Hi, I am trying to save power during low-load times on a dual quad-core 64-bit workstation by using CPU hotplugging. I recently switched to 64-bit Kubuntu Intrepid (2.6.27-11-generic, KDE 4.2), and this is one of the last few things I can't get to work.

All CPUs (except cpu0) do have an "online" entry under /sys/devices/system/cpu/cpu*/, but Powerdevil says there's no "CPU can be turned off" capability. The command-line way works,

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$ sudo sh -c "echo 0 > /sys/devices/system/cpu/cpu7/online"

but I'd like to get it working in Powerdevil so I can create a 2-cpu profile to use when I don't need all 8.

I'm not sure how to troubleshoot this and haven't found anything yet by searching.
Thanks much for any help.

Powerdevil uses Solid to get information about your system, which in turn uses HAL I believe. Please check that HAL returns the information needed.

Yes, you're right. I'm a novice with HAL, but I think it doesn't do CPU hotplug yet, based on:

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$ hal-device | grep cpu
  info.capabilities = { 'cpufreq_control' } (string list)
  info.addons = { 'hald-addon-cpufreq', 'hald-addon-acpi' } (string list)

Also, from the KDE 4.2 source:

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00351 bool HalPower::canDisableCpu(int /*cpuNum */) const
00352 {
00353     return false;
00354 }


Looks to me like the "turn off CPU" UI in Powerdevil is just a placeholder. Can anyone disable CPUs from the KDE Power Management UI?
I know for sure that my kernel supports CPU hotplug and it works from the commandline (offlined CPU disappears from /proc/interrupts).

Maybe I'll try a script workaround, unless someone tells me that I'm way off base.
Thanks for your help!

At this time it certainly looks like that. You will probably need to implement a script based method unforunately.