So you have a system with a processor with performance state control running Windows Vista/SP1/Server 08. On such systems, in the Control Panel "Power Options", in "Change advanced power settings" under your selected Power Plan (for example: "Balanced"), you'll see an expandable "Processor Power Management" setting, under which there is a "Minimum Processor State" and "Maximum Processor State" rated in percentages depending on the Power Plan. What can we do with this? Quite a bit: You can tweak the maximum and minimum frequency of your CPU based on the available p-states that fall within said chosen range that your CPU can run at.
Why would you not want your CPU to use all it can (100%)? Well, this may be a laptop where battery life is important, and limiting the maximum might lengthen how long your system can run off battery depending on use.
How can we see the CPU Frequency we're running at after changing the minimum and maximum percentages? Resource Monitor can help. This is available from Task Manager on the Performance tab. On a recent laptop, changing the Max Processor state setting had the following effects on running frequency:
Going further, it looked like we were hitting a P-State about every 12/13% (0-100%) for this particular CPU on this particular system. More testing:
Max Processor in Power Options vs: What we see in Resource Monitor for this CPU frequency on this system
After getting an idea of what mapped to what, we were able to set the Max to 87% on this system to cap frequency at 87%. This did mean the system ran slower with CPU intense operations, but the trade-off was not necessarily plugging in the system to grab a charge quite as often.
Want to know more? See the "Processor Power Management in Windows Vista and Windows Server 2008" whitepaper here, pages 10-11.