I have a Compaq 2195US. I notice a considerable performance hit when running on batteries vs. plugged into the wall. It has an AMD ATHALON XP-M processor, which I can set to performance and fix the speed at 1855 MHz. On wall power, with nothing on my desktop open, and at that speed, CPU usage averages around 15-20%. If I just unplug the adapter, CPU usage jumps to 50-60%??? The speed still shows as 1855 MHz, the only thing I can figure is maybe the voltage is being scaled down, but I cannot figure out how to check/affect that. It does the same thing whether I am running Windows on it or Linux, so I don't think it is an OS issue. Anyone have any ideas?

if there is a way to configure it you will find it in the BIOS also check for BIOS updates from Compaq

if that doesn't work I'm out of ideas sorry

This is almost certainly a design feature of the machine itself. Many laptops will step down the CPU when the machine in on battery power in order to extend the battery's charge. This is actually a desirable thing, as you'll be able to use the PC for a longer period of time than if it were running at full speed. (Along those lines, most laptops will also dim the display, etc, again to save power)

You are correct though that this is not Linux issue

There may be a BIOS setting that turns that feature off, but as already mentioned, your battery life will be substantially less.

There is no setting in the BIOS. It would be nice to be able to throttle up if needed, instead of the laptop deciding that it is going to extend the battery life. Most of the time, I would rather have 1 hour of full scale speed than 3 hours of reduced performance. You are correct, the display does dim automatically when I unplug from the wall, but I have control over that if I want to bring it back up. I can find no way to bring the processor performance back up, although the clock speed never changes.

well, if the option exists, it would be somewhere in your BIOS settings. As you know, there are any number of different BIOS and versions out there, but if you've got a section related to CPU voltage/scaling options or power management options, that's the logical place for it to be. Happy hunting

This implies frequency drop, how do you know it's still at 1855 MHz?

Sounds like CPU Frequency Scaling is occurring.
Possibly the powersave governor is kicking in.

You might check out the /sys/devices/system/cpu/cpu0/cpufreq directory for clues.

I'm running superkaramba which is giving me a real time readout of the CPU frequency updated every second. Kpowersave is set to performance to "lock-in" 1855 MHz. The only thing I can figure is the processor voltage is being dropped. I read somewhere about hacking the DSDT table to underpower a processor to squeeze more power out of the batteries. I guess it could be done in reverse, although that is a little more than I would want to take on. Everything on the laptop works with the exception of suspend to ram and the one-touch buttons. Suspend to disk works, although it is a bit spotty at times.

Are these KDE things trustworthy? Performance governor is in kernel. Run cpufreq-info from command line. This will tell you which governor is active and lots more.