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I just upgraded to 12.04. I noticed a new weird behavior that did not happen on 11.10.

When I lock the screen, or when it sleeps automatically after the computer is idle, the CPU fan starts working hard (which usually means the CPU itself is working hard). When I return from the sleep to re-enter my password, the fan subsides.

I noticed some bugs related to compiz (Compiz makes CPU run at 100% when in suspend mode in 12.04), but I tried the suggestions there, and none seem to help.

Any ideas on how to check what's making the CPU work hard, or how to stop it from doing so?

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Sometimes background processes are specifically set to run only when the system is otherwise idle. I have seen this setting with anti-virus and backup software on various systems.

To find out which process is busy, you just need to open a terminal window and run 'top'.

The default settings won't help much as they're designed to be watched as they run rather than read later. If you change the settings in top to update only once a minute, then make sure you wait AT LEAST a minute after the trouble starts, and stop top from running as soon as you have logged back in, there's a decent chance the problem process will be at the top of the list!

To be totally sure, I would re-run this trick at least 2 or 3 times.

It may also help to check the system log files in '/var/log'.

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I'm on Linux Mint and have what sounds like the same thing going on, but I haven't been able to figure it out, either (thread + link to bug report).

However, as a temporary fix, I've found that if I set the screen to never turn off, it doesn't happen. Of course then you'll have to manually lock and turn off your monitor if you don't want it on all the time.

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