There are certain popular system architectures that transcend the operating system and will turn your efforts to build a green network operating system brown, figuratively speaking.

Hypervisors, the underlying technology used to permit consolidation of software servers into virtual machines, uniformly push CPU pedals to the floor and never, ever let up.

Hypervisors thwart CPU green initiatives (especially ones where the operating system plays a role in conserving overall energy consumption by throttling back the CPU during low activity periods) because they subscribe to the get-the-best-performance-per-watt-used school of thought.

The hypervisors we tested from VMware (ESX 3.X), XenSource Xen, and Microsoft's Virtual Server (not Hyper-V, which isn't yet released) all prohibit the ability for the CPU/chipsets to reduce speed or go into 'green mode' as long as there's a virtual machine guest running atop the hypervisor — regardless of operating system flavor.

Hypervisors allocate system resources as specified by administrative constraints imposed on VM guest operating systems and applications. The action of actively monitoring and allocating specified resources generates a lot of work for the CPU. Hypervisor system clock ticks, resource controls, combined with host VM guest operating system ticks, simply prevent CPUs from resting, and therefore saving power.

The argument in favor of hypervisors as an energy-saving measure is that you can run multiple guest operating systems and their applications on a single hardware platform when they had previously been housed on multiple, aged servers.