Re: I'm just trying to understand the use of the acronym TDP. I thought TDP was a spec not a measurement. You seem to be using it the way someone would use MPG (Miles Per Gallon) as a way to state observations. I thought TDP was a fixed value, once you had a CPU in hand it would always have the same TDP no matter what heatsink you put on it.
TDP (the Thermal Design Point) is somewhat empirical, rather than an exact specification. In order to derive it, you have to test how real applications dissipate power. The end result is that if a system designer constructs a cooling solution that can dissipate that many watts, while keeping case temperature (also called junction temperature, or Tj) below a stated specification (usually in the range of 50-100C degrees).
Of course, most of the time, the cooling solution is below Tj, and cooler temperatures lower one very important element of power - leakage. You would generally expect that across a number of various chips a frequency F and temperature T, the vast majority will dissipate power well below TDP, which is consistent with most CPU measurements.
In the Pentium 4 and Pentium D era, many noticed that Intel's TDP value was far too aggressive, and that the CPU generally dissipated more power under normal conditions than what was rated as TDP. Of course, the processor is designed to throttle in these cases, but the consumer certainly doesn't want throttling to occur under normal workloads. It appears that besides a few early chips that exhibited this behavior, many newer chips became far more conservative. And by the time Intel launched Core 2, their chips were measuring far below TDP, just like AMD's chips.
For what it's worth, here is AMD's definition of TDP, word for word:
TDP. Thermal Design Power. The thermal design power is the maximum power a processor can
draw for a thermally significant period while running commercially useful software. The
constraining conditions for TDP are specified in the notes in the thermal and power tables.
amd.com
Notice that the notes usually mention which temperature the measurement assumes.
Intel summarizes their definition of TDP in 5.1.1 of this specification:
download.intel.com
Additional details on the measurement of TDP can be found here, too (specifically, note the text in section 4.2.6, starting on page 37):
download.intel.com |
