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Google's Liquid Cooling

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399 points giuliomagnifico | 5 comments | 25 Aug 25 17:57 UTC | HN request time: 0s | source
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michaelt ◴[25 Aug 25 18:51 UTC] No.45017512[source]▶
> TPU chips are hooked up in series in the loop, which naturally means some chips will get hotter liquid that has already passed other chips in the loop. Cooling capacity is budgeted based on the requirements of the last chip in each loop.

Of course, it's worth noting that if you've got four chips, each putting out 250W of power, and a pump pushing 1 litres of water per minute through them, water at the outlet must be 14°C hotter than water at the inlet, because of the specific heat capacity of water. That's true whether the water flows through the chips in series, or in parallel.

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foota ◴[25 Aug 25 18:54 UTC] No.45017544[source]▶
Hm... but in the case when the chips are in serial, the heat transfer from the last chip will be less than when the chips are in parallel, because the rate of heating is proportional to the difference in temperature, and the water starts at a lower temperature for the parallel case for this last theoretical chip.
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0x457 ◴[25 Aug 25 20:39 UTC] No.45018696[source]▶
Yes, but water is constantly moving in a loop. It's not like you use water to cool chip #1, and then it moves to chip #2, it's constantly moving, so temperature delta isn't that much.
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1. smachiz ◴[25 Aug 25 21:10 UTC] No.45019066[source]▶
in their first serial design, that's exactly what it was doing.
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2. Dylan16807 ◴[27 Aug 25 01:14 UTC] No.45034330[source]▶
They mean it doesn't sit around and wait until it's hot, which then makes it "used" and unable to cool something else. It keeps moving through the first chip and is only a small percent saturated.
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3. smachiz ◴[27 Aug 25 20:04 UTC] No.45044451[source]▶
depends how fasts it is moving - but it's all moving at the same speed. The last chip will get less cooling than the first chip, proportional to the amount of cooling the previous chips received. Delta T doesn't lie.

Whether it's significant or not, I can't know - but you want it to be significant, otherwise it's less efficient.

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4. Dylan16807 ◴[27 Aug 25 20:27 UTC] No.45044755{3}[source]▶
> depends how fasts it is moving - but it's all moving at the same speed. The last chip will get less cooling than the first chip, proportional to the amount of cooling the previous chips received. Delta T doesn't lie.

The watts of cooling per chip will be the same. The last chip in the loop will be a little warmer, but not by much in a reasonable setup. The difference in temperature between each chip and the water running across it will be the same.

And if you take a weak water supply and then split it to run in parallel, you can end up with a significant heat gradient across each waterblock which doesn't sound great either. If you have 4 high power chips please don't limit them to .25 liters per minute each.

> Whether it's significant or not, I can't know - but you want it to be significant, otherwise it's less efficient.

Keeping your fluid cool is good for long term reliability. And if you're doing that, then every block is getting cool fluid and the other details about loop layout won't matter.

5. 0x457 ◴[03 Sep 25 20:30 UTC] No.45120066{3}[source]▶
It moves fast enough that the temperature delta between first and last chip isn't significant. Not too fast, though, because if the pump is running full-tilt it becomes a major heat-source itself.

> but you want it to be significant, otherwise it's less efficient.

No? You want a large delta between heat-source and water, as well as water and whatever is on your cool side. Essentially just between hot and cold sides.

Delta between heat-sources is irrelevant unless you start cherry-picking some very odd combinations.