Some chipsets are designed for throttle-back, while others (especially older ones, predating 2007) always work at full pace and power full time. Only in the past three years have processors shipped from Intel, Advanced Micro Devices, Via Technology, and others in the x86 family have been specifically designed to cycle between fast (and higher power consuming) and slow (power savings) states.

The systems used in our testing ship with Intel Xeon multi-core CPUs, which can support throttle-back (manifested by slowing down the CPU clock so that the power strobes through the CPU more slowly against the slowed clock); but as we stated earlier the server's BIOS and firmware must be sufficiently upgraded to correctly supported this. The IBM x3550 and HP DL-160 house a single quad-core CPU, while the Dell 1950 and HP DL360G5 each housed two quad-core CPUs for a total of eight cores.

Once throttled back, the millions of transistors in CPUs can turn back on almost spontaneously at virtually the speed of the CPU (we could detect no latency issues in our testing), and can throttle-back down at nearly the same speed. The throttle back condition can save quite a bit of power in the four systems we tested, but most other electronics within the system remain on and therefore continue to consume power. Because the system must be ready to service application requests, it must have at minimum, some electronics running to monitoring application, user, network and other peripheral service requests. This minimal amount of power drawn is what you see measured in our quiescent state (sometimes referred to as minimal ready-state) results.

Operating systems must allow a CPU to throttle back to this minimal ready state, to be considered green from the power consumption perspective, and both Linux and Windows allow for this. However, there is a 'tickless' version of Linux on the horizon that may prove to have power savings characteristics. System interrupt ticks are 'time slices' that the operating system uses to queue activities, and they've been traditionally set in the past half dozen plus years to a 1,000 ticks per second, each of which serves as an interruption to the CPU. A tickless version of the Linux kernel now reportedly exists that interrupts the CPU less frequently, but was not part of the Linux distribution kernels we tested -- although that addition is planned in future editions of Red Hat and SUSE.

Choosing which level is green enough for your servers

In advance of setting up and running our testing, we talked with Novell/SUSE, Red Hat and Microsoft regarding their respective green initiatives.

We also asked IBM, HP, and Dell to supply the server samples they believed promoted their fullest green potential, although measuring the hardware elements was not our primary point here. There is much contention in the server marketplace over power supply efficiency, and other hardware energy conservation initiatives that often use vendor-specific hardware management APIs that have nothing to do with the deployed operating system. We weren't interested in a server hardware 'bake-off' that measured power consumption of the hardware elements, as this is the operating systems view of consumption.

Windows 2008 Server and Windows Vista power saving modes are essentially identical. They allow a system to fall back to increased resting states (principally in CPU power consumption and disk hibernation). These modes fit the Advanced Configuration and Power Interface V3 support that is more generally associated with personal computer, rather than server application use.

There are three states to these Windows power plans -- Power Savings, Balanced and High Performance -- which are selected through the Windows Control Panel Power Settings options. These options can also become enforced through the Active Directory through group policies. A program, powercfg.exe is also available to help establish very highly detailed performance policy settings, but the nearly endless permutations available with that executable were clearly beyond the scope of this test.