Comment by NickHoff

Comment by NickHoff 3 days ago

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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?

Paul_Clayton 3 days ago

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/

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  • twic 3 days ago

    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.

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    • pants2 3 days ago

      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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      • ddxxdd 2 days ago

        I could argue that ethanol has 1/3 the latent heat of vaporization of water, and would boil off 3 times quicker. However, what ultimately matters is the rate of heat transfer, so my nitpick may be irrelevant.

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    • Dylan16807 3 days ago

      > 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?

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throwup238 3 days ago

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.

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lostlogin 3 days ago

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.

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  • kristjansson 3 days ago

    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.

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    • ericye16 3 days ago

      What if you chose a refrigerant with a lower boiling point?

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      • kristjansson 3 days ago

        That's basically the principle of binary cycle[1] generators. However for data center waste heat recovery, I'd think you'd want to use a more stable fluid for cooling, and then pump it to a separate closed-loop binary-cycle generator. No reason to make your datacenter cooling system also deal with high pressure fluids, and moving high pressure working fluid from 1000s of chips to a turbine of sufficient size, etc.

        [1]: https://en.wikipedia.org/wiki/Binary_cycle

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  • renhanxue 3 days ago

    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.

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  • sebzim4500 3 days ago

    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.

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    • dylan604 3 days ago

      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

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  • bentcorner 3 days ago

    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.

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    • twic 3 days ago

      Hey, we're building artificial general intelligence, what's a little perpetual motion on the side?

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