Oct 6, 2012

sysstat vs top vs ps (II)

Following up on the previous article, sysstat vs top vs ps (I), a curious case that I would like to talk about is when you use more than one core. Let's create a simple script in Python which runs a couple of threads a little bit overloaded.

root@ubuntu-server:~# nproc 
2

root@ubuntu-server:~# cat script.py 
#!/usr/bin/python

import threading, time

def sleep():
    while True:
        time.sleep(0.000001)

t1 = threading.Thread(target=sleep)
t2 = threading.Thread(target=sleep)

t1.start()
t2.start()

t1.join()
t2.join()

root@ubuntu-server:~# ./script.py &
[1] 1561

If we take a look now at the status of this process by means of top, we can see as follows.

root@ubuntu-server:~# top -b -n 1 -p 1561
top - 19:41:19 up 24 min,  1 user,  load average: 0.74, 0.66, 0.48
Tasks:   1 total,   0 running,   1 sleeping,   0 stopped,   0 zombie
Cpu(s):  5.3%us, 39.7%sy,  0.0%ni, 55.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   1024800k total,   205596k used,   819204k free,    23192k buffers
Swap:  1048572k total,        0k used,  1048572k free,    98712k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 1561 root      20   0 29280 4180 2416 S  129  0.4  26:23.26 script.py

So what would be the first weird thing that you can observe from the previous screen? The script is consuming 129% of the CPU. This is right because you have to remember that this virtual machine has two cores and the script, rather, its two threads, are using the two ones and that figure is the combination of the CPU utilization from both cores. You can appreciate this situation much better if you execute top with the -H option.

root@ubuntu-server:~# top -b -H -n 1 -p 1561
top - 19:44:22 up 27 min,  1 user,  load average: 0.69, 0.72, 0.54
Tasks:   3 total,   2 running,   1 sleeping,   0 stopped,   0 zombie
Cpu(s):  4.3%us, 38.9%sy,  0.0%ni, 56.8%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   1024800k total,   205624k used,   819176k free,    23192k buffers
Swap:  1048572k total,        0k used,  1048572k free,    98712k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 1563 root      20   0 29280 4180 2416 R   65  0.4  15:10.19 script.py
 1562 root      20   0 29280 4180 2416 R   64  0.4  15:12.39 script.py
 1561 root      20   0 29280 4180 2416 S    0  0.4   0:00.05 script.py


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