Server Istanbul processors have pretty low ACP values, which suggests that they might be very efficient in terms of performance-per-watt. To check this out we tested the actual power consumption of all participating platforms. The following numbers show the total power consumption of the tested platforms (without the monitor). During our tests we used 64-bit LinX 0.6.3 utility to load the systems to the utmost extent. Moreover, to ensure that we estimate the power consumption in idle mode correctly we activated all power-saving technologies, such as C1E, Cool'nQuiet 3.0 and Enhanced Intel SpeedStep. It is also important to keep in mind that Socket F MSI K9NU Speedster mainboard we used doesn’t support separate power supply for the processor cores and North Bridge integrated into the CPU that is why the power consumption of the system built on it appears a few watts higher in idle mode.
In idle mode Istanbul based system consumes a little more that Socket AM2 platform. It can be explained by the reasons described above as well as by the fact that Nvidia nForce Professional 3400 chipset used in our Socket F platform is not one of the energy-efficient solutions. However, despite this fact the system with a six-core AMD CPU consumes less power in idle mode than an LGA1366 platform with an Intel processor.
During full CPU utilization Istanbul is incredibly energy-efficient. While six-core Opteron processors do not yield in performance to the top Phenom II X4 CPU in a number of applications optimized for multi-threaded architectures, their power consumption is way lower. Six-core processors also look very appealing compared with Core i7-920, but here we should keep in mind that the latter is considerably faster.
To get a better picture of the situation we also tested the power consumption of the processors and mainboards under heavy load without taking into account the rest of the system components. To be more exact, we measured the power consumption along the 12 V power line connected directly to the processor voltage regulator on the mainboard and along the mainboard power lines.
As we see, in reality six-core Opteron processors working at 2.4-2.6 GHz frequency consume by about 1/3 less power than quad-core CPUs with the same microarchitecture, but working at 3.4 GHz clock speed.
It is interesting to see the mainboards power consumption first of all because low power consumption readings taken off the Nehalem based CPUs are partially explained by their voltage regulator design. The thing is that only processor cores are connected to the 12 V power line. Uncore part of the CPU is powered from the mainboard via 24-pin ATX power connector. As a result, if we sum up the numbers on this and the previous diagram, we can conclude that six-core Opteron processors win from the Core i7-920 in power consumption. So, looks like the major trump of the upcoming six-core AMD processors targeted for desktop computer systems will be performance increase-per-watt.