[dietrichepp]

I didn't get the impression at all that Facebook was "dumping" Xeon, merely that they were adding ARM.

Throughput is not really relevant. Throughput per unit power is better. ARM manufacturers have been optimizing for power consumption and unit cost for a long time, Intel has been optimizing for throughput and speed for a long time. Both sides are aiming for the same target, from different angles. A "low power" Xeon might be 40W for four cores, but the typical range is 60W - 140W. Calxeda sells a sixteen-core, 20W ARM board. By these numbers, Xeon needs to have 8x throughput of the ARM to beat it in throughput per watt.

Now, this is not entirely unreasonable. I've done benchmarking of my Core i5 against my Atom on an audio sample rate conversion library I wrote, and the Core i5 has 6-7x the throughput of the Atom. So, I would believe you if you said that a Xeon core is more than 8x as fast as an ARM core, and therefore the throughput per watt is better. This is all guessing, based on one benchmark I did for a library I wrote. I'm sure that Facebook has done some experiments with the applications they'll actually be running on the ARM.

I'm not sure where you get the idea that these ARM computers will be centered around specialized hardware, Calxeda's website only mentions general-purpose hardware built around ARM cores.