AMD Clarifies ACP, TDP Enigma

AMD’s Average CPU Power rating scale underwent its first test of scrutiny yesterday when two nearly-identical whitepapers discussing ACP caught the attention of Global Tech News and other technology forums. 

The inconsistencies between these two whitepapers included a table where the Thermal Design Power of AMD’s new quad-core Opterons increased, without an increase in the Average CPU Power rating.

Several processor architectures ago, AMD and rival Intel used the same methods for calculating Thermal Design Power with regard to microprocessors.   From an engineering standpoint, the TDP represents the amount of power the cooling mechanism for the CPU must dissipate before failure.

AMD and Intel now differ with TDP calculations, and for different reasons.  Intel’s current architecture, for example, allows the CPU to exceed the TDP rating for a small period of time before the processor throttles its frequency clock in order to reduce the temperature at the processor level.  AMD’s current-generation processors do not practice this method, and thus AMD intentionally publishes conservative TDP ratings.

AMD’s 2008 Phenom roadmap clearly illustrates the increase in TDP for upcoming K10 processors. Even though Phenom processors are desktop units, equivalently clocked Barcelona Opteron processors share almost all the same attributes and specifications.

AMD’s Brent Kerby, author of both whitepapers, explains the inconsistency with timing and the nature of ACP itself.  “The measured value of ACP already included the changed TDP values,” he explains. 

So are the ACP measurements represented in both documents wrong, or mutually exclusive? “No,” says Kerby.  Even though the maximum thermal envelope has increased by as much as 21% between the two whitepapers, the algorithm for calculating the ACP is not entirely affected by this upperbound.

“When we published the first whitepaper, we had to anticipate TDP changes,” adds Kerby.  The newest whitepaper, Kerby states, is relevant to both published TDPs.  The TDP references in the first document can be replaced with the TDP changes from the second, and in fact should have been.

Kerby could not comment on the specific margins built into the ACP rating, but mentions it’s an integral part of the ACP process.  His whitepaper details the margin as follows:

The results across the suite of workloads are used to derive the ACP number. The ACP value for each processor power band is representative of the geometric mean for the entire suite of benchmark applications plus a margin based on AMD historical manufacturing experience.

However, Kerby could confirm that if the TDP were to increase again, the company’s ACP values would need to be recalculated. 

Will AMD’s thermal envelope increase again?  AMD won’t say, but at least for now AMD’s ACP/TDP mystery appears solved.