Sep 30, 2012

sysstat vs top vs ps (I)

I have always been using several tools to get the CPU utilization of Linux processes through different tools such as top, ps, sar, etc., but so far, I did not realise that the results obtained from them can vary considerably.

For example, I am going to run a job and measure its CPU usage afterwards. Also say that a virtual machine (Ubuntu Server 12.04) with only one core will be used.

root@ubuntu-server:~# nproc 
1

root@ubuntu-server:~# while [ 1 ] ; do sleep 0.0001 ; done &
[1] 5605

Now let's show its performance by means of top, ps and pidstat (this command belongs to the sysstat package, which also provides the sar utility, used to collect, report, or save system activity information).

root@ubuntu-server:~# top -b -n 1 -p 5605 | grep bash | awk '{print $9}'
37.9

root@ubuntu-server:~# ps -eo pid,pcpu | grep '^ 5605' | awk '{print $2}'
37.8

root@ubuntu-server:~# pidstat -u -p 5605 1 1 | grep bash | head -n 1 | awk '{print $7}'
45.12

If you go over the definitions of these measures, you can read as follows:

  • top (%CPU): the task's share of the elapsed CPU time since the last screen update, expressed as a percentage of total CPU time.
  • ps (%CPU): cpu utilization of the process in "##.#" format. Currently, it is the CPU time used divided by the time the process has been running (cputime/realtime ratio), expressed as a percentage.
  • pidstat (%CPU): total percentage of CPU time used by the task. In an SMP environment, the task's CPU usage will be divided by the total number of CPU's if option -I has been entered on the command line.

What is my opinion? All data turned out try to display the CPU utilization of a process during a period of time, but the key is that period of time taken to work out the result, and I think that for pidstat is different that for top and ps.

So my conclusion is that all aforementioned tools are correctly valid, and they will give back you a correct idea about the behaviour of a process in terms of CPU.


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