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There are two things to take from the above. First, as a general point, every chip on a given manufacturing process has a peak efficiency clock, below which you lose power efficiency by reducing clocks. Secondly, we have the data from Orin to know pretty well where this point is for a GPU very similar to T239's on a Samsung 8nm process, which is around 470MHz.
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That is, if the power budget is 3W for the GPU, and the peak efficiency clock is 470MHz, and the power consumption per SM at 470MHz is 0.5W, then the best possible GPU they could include would be a 6 SM GPU running at 470MHz. Using a smaller GPU would mean higher clocks, and efficiency would drop, but using a larger GPU with lower clocks would also mean efficiency would drop, because we're already at the peak efficiency clock.
In reality, it's rare to see a chip designed to run at exactly that peak efficiency clock, because there's always a financial budget as well as the power budget. Running a smaller GPU at higher clocks means you save money, so the design is going to be a tradeoff between a desire to get as close as possible to the peak efficiency clock, which maximises performance within a fixed power budget, and as small a GPU as possible, which minimises cost. Taking the same example, another option would be to use 4 SMs and clock them at around 640MHz. This would also consume 3W, but would provide around 10% less performance. It would, however, result in a cheaper chip, and many people would view 10% performance as a worthwhile trade-off when reducing the number of SMs by 33%.
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