Mobile Core i7-920XM: Power Is The Price For Better Performance

Power Consumption: Core i7 Versus Core 2

So, now we know that shifting from Windows Vista to Windows 7 is no alchemy for cutting battery use. But in an effort to further-quantify the difference between the Calpella and Montevina platforms, I plotted them both on the same chart under Windows 7.

For all the talk of extensive power gating throughout the Clarksfield die, it’s hard to get past the bottom line: Clarksfield is a more power-hungry processor, regardless of whether you’re at idle or under load. Just follow the peaks and valleys here—at every point, mobile Core i7 is sucking down more juice than its predecessor.

Now, we pushed this story back for more than a week as we worked with Intel to explain the power results (after all, a number of other sites have already reported power numbers at idle and under load suggesting that Clarksfield uses less power). Intel’s very valid concern is that our test platform sports a power-hungry graphics module, so any gains related to Clarksfield’s architecture are muffled by G92’s demands. Eurocom, the company who built both of our test platforms, astutely counters that these are desktop replacement notebooks; they’re not intended to be five-hour mobile machines.

If the lesson of balanced system building has taught us anything, though, it’s that you want to complement high-end CPUs with plenty of memory, a capable I/O subsystem, and respectable graphics—especially if you’re pegging the platform to the high-end/enthusiast market.

More Power Testing

We expected both machines to finish this test faster thanks to Windows 7’s more aggressive P-state promotion policies, but it was a little more surprising to see them finish up at roughly the same time.

Of course, none of the preceding three charts really relate to battery life or run time. So, I also took a few other measurements. The first was idle on the Windows 7 desktop, where our mobile Core 2 Extreme pulled 40.7W compared to the Core i7’s 54.7W. That was with a battery. You have to bear in mind that with charge running to the battery, power draw will increase, unrelated to the platform’s actual needs. Pulling the battery out to gauge pure power draw yielded a 45W Core i7 and 34W Core 2 Extreme. This is still higher than what Intel expected Clarksfield to be running at, but Eurocom confirmed this was in line with expectations.

Then I took a reading with the opening chapter of 300 playing (hardware acceleration enabled), and found the Core 2 Extreme at 50W to the Core i7’s 62W. Next, I completely charged both notebooks and let them play through as much of the movie as possible before being automatically shut down at 7% battery life. The mobile Core i7 lasted 44 minutes to the Core 2 Extreme’s one hour and 32 minutes (neither long enough to last for a full movie). That’s only part of the story—the i7’s battery holds 3,800 mAh of charge, while the Core 2’s holds 4,400 mAh. If you divide that out into minutes of DVD playback per unit of charge, Core i7 delivers 11.5 min/Ah, while the Core 2 Extreme enables 20.9 min/Ah.