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 dual Opteron 252, 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 PowerNow/Cool'n'Quiet. (In the case of the Pentium 4 600-series processors and the XE 840, the C1E halt state is always active, even in the "no power management" tests.) Sadly, the beta BIOS we used for our Tyan S2895 motherboard didn't support AMD's PowerNow, so we couldn't report scores for the Opterons with power management enabled.

Intel has tamed the Prescott and Smithfield cores' power consumption at idle through the use of SpeedStep and the C1E halt state. As a result, the dual-core Pentium XE 840 system consumes fewer watts at idle than the system based on AMD's Opteron 175 (without the benefit of CPU power management.) The picture changes dramatically, though, when the processors are under load. The XE 840-based system pulls 313W at the wall socket, while the Opteron 175 box only requires 201W. Indeed, the XE 840 setup uses more power under load than the test rig that's housing a pair of dual-core Opteron 275 processors.

Notably, there's a 26W delta between the Pentium D 840 and the XE 840 under load, simply because of the addition of Hyper-Threading in the Extreme Edition.