The outcome of this is a capability Microsoft calls Windows 7 timer coalescing. The idea behind this mouthful is to keep the CPU in as low a power state as possible for as long as possible. For laptops and netbooks, this means longer battery life. For desktop PCs, it means less overall power usage over time. Current generation CPUs from Intel support C6 deep sleep, so that means keeping the CPU in that state as much as possible.
That’s actually pretty tricky. C6 essentially turns the CPU off; the power draw on a per-core basis is essentially zero. What that also means is that dropping into C6 and coming out of C6 actually costs power. What you want is for the overall power savings to exceed the cost of the transition to and from the deep sleep state.
Of course, what’s considered a long time in CPU land is a very short time to normal humans. Even a seemingly constant application, like DVD playback, creates situations where individual cores may power down for relatively long intervals. That results in an overall power savings.
Microsoft has actually built an API around time coalescing, so that applications can take advantage of this. Rather than request that the OS wake up the app at specific intervals (say, ten timer ticks), the API lets the app give the OS a range – 5 – 20 timer ticks. Windows 7 can then defer waking up the app until the maximum time, or wake it up sooner, depending on the application load and power draw.
Of course, you have to wonder what part the PCU (power control unit) built into Nehalem plays in all this. Intel has been loath to expose the PCU to applications, but an operating system may be another matter. So it would be interesting to find out if Windows 7 has some inside knowledge as to the behavior of the PCU.
One demo shown was DVD playback. Two identical Lenovo 400 laptops were running identical DVDs (Lord of the Rings: the Fellowship of the Ring, I think). One system had Vista installed; the other Windows 7. The average power drain over thirty minutes for the Vista system was 20.5W. The Windows 7 system consumed just 15.6W.
Of course, different applications mixes will result in greater or lesser power savings.
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6 comments
Alejandro Hernandez says:
September 1, 2009 at 6:57 pm (UTC -7 )
Oh nice, to see the new scheduler learning how to know the diference between virtual and phisical cores. Less power is ok too. But will that new CPU architecture require a new socket? cus Im still on 775 and Im planing to go i7 next holydays, should I wait?, I dont wanna get stuck on an expensive great mobo but with the wrong socket (no scalability) like happend to me on 478 times.
Loyd Case says:
September 1, 2009 at 7:24 pm (UTC -7 )
Intel is coming out with a new socket, but the old socket will be around, too. See my earlier post on Socket 1366 and 1156:
http://www.improbableinsights.com/2009/08/19/the-future-of-socket-1366/
Markeyse says:
September 1, 2009 at 9:51 pm (UTC -7 )
I can’t wait for Windows 7! And with the new Intel i5 platform right around the corner, will have a lot of possibilities. I can use a i7 chip for my professional workstation, and the i5 as a home computer.
Chris Johnson says:
September 2, 2009 at 6:16 am (UTC -7 )
Very cool stuff. Computers are so beastly hot nowadays to achieve higher and higher levels of performance. But many of us don’t need all that power all of the time. The same has been true of cars for many years. Timer coalescing actually reminds me of that Chrysler innovation where when you’re cruising on the highway the engine will shut off half the cylinders to save gas.
FH says:
September 2, 2009 at 8:24 am (UTC -7 )
Good report. Windows is a better deal on laptops/netbooks than Linux because of this, and laptops are the new majority market.
Give Apple credit, though, for having a good S3-style sleep for years, while Windows machines were hanging every time you closed the lid. There’s lot’s of history behind this New Religion.
And be ready for side effects. Do mb vendors realize they now need accurate HW clocks? Perhaps your mb coin cell won’t last long, now? Who knows?
mark says:
September 16, 2009 at 10:26 pm (UTC -7 )
Great article, I’m looking forward to 7 for a number of reasons, if it will actually have a decent sleep mode that would be great.
Any word when we might see Core i7 or i5 notebooks anytime?