From a report on a recent study:
“The sweet spot is often at low power processors, especially when system power and price are considered.”
The examples considered include a 50 watt, 2.33 gHz CPU giving twice the performance per watt of a 120 watt 3.167 gHz CPU. No mention of ARM at all… this report contains data from M$’s own centres. They are huge. They don’t include licensing fees for that other OS because they don’t charge themselves. For the rest of the world, the price per server goes way up and the performance per dollar drops. GNU/Linux on ARM should be a no-brainer for data-centres. There are outfits now jamming hundreds of ARMed cores into 2U.