Why does top report the incorrect CPU usage?

Question

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?

Solution 1

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.

Solution 2

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.

Comments

[Admin]

you may try atop and change the top to report usage of each CPU individually (by pressing 1)

[Admin]

@HubertKario the usage reported for each core is correct so why then is the total wrong?

[Admin]

If the usage levels of individual cores are correct then this would suggest a bug in top as unlikely as it sounds...

[Admin]

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?

[Admin]

@Nils Xen with an Ubuntu/2.6.32-21-server guest. No information about the host OS.

[Admin]

use htop utility.