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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.
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.
We chose to test the Windows Power Savings and High Performance power plans because they offer the highest comparability to power consumption parameters available in Linux — a full throttle performance mode and a rational, low-latency power savings mode.
The energy saving choices available with the Linux 2.6 kernels shipped with RHEL 5.1 and SUSE Enterprise Linux 10 center on the ability to throttle back CPU clock speed through a kernel module called cpufreq. These modes are called "governors", and are officially referred to as performance, ondemand, powersave, and conservative. There is a fifth, user-space governor, but it homes in on only specific, policy-defined root objects, and we're testing the operating system rather than discrete processes governed by the operating system.