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Comment by pjc50 | Hacker News Reader

Let's say you're using 2x 10 core CPUs per 1U PC, in a 42U rack. That's 840 cores per rack, 262 racks. You won't fit that in a house, you need a moderately large datacenter.

At 100W per CPU that's 22kW, which feels low; about a tenth of a Tesla and a fraction of a plane. That doesn't account for the cooling you'll need!

(You can probably do a lot better, but that's my piano tuners estimate)

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22kW of CPU heat would need 22kW of cooling. Refrigerated air conditioning is has about 1:3 thermal efficiency, so your cooling system will require ~7.5kW electricity to generate 22kW cooling capacity.

That makes about 30kW. At $0.15 / kWh we're talking about $4.5 per hour for the electricity.

Other costs dwarf the energy costs.

If it were 10 cores/CPU then 220,000 cores is 22,000 CPUs. At 100W per CPU that's 2,200,000W = 2.2MW. So more like a train than a Tesla!

So ~$300/hour in electricity.

Yes, and based on the $0.15/kWh comment, about $450/hour of electricity.

This is more in line with what I would have guessed for 220k cores.

Note that 22kW is right about what a Tesla will consume cruising at highway speeds. So you could label that as "an efficient cruising car."

I would be shocked if Google wasn't getting 2x 32 core CPUs per 1U at a minimum, which brings down your rack count a bit.

Even if those were ARM cores (currently 192 cores/U @ 1.8 GHz), it would still take about 30 racks. ARM sucks at single-core performance, but it handles parallel workloads very well.

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