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Power consumption
We measured the power consumption of our entire test systems, except for the monitor, at the wall outlet using a Watts Up PRO watt meter. The test rigs were all equipped with OCZ PowerStream 520W power supply units. The idle results were measured at the Windows desktop, and we used SMPOV and the 64-bit version of the POV-Ray renderer to load up the CPUs. In all cases, we asked SMPOV to use the same number of threads as there were CPU front ends in Task Manager—so four for the Pentium XE 840, two for the Opteron 175, and so on.

The graphs below have results for "power management" and "no power management." That deserves some explanation. By "power management," we mean SpeedStep or Cool'n'Quiet. (In the case of the Pentium 4 600-series processors and the Pentium D 840 and Pentium XE 840 CPUs, the C1E halt state is always active, even in the "no power management" tests.) Sadly, the beta BIOS for our Asus A8N-SLI Deluxe mobo wouldn't support Cool'n'Quiet on the Athlon 64 X2 processors. AMD says all of its dual-core chips will support power management once the proper BIOS support becomes available.

The Pentium D 820 doesn't support power management, as we've noted.

The AMD processors simply consume less power, both at idle and under load, than the Pentiums do. That's been the case for some time now, although Intel has made progress with Speedstep and the like.

Interestingly enough, the Pentium D 820's pair of 2.8GHz cores consume almost exactly as much power under load as the Pentium 4 670's single 3.8GHz core. This is one of the main reasons why Intel is moving away from higher clock speeds towards multi-core CPUs. We've already seen that the Pentium D 820 can often outperform the P4 670 in multithreaded applications, though they share the same basic power needs.

I sure wish the Pentium D 820 supported lower multipliers for use with Speedstep, though. There's no good reason why this CPU should consume as much power at idle as it does.