3 ms·

> Since the benchmark in question (Geekbench 5) only runs for a minute or two, it does most of its work before the chip even attempts to throttle down to anywhe

by ajross 4y ago

> Since the benchmark in question (Geekbench 5) only runs for a minute or two, it does most of its work before the chip even attempts to throttle down to anywhere near 35W. That doesn't sound right at all. Nothing in a PC system (absent water systems with huge reservoirs I guess) is going to buffer excess heat on minute-long timescales, not even close. There's literally nowhere for that energy to go; 125W for a minute would be melting the solder joints. CPU throttling works on at most small-integer-second scales.

wtallis 4y ago
CPU throttling happens on many different timescales. That's why Intel processors have multiple power limits (PL1, PL2, PL3, PL4), in addition to current and temperature-based throttling mechanisms. The time constant for PL2 is usually either 28 seconds or 56 seconds. At those timescales, the concern is usually not with power delivery or CPU die temperature but rather with exterior case temperatures that the user may be directly touching. Based on the reported benchmark performance, it seems very unlikely that temperature-based throttling kicked in, and it's clear that the chip was operating well above 35W for at least a large portion of the benchmark run. So the PL1 and PL2 turbo power limits are the relevant controls at play here.
- ajross 4y ago
  Yeah yeah, I know. I'm just saying that there's zero chance that throttling is affecting this measurement. The idea that an Intel machine is significantly faster for 2-3 minutes after the start of a benchmark is just silly, that's not the way these things work. Go start a benchmark of your own to see. Again, the thermodynamic argument is fundamental here. You're saying that a "35W" CPU is "actually" drawing power equivalent to a 125W CPU for exactly the time of a benchmark, which is several minutes. That excess would have nowhere to go! There's no reservoir to store it. (Obviously the cooling system could take it away, but part of your argument is that the cooling system is only good for 35W!).
  - wtallis 4y ago
    > The idea that an Intel machine is significantly faster for 2-3 minutes after the start of a benchmark is just silly, that's not the way these things work. Go start a benchmark of your own to see. I've done so, on many occasions, with actual power meters rather than trusting software power estimates. You really do commonly see a laptop's power consumption drop significantly ~28 seconds into a multithreaded CPU benchmark. > (Obviously the cooling system could take it away, but part of your argument is that the cooling system is only good for 35W!). I make no such claim that the cooling system is limited to 35W. I only claim that the default platform power management settings from Intel impose a 35W long-term power limit, unless the system builder has adjusted the defaults to account for whatever form factor and cooling choices they've made. Perhaps you haven't realized that the turbo power limits will still kick in even if the CPU die temperature is not too hot—because they're not actually a temperature-based control mechanism?
    - ajross 4y ago
      Uh... this is a socketed CPU: https://www.intel.com/content/www/us/en/products/sku/230498/intel-core-i913900t-processor-36m-cache-up-to-5-30-ghz/specifications.html https://www.intel.com/content/www/us/en/products/sku/230498/... Now I see where the disconnect was. You're right, if this was a laptop that could happen. It isn't, and it didn't.
      - ribit 4y ago
        Even in a laptop CPUs regularly draw more power than the TDP for non-trivial amounts of time.
      - wtallis 4y ago
        It's a socketed CPU intended for low-power small form factor systems and thus will usually be running with Intel-recommended power limits or lower, for all the same reasons that laptop CPUs are usually running with low power limits. The control mechanisms don't actually function any differently between their laptop and desktop CPUs, they just have different default parameters (the various turbo limits). The only relevance of the socketed nature of this part is that it is easy to put it in a normal desktop form factor where a big heatsink and possibly tweaked turbo limit settings can be used to generate misleading benchmark results. But it's not actually certain that this is what's happening; the Intel-recommended default behavior for this chip can plausibly produce the reported results—just not in any way that could be reasonably described as "35W".
neilmovva 4y ago
I was curious about exactly how much burst heat could be absorbed, so I asked WolframAlpha [0]. In a 15" workstation laptop, I think the CPU could quite reasonably pull +100 watts over steady-state TDP for 30 seconds. (100-gram aluminum heatsink absorbing 100W * 30sec -> ∆T = +33ºC) [0] https://www.wolframalpha.com/input?i=%28100w+*+30+sec%29+%2F+%28specific+heat+aluminum+*+100g%29+ https://www.wolframalpha.com/input?i=%28100w+*+30+sec%29+%2F...

