The more I dig, the more murky this looks. The 'TDP' that AMD lists for the 3900x is 105w, and then the actual full CPU package power draw (the 'PPT') is more like 142w. Then AMD lists 280w as the TDP for the 3970x, but that Threadripper ‘TDP’ is actually the full package power envelope (PPT) for this chip, so a completely (more fully inclusive) different metric, with less need to overrun it.
Then, looking back over the Ryzen chips and generations, some are real sticklers about staying within their PPT limits and others are happy to overrun them for a higher performance.
Just having a play, loaded with 64 Rosetta threads and under excellent cooling, I can see the 3970x doing all core boosts as high as 4.17GHz (vs base clock of 3.7GHz, so like you were suggesting, Chris) AND it is well inside its 280w PPT limit. Some other times I've seen it butting up against 280w and ‘only’ clocking 3.85Ghz, only marginally better than the base clock. It does then fluctuate with workloads and probably ambient temps, etc.
To examine this further, I wound the power limits down to a level of 3w per core, which is what Anandtech suggested might be the actual per-core power available to a 3990x (the rest of the 280w PPT power going to the 'un-core', or all of the data transfer interconnects etc). For this particular Rosetta workload (which seems a bit of a best case) ... the chip is actually still clocking 3.66GHz!! In other workload and conditions, I have to assume that it might clock closer to its base clock?
Perhaps this architecture at 7nm is a real step-change and plays by new rules ... it seems pretty different to the way Intel works in any case. There is less need for very high watts than I had thought ...
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