Why doesn't the "% CPU" in System Monitor add up to 100? (% CPU is the third column on the following screenshot)
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Furthermore it is unusual for 100% of any processor to be in use. See the resources tab for the total in use per processor.
Does your do you have more than one CPU core? Or Hyper-Threading (or the AMD equivalent)?
You can see how many CPUs Ubuntu thinks you have by running this:
cat /proc/cpuinfo | grep "^vendor" | wc -l
Ubuntu will count each of these. So if you have 8 CPU's (as I do, quad-core with hyperthreading), you can reach 800% CPU.
There is more information regarding this issue at the askubuntu link below (a question I asked):