Vista power management & CPU frequency
For those that have taken the leap and moved to Windows Vista, you might have noticed that when you click the battery icon in your start bar, you now have three batter profiles to choose from: Balanced, Power saver and High performance.
Whilst you can explore the different settings that let you control when the PC turns off your screen, your hard drives or puts your computer to sleep, for example, it might not be quite as apparent how your machine's CPU is affected by these different profiles.
If you open the Power Options dialog and hit "Change plan settings" for any of the plans and then hit "Change advanced power settings", you'll see a number of knobs you can tweak.
One of these settings in particular is the Processor power management item. If you expand out the "Processor power management" settings, you'll see values similar to the following:
But how does this map out in the real world? Does Vista REALLY enforce these limits in the CPU? Let's take a look. First of all, I open up the Resource Monitor. Then, I set the power profile to "High performance" - here's the CPU chart (note - the blue line shows you the CPU frequency - the green line shows you CPU utilization):
As you can see, my CPU's frequency is indeed pegged at 100% even though the CPU utilization is only averaging around 27%. This is because I am plugged in - what happens when I unplug the power? You can see that Vista automatically enters a state where it varies the CPU's operating frequency based on CPU utilization and that the operating frequency pretty closely tracks the CPU utilization.
Now, this behavior should be pretty close to what happens with "Balanced" mode, right? Absolutely! In the chart below, we can see that the CPU's frequency tracks the CPU's utilization, never dropping when the CPU is doing more work:
Note that this chart was captured from my machine whilst it was plugged in, and operates within the frequency tolerances dictated in settings we saw earlier.
But what happens when we go into "Power saving mode"?
Well, as we can see - Vista caps the operating frequency of the CPU to a maximum of 40% regardless of how busy the CPU might be.
This clearly shows that Vista's power management works a treat. Tips: if you're doing anything fairly CPU intensive note that your CPU will take longer to complete the task if on "Power saver" mode. If you need your machine to work as quickly and as responsively as possible, use "High performance" mode, and if you want the best of both worlds, choose "Balanced"!
Duh! ;)
Oh, and don't forget - you can, if you should so wish, go tweak your power profiles to give you the best performance when plugged into the power and to back off when on battery ... take it from me - this comes in real handy when trying to resuscitate a machine whose battery is on its last legs!
