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I have a machine with Intel(R) Xeon(R) CPU E5520 visible as 16 logical cores (8 with HT). If I run something to eat CPU time (single threaded) top correctly reports 100% CPU usage for the process but only around 3-4% total usage. 

Cpu(s):  3.6%us,  0.0%sy,  0.0%ni, 96.4%id,  0.0%wa,  0.0%hi,  0.0%si,  0.1%st

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 1567 www       20   0 65632   84    4 R  100  0.0   0:05.96 burnMMX

Surely the reported usage should be ~6.25% (1 of 16 cores).

  • Is it incorrect because it's a virtual machine?
  • How could I calculate a correct value?
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    you may try atop and change the top to report usage of each CPU individually (by pressing 1)
    – Hubert Kario
    Commented Jan 7, 2012 at 15:07
  • @HubertKario the usage reported for each core is correct so why then is the total wrong?
    – Yotaware
    Commented Jan 7, 2012 at 15:09
  • If the usage levels of individual cores are correct then this would suggest a bug in top as unlikely as it sounds...
    – Hubert Kario
    Commented Jan 7, 2012 at 15:12
  • This would not be the first post about incorrect top results in VMs. Can you describe your VM-technology and your host and guest operating systems?
    – Nils
    Commented Jan 7, 2012 at 20:57
  • @Nils Xen with an Ubuntu/2.6.32-21-server guest. No information about the host OS.
    – Yotaware
    Commented Jan 7, 2012 at 21:20

2 Answers 2

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top in the summary reports usage relative to the CPU with all cores added up. On the other hand, when you look in the list of processes, top reports percent referenced to one core (real or HT). Try running a process with 4 threads that can eat up 4 CPU cores, you will find it using 400% CPU.

I can't tell why, but that's how top works on all Linux system I've seen.

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Most often top works in 'Irix' mode by default, just as you describe it. If you want to obtain a percentage of the total CPU capabilities (i.e. when the sum of the possible CPU usage adds up to 100% and not cpu_cores_number * 100%), just toggle the 'Irix' mode by pressing I.

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