interrupt to the CPU to maintain its sync. The default VSync rate for most users is 60Hz which means an interrupt occurs once every 16 milliseconds. As you might have guessed, this continuous interruption can prevent the CPU from entering a low-performance state where it is conserving energy.

Starting from SP1, the VSync functionality no longer runs continuously. Instead, the interrupts are disabled after a short timeout period where no screen updates has occurred. The default timeout period is 10 VSync periods which translates to 160 milliseconds (0.16 second) on a 60Hz screen. The catch is that a screen update can be anything from a blinking cursor to a flashing network icon, so the chances of a screen idle might be a lot less than you would imagine. Microsoft estimates VSync can increase power consumption from 1 to 2 percent.

The second improvement is in the audio subsystem and only concerns those with HD Audio codecs (not AC97). In Vista RTM, the audio device will never idle regardless of whether or not it is mute or if there’s any sound processing being done on the device, thus sucking power even when you’re not hearing any sound at all.

Starting from SP1, the default idle timeout for has been changed to 30 seconds. This means on battery power if the audio device is not rendering audio for a continuous period of 30 seconds, it will actually switch the audio device to a D3 power state. By definition, this means the device is off and should not consume any energy at all. Whilst Microsoft doesn’t give any indication of potential power savings, I presume this will have more of an effect than the VSync enhancement.

Before any of this matters, both of these changes also require some work on behalf of the hardware vendor. In the case of the graphics driver, it involves adding an additional flag in the source code and recompiling it with the new framework. In the case of the audio driver, it will have to direct the hardware to enter into a low-power state after a period of audio idle. Besides Microsoft’s default drivers, I’m not aware of any third-parties already supporting these enhancements but I hope they will soon.

If any laptop users with Vista SP1 have noticed a dramatic increase in battery life after SP1, please share your system specification and how much of an improvement you saw. If you don’t have SP1 already, it only means you’re not looking hard enough.

21 insightful thoughts

  1. What do you mean with: “If you don’t have SP1 already, it only means you’re not looking hard enough.” 😀

    I’m looking *very* hard, as hard as checking in my MSDN Subscription every once in a while to see if they changed their mind and made SP1 available. On the other hand, I don’t want to download something illegal. Was that a sarcastic allusion?

  2. “If you don’t have SP1 already, it only means you’re not looking hard enough.”

    Or you have one of the millions of non-English/German/French/Japanese editions of Vista out there, of course.

    That being said, would installing SP1 on a localised Vista Ultimate with English language pack work? Or what if you have an originally English version of Ultimate, but are running an, I don’t know, Swahili language pack?

  3. Very interesting info, I’d like to see more articles like this! :-)

    I’m using SP1 on my laptop. I can’t comment on any differences in battery life, but entering and resuming from Sleep is a lot faster!

  4. “If you don’t have SP1 already, it only means you’re not looking hard enough.”
    Some of us have to use our systems in a production environment, so I will wait until Microsoft releases the SP1 to the public via Windows Update, so that I can be sure that the driver problems are resolved.

  5. My laptop battery life is now more consistently 4 hours, it is a dell Inspiron 1520 2.0Ghz w/ 8600M GT 2GB Ram 5400 RPM hard drive and the WSXGA+ screen.

  6. My power consumption has only gone up because of the stability that Vista has gained. I’ve been playing more, sucking more juice out of my wall. Damn you MS!

  7. Fred: Wave 1 of Vista SP1 RTM is ready. I just installed Vista SP1 through Windows Update, using the Windows Update script, for my friend’s Asus F5N with the Traditional Chinese (Taiwan) version of Vista RTM installed.

  8. And the newer, probably SP1 compatible (it has a date of Feburary 2008) Atheros wireless drivers and some prerequisite updates are marked as important updates. SP1 did not appear in the update list until they were installed. This behavior is just like what Microsoft intends to implement when Vista SP1 is officially released to the public. Nothing is broken after installation.

  9. Great that I get a few more minutes of battery life. If only I could see it, as the battery icon has disappeared and I have no way of knowing how much life I have at all.

  10. Since i always use my computer in class, I usually use 1.5 hours as a standard length of time my battery should last. Pre-SP1, I always booted into Ubuntu because whenever I tried to use Vista unplugged from the wall it always displayed something around 1:04 or :10 of estimated battery life.

    The first few days after installing to SP1, I noticed battery power climbing upwards of two hours, which seems strange for a 5% increase in battery power. Back when I used XP Tablet, it was just over 1:35 or so, and that was if the screen was dimmed as far as it would go and both wireless and Bluetooth antennas were turned off. Now, I can actually see what I’m typing, and browse the Internet (or rather, in my case, search for campus WAPs). Vista still sucks in my opinion – I hate that I can’t do things like browse the Internet and play music at the same time, and it skips whenever I try to record things – but for classroom notes, Vista actually gained a tiny notch in my respect.

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