Those familiar with the processor business will be familiar with the term Thermal Design Power (TDP), which refers to the number of watts that a cooling system must dissipate under a maximum real program load. AMD, however, has adopted a different power measurement system to rate its processors.
The new system, dubbed Average CPU Power (ACP), was championed by AMD for the first time with the release of its K10 Phenom family. AMD claims ACP is comparable to TDP measurements in Intel’s benchmarks; and is in fact a superior measuring stick for power consumption. ACP is described as being calculated based on an “average” daily use scenario for desktop users or for a server.
Despite AMD’s claims that ACP is directly comparable with TDP, many began to have doubts when AMD struggled to raise the speeds on its Phenom processors. This could indicate, among other things, thermal issues due to high power consumption.
The results are startling. The story is best explained by two versions of a white paper ironically titled “ACP – The Truth About Power Consumption Starts Here,” — the old version can be found here and the new version here. The key information in these papers can be seen in the table below.
K10 Family ACP and TDP Processor
TDP Whitepaper 1 TDP Whitepaper 2
ACP Whitepaper 1
ACP Whitepaper 2*
Quad-core AMD Opteron SE
120 W 137 W
Quad-core AMD Opteron
Quad-core AMD Opteron HE
Look at those numbers closely. The first thing to notice is that TDP measurements are significantly higher than ACP. When AMD compared its power consumption figures to Intel’s TDP, ACP measurements significantly underestimate power consumption. TDP differed between the two versions of the white paper by as much as 20 W, which is a 21% increase in the case of the quad-core Opteron. AMD did not increased its ACP estimates, emphasized in bold, despite the TDP increase.
Either the ACP is an arbitrarily measured system, and AMD changed it at will for its convenience, or AMD’s document team failed to update the document properly. There is no other feasible explanation of why a 20 Watt TDP increase would be accompanied by no increase in ACP.
Whitepapers are a very important tool for an electronics firm to inform the public of its products’ performance. Glaring inaccuracies such as these certainly reflects negatively on AMD. Furthermore, the admission that ACP can be as much as 20 Watts lower than the TDP on the same processor indicates that AMD is spreading misinformation using these ACP numbers in its advertising.
To put this information in perspective, a 3.16 GHz Xeon X5460 from Intel squeaks in at a still weighty 120 W. While AMD failed to disclose in the white paper on what frequencies its selected processors operate, it is almost surely 3.0 GHz or lower, as 3.0 GHz is the highest speed K10 processor currently demonstrated. The best case scenario is that a 166 MHz slower AMD processor consumes 17 more watts. If this was truly the case, it would not be the end of the world, but it would indicate AMD falling behind further in performance.
However, if the samples tested were lower than 3.0 GHz, obviously the picture becomes far worse. And since AMD’s 2008 roadmap states that its 2.4 GHz processors are rated at 125 Watts TDP, this is almost certainly the case. Architecture and design advantages aside, K10 is a chip that is almost a gigahertz slower but with a significantly higher power consumption rating.
AMD’s Phenom processor already underwent lackluster reception at launch, and the company faces ever-higher-piling losses. The problems have led AMD to try to focus on expanding its K8 platform offerings, instead of addressing the compounding issues of its K10 platform.
The ACP/TDP issues are just one of many issues with the K10, but have many ramifications — everything from increased rate of chip failure, to simple higher energy bills at no additional speed benefit. Expect AMD to fix its documents soon, but it will take far longer for AMD to salvage the physical hardware of its K10 platform.
AMD has clarified its position on ACP and TDP. Read here for further information.
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