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Back when my daily driver was a Core 2 laptop, someone told me that capping the clock frequency would make it unusable. As a petty "Take that", I dropped the m

Back when my daily driver was a Core 2 laptop, someone told me that capping the clock frequency would make it unusable. As a petty "Take that", I dropped the max frequency from 2.0 GHz to 1.0 GHz. I ran a couple benchmarks to prove the cap was working, and then just kept it at 1.0 for a few months, to prove my point. It made a bigger difference on my ARM SBC, where I tried capping the 1,000 MHz chip to 200 or 400 MHz. That chip was already CPU-bound for many tasks and could barely even run Firefox. Amdahl's Law kicked in - Halving the frequency made _everything_ twice as slow, because almost everything was waiting on the CPU.

mlyle 6y ago
The funny thing is, on modern processors-- throttling TDP only affects when running flat out all-core workloads. A subset of cores can still boost aggressively, and you can run all-core max-boost for short intervals. And the relationship between power and performance isn't linear as processor voltages climb trying to squeeze out the last bit of performance. So if you want to take a 105W CPU and ask it to operate in a 65W envelope, you're not giving up even 1/3rd of peak performance, and much less than that of typical performance.
- lynguist 6y ago
  You’re giving up 0 of peak single thread performance. A single core in turbo across Intel and AMD, mobile and desktop uses max 50W.
  - chrismorgan 6y ago
    Here are some numbers on single core power consumption, ⅌ https://www.anandtech.com/show/16214/amd-zen-3-ryzen-deep-dive-review-5950x-5900x-5800x-and-5700x-tested/8 https://www.anandtech.com/show/16214/amd-zen-3-ryzen-deep-di...: AMD Ryzen 9 5950X: 20.6W for a single core at 5050MHz, 49W for the whole package. (And it’s generally the package figure that you care about.) AMD Ryzen 9 5900X: 17.9W/54W at 4875MHz. AMD Ryzen 7 5800X: 17.3W/37W at 4825MHz. AMD Ryzen 5 5600X: 11.8W/28W at 4650MHz (though the highest core reading is 13W, at three cores loaded). You’re both correct: by simply restricting that power envelope by 40%, you shed a lot less multi-threaded performance than people realise, and no single-threaded performance. Look at the 5950X figures, and you observe that at about 120W, it can run 6 cores at 4,650MHz (27,900 core–MHz), or 16 cores at 3,775MHz (60,400 core–MHz). Expressed one way: by dropping the frequency by 20%, power per watt increased by around 2.7×. Expressed another way: let’s skip a 65W envelope—put this particular 105W chip in a 40W envelope and you lose only 20% of your six-cores performance. Seriously. But I’m not sure what the curve would look like if you load all 16 cores at a 40W envelope, what speed they’d be going at. (But do remember that “TDP” is a bit of a mess as a concept, and that we’re depending on non-core power consumption being generally fairly consistent regardless of load.)

mlyle 6y ago

The funny thing is, on modern processors-- throttling TDP only affects when running flat out all-core workloads. A subset of cores can still boost aggressively, and you can run all-core max-boost for short intervals. And the relationship between power and performance isn't linear as processor voltages climb trying to squeeze out the last bit of performance. So if you want to take a 105W CPU and ask it to operate in a 65W envelope, you're not giving up even 1/3rd of peak performance, and much less than that of typical performance.

lynguist 6y ago
You’re giving up 0 of peak single thread performance. A single core in turbo across Intel and AMD, mobile and desktop uses max 50W.
- chrismorgan 6y ago
  Here are some numbers on single core power consumption, ⅌ https://www.anandtech.com/show/16214/amd-zen-3-ryzen-deep-dive-review-5950x-5900x-5800x-and-5700x-tested/8 https://www.anandtech.com/show/16214/amd-zen-3-ryzen-deep-di...: AMD Ryzen 9 5950X: 20.6W for a single core at 5050MHz, 49W for the whole package. (And it’s generally the package figure that you care about.) AMD Ryzen 9 5900X: 17.9W/54W at 4875MHz. AMD Ryzen 7 5800X: 17.3W/37W at 4825MHz. AMD Ryzen 5 5600X: 11.8W/28W at 4650MHz (though the highest core reading is 13W, at three cores loaded). You’re both correct: by simply restricting that power envelope by 40%, you shed a lot less multi-threaded performance than people realise, and no single-threaded performance. Look at the 5950X figures, and you observe that at about 120W, it can run 6 cores at 4,650MHz (27,900 core–MHz), or 16 cores at 3,775MHz (60,400 core–MHz). Expressed one way: by dropping the frequency by 20%, power per watt increased by around 2.7×. Expressed another way: let’s skip a 65W envelope—put this particular 105W chip in a 40W envelope and you lose only 20% of your six-cores performance. Seriously. But I’m not sure what the curve would look like if you load all 16 cores at a 40W envelope, what speed they’d be going at. (But do remember that “TDP” is a bit of a mess as a concept, and that we’re depending on non-core power consumption being generally fairly consistent regardless of load.)