by ajross 4y ago

wtallis 4y ago

CPU throttling happens on many different timescales. That's why Intel processors have multiple power limits (PL1, PL2, PL3, PL4), in addition to current and temperature-based throttling mechanisms. The time constant for PL2 is usually either 28 seconds or 56 seconds. At those timescales, the concern is usually not with power delivery or CPU die temperature but rather with exterior case temperatures that the user may be directly touching. Based on the reported benchmark performance, it seems very unlikely that temperature-based throttling kicked in, and it's clear that the chip was operating well above 35W for at least a large portion of the benchmark run. So the PL1 and PL2 turbo power limits are the relevant controls at play here.

ajross 4y ago
Yeah yeah, I know. I'm just saying that there's zero chance that throttling is affecting this measurement. The idea that an Intel machine is significantly faster for 2-3 minutes after the start of a benchmark is just silly, that's not the way these things work. Go start a benchmark of your own to see. Again, the thermodynamic argument is fundamental here. You're saying that a "35W" CPU is "actually" drawing power equivalent to a 125W CPU for exactly the time of a benchmark, which is several minutes. That excess would have nowhere to go! There's no reservoir to store it. (Obviously the cooling system could take it away, but part of your argument is that the cooling system is only good for 35W!).
- wtallis 4y ago
  > The idea that an Intel machine is significantly faster for 2-3 minutes after the start of a benchmark is just silly, that's not the way these things work. Go start a benchmark of your own to see. I've done so, on many occasions, with actual power meters rather than trusting software power estimates. You really do commonly see a laptop's power consumption drop significantly ~28 seconds into a multithreaded CPU benchmark. > (Obviously the cooling system could take it away, but part of your argument is that the cooling system is only good for 35W!). I make no such claim that the cooling system is limited to 35W. I only claim that the default platform power management settings from Intel impose a 35W long-term power limit, unless the system builder has adjusted the defaults to account for whatever form factor and cooling choices they've made. Perhaps you haven't realized that the turbo power limits will still kick in even if the CPU die temperature is not too hot—because they're not actually a temperature-based control mechanism?
  - ajross 4y ago
    Uh... this is a socketed CPU: https://www.intel.com/content/www/us/en/products/sku/230498/intel-core-i913900t-processor-36m-cache-up-to-5-30-ghz/specifications.html https://www.intel.com/content/www/us/en/products/sku/230498/... Now I see where the disconnect was. You're right, if this was a laptop that could happen. It isn't, and it didn't.
    - ribit 4y ago
      Even in a laptop CPUs regularly draw more power than the TDP for non-trivial amounts of time.
    - wtallis 4y ago
      It's a socketed CPU intended for low-power small form factor systems and thus will usually be running with Intel-recommended power limits or lower, for all the same reasons that laptop CPUs are usually running with low power limits. The control mechanisms don't actually function any differently between their laptop and desktop CPUs, they just have different default parameters (the various turbo limits). The only relevance of the socketed nature of this part is that it is easy to put it in a normal desktop form factor where a big heatsink and possibly tweaked turbo limit settings can be used to generate misleading benchmark results. But it's not actually certain that this is what's happening; the Intel-recommended default behavior for this chip can plausibly produce the reported results—just not in any way that could be reasonably described as "35W".

neilmovva 4y ago

I was curious about exactly how much burst heat could be absorbed, so I asked WolframAlpha [0]. In a 15" workstation laptop, I think the CPU could quite reasonably pull +100 watts over steady-state TDP for 30 seconds. (100-gram aluminum heatsink absorbing 100W * 30sec -> ∆T = +33ºC) [0] https://www.wolframalpha.com/input?i=%28100w+*+30+sec%29+%2F+%28specific+heat+aluminum+*+100g%29+ https://www.wolframalpha.com/input?i=%28100w+*+30+sec%29+%2F...