We put Microsoft Windows 7, Vista and XP to the test, to examine just how much the changes to the way the operating system deals with multithreaded applications effects its speed.
Windows 7 performs several tricks to keep threads running on the same execution pipelines so that the underlying Nehalem processor can turn off transistors on lesser-used or inactive pipelines.
The primary benefit of this feature is reduced energy consumption. To quantify this benefit, I ran the four-thread version of Viewperf with SMT enabled.
This configuration meant that roughly half the pipelines would see little or no activity. I expected, therefore, to see some power savings for Windows 7. My results appear below.
Watts consumed at three points in Viewperf benchmark
The Windows 7 advantage is indeed significant. Note that this 17 percent decrease in power consumption is for the exact same software running unchanged on the same machine.
Only the versions of Windows are different. That's a substantial savings, and there is every reason to believe that other software will similarly benefit from Windows 7's ability to leverage Intel's processor magic.
Wrapping it all up
Tight integration between Intel processors and Microsoft operating systems has been a constant thread in the history and evolution of the PC.
This linkage has been dubbed by some a virtuous circle, although not every iteration of the cycle has produced substantial end-user benefits.
This time, though, the cycle indeed delivers key advantages: Nehalems are much more powerful than predecessors, and they provide, as we have seen, considerable energy savings when teamed up with an OS that leverages them effectively.
Among Microsoft offerings, Windows 7 is the software that does this best.
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See also: The 7 deadly sins of Windows 7