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I have been running for a day at 100% CPU and suddenly seems that for the same tasks as before its usage has been limited to 50%. I suspect that overheating may be the cause.

I am using 11.10. Is this throttle automated?

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No, Linux does not do this. Theoretically your hardware is supposed to, but I have not heard of any that does. You may be able to run sensors to check the temperature and see if it is getting too high ( near 100 C ).

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Are you sure Linux does not do this? I've run processes that take up all available CPU cycles on a machine that had bad cooling, and found the clock speed as shown in /proc/cpuinfo decrease when I did. I was able to increase it back manually, using cpufreq-selector, which I consider to suggest (but not prove) that it was Linux and not my machine that had been throttling back. (Subsequently I blew the dust out of the machine, and this decrease no longer occurs.) – Eliah Kagan Jun 11 '12 at 2:28
I do not see any clock speed reduction in /proc/cpuinfo. The cpu is a P4@3.4GHz and the cpu MHz is 3391.607. The sensors program tells me that no sensors can be detected. After reboot same behaviour. I now running a query script with nice -15 which is a lot better with average cpu usage 70-85%. But again doesn't seems to stay at 100% for a long time. – Alex Jun 11 '12 at 5:26

are you sure it was not a faulty process eating up all your resources before? Unless you are compiling code, video editing or things alike it is difficult to use 100% CPU, even less nowadays with dual cores or quadcores.

try again after a reboot...if you get 50% again it was a ghost process that was taking the CPU time.

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100% CPU on a multiple core CPU means that you use the equivalent of one core. A quad core with all four cores used to the limit would show 400% CPU. – JanC Jun 11 '12 at 0:09
@JanC Right. This is a matter of confusion for many users coming from Windows, since in Windows the percentage values for CPU resource utilization reflect the portion of all available CPU's. – Eliah Kagan Jun 11 '12 at 2:26
No this is not the case. I am running some SQL queries...! – Alex Jun 11 '12 at 4:37

Canonical claimed to had fixed the unity in 11.10. But it still din't work well. For me, it was unity destkop environment which was eating up the processor all the time. The temperature of cores used to be high (68+) all the time.. Compiz was another role player in throttling the cpu.
You may check if its the case for you. You may try different desktop environment say light weight version of desktop environments. Examples would be DWM (from or Awesome (a light weight DE, a fork of DWM). If that helps, I would suggest to install 12.04 in which, unity seems to have become stable and less annoying.

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