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I am using freescale IMX6 quad processor. I want to know if the top command lists the CPU usage of all 4 cores or of a single core. I am seeing an application's CPU usage being the same with 4 cores and with a single core. I was guessing the CPU usage by the application will increase on a single core and decrease on 4 cores but it has not changed.

Jeff Schaller
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user3818847
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    try pressing 1 while top is running – Dani_l Jul 23 '14 at 10:57
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    Could you please elaborate. As in what happens if I press 1. Because I am getting this inconsistent result since 2 days. – user3818847 Jul 23 '14 at 10:59
  • you may find this link useful: http://unix.stackexchange.com/questions/41311/cpu-and-core-usage-stats – TPS Jul 23 '14 at 11:05
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    Which application is that? Why do you think your application should use multiple cores if available? There are many applications out there that work on a single CPU/core and for which nobody bothered to take the time to parallize them. – Anthon Jul 23 '14 at 11:05
  • @Anthon well that's the question, if top gives you a percentage of 4core or a percentage of a core, because if the percentage comes from 4 cores, then the value (even with a non threaded app should be 4 time fewer – Kiwy Jul 23 '14 at 11:27
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    no. if multiple cores, they accumulate to over 100%. 4 cores can get as high as 800% with hyperthreading on each core – Dani_l Jul 23 '14 at 11:35
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    I like to use `htop` for this. – Richard Apr 25 '17 at 01:34
  • pressing 1 does not help is I grep a specific process. i.e top | grep followed by 1 does not show cpu core info – Naveen Jan 02 '19 at 18:23
  • Then `top | awk '/(Cpu|)/'` will show you Cpu and that one process. Then press `1` to list Cpus separately. But `top` is not designed for piping. You can specify list of pids (by number) on command line. But Cpu is still total of all processes, not just the ones you list. – Paul_Pedant Jul 18 '20 at 21:54
  • related post https://stackoverflow.com/questions/3342889/how-do-i-measure-separate-cpu-core-usage-for-a-process – red0ct May 31 '23 at 15:21

3 Answers3

106

I'm not entirely sure what you're asking here. Yes, top shows CPU usage as a percentage of a single CPU by default. That's why you can have percentages that are >100. On a system with 4 cores, you can see up to 400% CPU usage.

You can change this behavior by pressing I (that's Shift + i and toggles "Irix mode") while top is running. That will cause it to show the pecentage of available CPU power being used. As explained in man top:

    1. %CPU  --  CPU Usage
       The task's share of the elapsed CPU time since the last screen
       update, expressed as a percentage of total  CPU  time.   In  a
       true  SMP environment, if 'Irix mode' is Off, top will operate
       in 'Solaris mode' where a task's cpu usage will be divided  by
       the  total  number  of  CPUs.  You toggle 'Irix/Solaris' modes
       with the 'I' interactive command.

Alternatively, you can press 1 which will show you a breakdown of CPU usage per CPU:

top - 13:12:58 up 21:11, 17 users,  load average: 0.69, 0.50, 0.43
Tasks: 248 total,   3 running, 244 sleeping,   0 stopped,   1 zombie
%Cpu0  : 33.3 us, 33.3 sy,  0.0 ni, 33.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu1  : 16.7 us,  0.0 sy,  0.0 ni, 83.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu2  : 60.0 us,  0.0 sy,  0.0 ni, 40.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu3  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   8186416 total,  6267232 used,  1919184 free,   298832 buffers
KiB Swap:  8191996 total,        0 used,  8191996 free,  2833308 cached
terdon
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    with hyperthread I believe you can see up to 800% as /proc/cpuinfo will show each thread as a cpu – Dani_l Jul 23 '14 at 11:37
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    @Dani_l yes, whether the "core" is physical or virtual is irrelevant, it is treated as a "CPU" by `top`. The output I show is from my laptop which has a single physical CPU with two cores, each of which has a 2nd logical core. The result is that `top` sees 4 cores. – terdon Jul 23 '14 at 11:44
  • Sorry for the nitpicking, in my dayjob we have to distinguish between sockets, cores and threads when reserving resources. I guess the habit stuck. – Dani_l Jul 23 '14 at 11:48
  • so is it correct to assume that if 'nvproc --all' returns 20 that the CPU % reported by htop per-process should be divided by 20 to get the actual percentage of total CPU capability available? e.g. htop reporting 54.2% for a process in this context would actually represent 2.71% total CPU usage by that process? – CCJ Dec 10 '20 at 01:57
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    @CCJ `htop` should show a breakdown by CPU anyway (this was about `top`), but yes, in the percentage column you should divide by the number of available cores. – terdon Dec 10 '20 at 14:24
15

If you're wanting to open top immediately displaying separate CPUs without needing to press 1, you can use the -1 option.

e.g.:

top -1

...
%Cpu0  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu1  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu2  : 44.7 us, 55.3 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu3  : 46.7 us, 53.3 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
...

Note: This works on Debian, but the variant of top installed may vary depending on distro.

Leigh McCulloch
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5

If you want top command to display per CPU usage everytime you run top.

  • run top command
  • press 1, this will display per CPU usage
  • Type W and press Enter to save configuration to file
  • Next time running top will display per CPU usage.
  • This way we can configure top to our custom requirement

(Above steps works with top version procps-ng 3.3.12)

ChethanSuresh
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