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.

We were able to initially test all of the servers in all of the savings modes that the cpufreq module supports to determine which would be most applicable to our test. We chose performance for our full throttle tests and ondemand for our power saving mode.

We rejected conservative mode because it introduces unnecessary latency to a server in random-accessible, 24/7 operations service. And we rejected the powersave mode because it slows down the processor, and everything takes longer -- not a representative application for this test as there aren't many situations where 'stepping on the garden hose' makes sense, except in periodic batch processing applications and other places where you're simply willing to wait much longer for a required result.

Both Linux's cpufreq kernel module and Windows' power settings can be changed dynamically if needed, although we didn't change them during the duration of our tests.

We chose two tests to measure power consumption. The first was a quiescent server test in which each operating system and hardware server pair sat idle for four hours in both performance (high power use) and the power savings mode for each operating system. The second active test sought to gauge consumption under load in which we sent a continuous stream of e-mails to each server and operating system pair for the duration of the four-hour test in both performance and power savings profile.

The active test used an e-mail test script to continuously send e-mails to the server and operating system pair under test. In the case of the two Linux distributions, we used sendmail/procmail as our SMTP server with 1,000 users. We installed Microsoft Exchange Server 2007 under Windows 2008 Enterprise Server Edition, imported the same 1,000 users, and assaulted it in the same way, by using scripts we generated. We let the operating system and application pick the number of cores to use as a default.

Summary results

With the IBM x3550, HP DL360G5, and Dell 1950 servers we performed our quiescent and active state tests twice: once with High Performance settings in place; and, the other with the Power Saver mode applied. For the HP DL-160G5 server, we could only complete the tests without the maximum power saving setting applied as the server crashed each time we attempted to toggle between the power saving settings. HP says a fix for this issue should be available by the time this test publishes.

In most cases when sitting idle, Windows Server 2008 drew slightly more power than either Linux did on the same server. The exception was when Windows Server 2008 was running in power savings mode on the Dell server, where it drew on average 3 per cent less power.