Oct 21, 2012

sysstat vs top vs ps (III)

Let's finish the series of articles related to the differences in the measure of sysstat, top and ps.

What about the system CPU time (%sy) of the first top from the previous article? It is 39.7%. It is right as well. You have to take into account that, because this server has a couple of cores, that data represents the average from both cores. You can see this point by running top into interactive mode and pressing the number 1. Then you will be able to obtain the consumption for both cores.

root@ubuntu-server:~# top
top - 20:04:37 up 47 min,  1 user,  load average: 0.40, 0.54, 0.60
Tasks:  87 total,   1 running,  86 sleeping,   0 stopped,   0 zombie
Cpu0  :  2.9%us, 37.7%sy,  0.0%ni, 59.4%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu1  :  1.6%us, 40.8%sy,  0.0%ni, 57.1%id,  0.0%wa,  0.0%hi,  0.5%si,  0.0%st
Mem:   1024800k total,   207680k used,   817120k free,    23532k buffers
Swap:  1048572k total,        0k used,  1048572k free,   100320k cached

 1561 root      20   0 29280 4180 2416 S  128  0.4  56:57.23 script.py

At any rate, the sum of both percentages does not match the %CPU used by the script. This might be due to the sampling frequencies commented in the preceding article.

Finally, I would like to remark that in order to monitor threads, I like to use pidstat with the -t parameter.

root@ubuntu-server:~# pidstat -u -t -p 1561 1 1
Linux 3.2.0-30-generic-pae (ubuntu-server)     09/30/2012     _i686_    (2 CPU)

08:14:47 PM      TGID       TID    %usr %system  %guest    %CPU   CPU  Command
08:14:48 PM      1561         -    9.00  119.00    0.00  128.00     1  script.py
08:14:48 PM         -      1561    0.00    0.00    0.00    0.00     1  |__script.py
08:14:48 PM         -      1562    4.00   60.00    0.00   64.00     0  |__script.py
08:14:48 PM         -      1563    4.00   59.00    0.00   63.00     1  |__script.py

This tool is great because you can see the core where the thread (or process) is being executed.

Another interesting choice is to use directly the ps command with the suitable parameters (-L shows threads and sgi_p the processor where the process is currently executing on).

root@ubuntu-server:~# ps -Leo pid,pcpu,sgi_p,command | grep '^ 1561'
 1561  0.0 * /usr/bin/python ./script.py
 1561 65.6 0 /usr/bin/python ./script.py
 1561 65.2 0 /usr/bin/python ./script.py

Also point out for the above output that, ps does not offer the number of TID (Thread ID), in contradistinction to top and pidstat (remember that a thread does not have PID).

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