Here is another example CPU throttling from someone who had a server running with the default “Balanced” Power Plan in Windows Server 2008 R2 (which works the same way in Windows Server 2008).
As you can see in Figure 1 below, the Core Speed with the Balanced Power Plan is 1599.5 MHz, because the operating system has reduced the multiplier to 12, which reduces the clock speed from the rated speed of 2.53 GHz. This is done in the interests of minimizing the electrical usage of the CPU(s) in the server, but it has the unfortunate side effect of also reducing CPU and memory performance in some benchmarks (and in real life, especially with OLTP workloads). The performance reduction on Geekbench is typically around 20% in my testing.
Balanced Power Plan
Figure 1: Core Speed of 1599.5 MHz on Intel Xeon E5540
In Figure 2 below, we see the multiplier at 21, with a resultant Core Speed of 2799.1 MHz, which is actually above the rated speed of 2.53 GHz. This is due to the effect of the first generation Turbo Boost technology in the Nehalem-EP processor, which boosts the speed of some cores when the overall processor is not working that hard. This technology is enhanced with Turbo Boost 2.0 in Sandy Bridge to be much more aggressive based on the overall temperature of the system.
High Performance Power Plan
Figure 2: Core Speed of 2799.1 MHz on Intel Xeon E5540
You can run CPU-Z on individual systems to detect this, but that can be time consuming if you have dozens or hundreds of servers to manage. Fellow SQL MVP Aaron Bertrand (blog|twitter) has a nice article about how to use PBM to detect servers that are using the Balanced Power Plan. Greg Gonzalez also recently blogged about how to use Group Policy to enforce the use of the High Performance Power Plan for your servers.
Don’t forget that you may need to also go into your main system BIOS and change the Power Management settings there to get your processors to run at full speed all of the time (which will require a reboot). I always try changing the Windows Power Plan first, and if that does not work, then you need go into your main BIOS and spelunk around to find the right settings to either disable power management or set it to OS Control.
Finally, if you have a few minutes, run CPU-Z on a system or two, and send me a screenshot of the results. My e-mail address is glennb at newsgator dot com (which might slow down the spambots). I always like adding to my CPU-Z screenshot collection!