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Why doesn't the "% CPU" in System Monitor add up to 100? (% CPU is the third column on the following screenshot)

cpu time

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3 Answers 3

up vote 7 down vote accepted

Two reasons:

  1. The calculations are per-core. So if you have a dual-core system then the maximum will be 200%.
  2. You will not necessarily see all processes which are using your CPU in System Monitor, so the CPU usage might be below 100% for each processor.

Furthermore it is unusual for 100% of any processor to be in use. See the resources tab for the total in use per processor.

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1  
Ah ok, I'm on a dual core that's why! A bit unintuitive though. Thx. –  Olivier Lalonde Dec 5 '10 at 1:22

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.

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2 cores. Nice piping btw :) –  Olivier Lalonde Dec 5 '10 at 1:23

There is more information regarding this issue at the askubuntu link below (a question I asked):

'htop' does not show the correct CPU%, but 'top' does.

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