Comment by NickHoff
3 days ago
Neat. What about power density?
An H100 has a TDP of 700 watts (for the SXM5 version). With a die size of 814 mm^2 that's 0.86 W/mm^2. If the cerebras chip has the same power density, that means a cerebras TDP of 37.8 kW.
That's a lot. Let's say you cover the whole die area of the chip with water 1 cm deep. How long would it take to boil the water starting from room temperature (20 degrees C)?
amount of water = (die area of 46225 mm^2) * (1 cm deep) * (density of water) = 462 grams
energy needed = (specific heat of water) * (80 kelvin difference) * (462 grams) = 154 kJ
time = 154 kJ / 39.8 kW = 3.9 seconds
This thing will boil (!) a centimeter of water in 4 seconds. A typical consumer water cooler radiator would reduce the temperature of the coolant water by only 10-15 C relative to ambient, and wouldn't like it (I presume) if you pass in boiling water. To use water cooling you'd need some extreme flow rate and a big rack of radiators, right? I don't really know. I'm not even sure if that would work. How do you cool a chip at this power density?
The enthalpy of vaporization of water (at standard pressure) is listed by Wikipedia[1] as 2.257 kJ/g, so boiling 462 grams would require an additional 1.04 MJ, adding 26 seconds. Cerebras claims a "peak sustained system power of 23kW" for the CS-3 16 Rack Unit system[2], so clearly the power density is lower than for an H100.
[1] https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#Other... [2] https://cerebras.ai/product-system/
On a tangent: has anyone built an active cooling system which operates in a partial vacuum? At half atmospheric pressure, water boils at around 80 C, which i believe is roughly the operating temperature for a hard-working chip. You could pump water onto the chip, have it vapourise, taking away all that heat, then take the vapour away and condense it at the fan end.
This is how heat pipes work, i believe, but heat pipes aren't pumped, they rely entirely on heat-driven flow. I would have thought there were pumped heat pipes. Are they called something else?
It's also not a refrigerator, because those use a pump to pressurise the coolant in its gas phase, whereas here you would only be pumping the water.
No need to bother with a partial vacuum when ethanol boils at around 80 C as well and doesn't destroy electronics. I'm not aware of any active cooling systems utilizing this though.
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I found this review from 2019 of mechanically pumped heat pipe technologies. I skimmed the intro. Looks like it already has a foothold in aerospace.
https://www.sciencedirect.com/science/article/abs/pii/S13594...
> This is how heat pipes work, i believe, but heat pipes aren't pumped, they rely entirely on heat-driven flow. I would have thought there were pumped heat pipes.
Do you have a particular benefit in mind that a pump would help with?
A Very Fancy cooling engine: https://www.eetimes.com/powering-and-cooling-a-wafer-scale-d...
A good talk on how Cerebras does power & cooling (8min) https://www.youtube.com/watch?v=wSptSOcO6Vw&ab_channel=Appli...
The machine that actually holds one of their wafers is almost as impressive as the chip itself. Tons of water cooling channels and other interesting hardware for cooling.
Minor correction, the keynote video says ~20 kW
If rack mounted, you are ending up with something like a reverse power station.
So why not use it as an energy source? Spin a turbine.
If you let the chip actual boil enough water to run a turbine you're going to have a hard time keeping the magic smoke inside. Much better to run at reasonable temps and try to recover energy from the waste heat.
What if you chose a refrigerant with a lower boiling point?
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There's a bunch of places in Europe that use waste heat from datacenters in district heating systems. Same thing with waste heat from various industrial processes. It's relatively common practice.
If my very stale physics is accurate then even with perfect thermodynamic efficiency you would only recover about a third of the energy that you put into the chips.
1/3 > 0, so even if you don't get a $0 energy bill I'd venture that any company that could get 1/3 of energy bill would be happy
I'm aware of the efficiency losses but I think it would be amusing to use that turbine to help power the machine generating the heat.
Hey, we're building artificial general intelligence, what's a little perpetual motion on the side?