10 Notes on selecting processes in proc.mem and proc.num items

Processes modifying their commandline

Some programs use modifying their commandline as a method for displaying their current activity. A user can see the activity by running ps and top commands. Examples of such programs include PostgreSQL, Sendmail, Zabbix.

Let’s see an example from Linux. Let’s assume we want to monitor a number of Zabbix agent processes.

ps command shows processes of interest as

  1. $ ps -fu zabbix
  2.        UID        PID  PPID  C STIME TTY          TIME CMD
  3.        ...
  4.        zabbix    6318     1  0 12:01 ?        00:00:00 sbin/zabbix_agentd -c /home/zabbix/ZBXNEXT-1078/zabbix_agentd.conf
  5.        zabbix    6319  6318  0 12:01 ?        00:00:01 sbin/zabbix_agentd: collector [idle 1 sec]                          
  6.        zabbix    6320  6318  0 12:01 ?        00:00:00 sbin/zabbix_agentd: listener #1 [waiting for connection]            
  7.        zabbix    6321  6318  0 12:01 ?        00:00:00 sbin/zabbix_agentd: listener #2 [waiting for connection]            
  8.        zabbix    6322  6318  0 12:01 ?        00:00:00 sbin/zabbix_agentd: listener #3 [waiting for connection]            
  9.        zabbix    6323  6318  0 12:01 ?        00:00:00 sbin/zabbix_agentd: active checks #1 [idle 1 sec]                   
  10.        ...

Selecting processes by name and user does the job:

  1. $ zabbix_get -s localhost -k 'proc.num[zabbix_agentd,zabbix]'
  2.        6

Now let’s rename zabbix_agentd executable to zabbix_agentd_30 and restart it.

ps now shows

  1. $ ps -fu zabbix
  2.        UID        PID  PPID  C STIME TTY          TIME CMD
  3.        ...
  4.        zabbix    6715     1  0 12:53 ?        00:00:00 sbin/zabbix_agentd_30 -c /home/zabbix/ZBXNEXT-1078/zabbix_agentd.conf
  5.        zabbix    6716  6715  0 12:53 ?        00:00:00 sbin/zabbix_agentd_30: collector [idle 1 sec]                          
  6.        zabbix    6717  6715  0 12:53 ?        00:00:00 sbin/zabbix_agentd_30: listener #1 [waiting for connection]            
  7.        zabbix    6718  6715  0 12:53 ?        00:00:00 sbin/zabbix_agentd_30: listener #2 [waiting for connection]            
  8.        zabbix    6719  6715  0 12:53 ?        00:00:00 sbin/zabbix_agentd_30: listener #3 [waiting for connection]            
  9.        zabbix    6720  6715  0 12:53 ?        00:00:00 sbin/zabbix_agentd_30: active checks #1 [idle 1 sec]                   
  10.        ...

Now selecting processes by name and user produces an incorrect result:

  1. $ zabbix_get -s localhost -k 'proc.num[zabbix_agentd_30,zabbix]'
  2.        1

Why a simple renaming of executable to a longer name lead to quite different result ?

Zabbix agent starts with checking the process name. /proc/<pid>/status file is opened and the line Name is checked. In our case the Name lines are:

  1. $ grep Name /proc/{6715,6716,6717,6718,6719,6720}/status
  2.        /proc/6715/status:Name:   zabbix_agentd_3
  3.        /proc/6716/status:Name:   zabbix_agentd_3
  4.        /proc/6717/status:Name:   zabbix_agentd_3
  5.        /proc/6718/status:Name:   zabbix_agentd_3
  6.        /proc/6719/status:Name:   zabbix_agentd_3
  7.        /proc/6720/status:Name:   zabbix_agentd_3

The process name in status file is truncated to 15 characters.

A similar result can be seen with ps command:

  1. $ ps -u zabbix
  2.          PID TTY          TIME CMD
  3.        ...
  4.         6715 ?        00:00:00 zabbix_agentd_3
  5.         6716 ?        00:00:01 zabbix_agentd_3
  6.         6717 ?        00:00:00 zabbix_agentd_3
  7.         6718 ?        00:00:00 zabbix_agentd_3
  8.         6719 ?        00:00:00 zabbix_agentd_3
  9.         6720 ?        00:00:00 zabbix_agentd_3
  10.         ...

Obviously, that is not equal to our proc.num[] name parameter value zabbix_agentd_30. Having failed to match the process name from status file the Zabbix agent turns to /proc/<pid>/cmdline file.

How the agent sees the “cmdline” file can be illustrated with running a command

  1. $ for i in 6715 6716 6717 6718 6719 6720; do cat /proc/$i/cmdline | awk '{gsub(/\x0/,"<NUL>"); print};'; done
  2.        sbin/zabbix_agentd_30<NUL>-c<NUL>/home/zabbix/ZBXNEXT-1078/zabbix_agentd.conf<NUL>
  3.        sbin/zabbix_agentd_30: collector [idle 1 sec]<NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL>...
  4.        sbin/zabbix_agentd_30: listener #1 [waiting for connection]<NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL>...
  5.        sbin/zabbix_agentd_30: listener #2 [waiting for connection]<NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL>...
  6.        sbin/zabbix_agentd_30: listener #3 [waiting for connection]<NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL>...
  7.        sbin/zabbix_agentd_30: active checks #1 [idle 1 sec]<NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL><NUL>...

/proc/<pid>/cmdline files in our case contain invisible, non-printable null bytes, used to terminate strings in C language. The null bytes are shown as “<NUL>” in this example.

Zabbix agent checks “cmdline” for the main process and takes a zabbix_agentd_30, which matches our name parameter value zabbix_agentd_30. So, the main process is counted by item proc.num[zabbix_agentd_30,zabbix].

When checking the next process, the agent takes zabbix_agentd_30: collector [idle 1 sec] from the cmdline file and it does not meet our name parameter zabbix_agentd_30. So, only the main process which does not modify its commandline, gets counted. Other agent processes modify their command line and are ignored.

This example shows that the name parameter cannot be used in proc.mem[] and proc.num[] for selecting processes in this case.

Using cmdline parameter with a proper regular expression produces a correct result:

  1. $ zabbix_get -s localhost -k 'proc.num[,zabbix,,zabbix_agentd_30[ :]]'
  2.        6

Be careful when using proc.mem[] and proc.num[] items for monitoring programs which modify their commandlines.

Before putting name and cmdline parameters into proc.mem[] and proc.num[] items, you may want to test the parameters using proc.num[] item and ps command.

Linux kernel threads

Threads cannot be selected with cmdline parameter in proc.mem[] and proc.num[] items

Let’s take as an example one of kernel threads:

  1. $ ps -ef| grep kthreadd
  2.        root         2     0  0 09:33 ?        00:00:00 [kthreadd]

It can be selected with process name parameter:

  1. $ zabbix_get -s localhost -k 'proc.num[kthreadd,root]'
  2.        1

But selection by process cmdline parameter does not work:

  1. $ zabbix_get -s localhost -k 'proc.num[,root,,kthreadd]'
  2.        0

The reason is that Zabbix agent takes the regular expression specified in cmdline parameter and applies it to contents of process /proc/<pid>/cmdline. For kernel threads their /proc/<pid>/cmdline files are empty. So, cmdline parameter never matches.

Counting of threads in proc.mem[] and proc.num[] items

Linux kernel threads are counted by proc.num[] item but do not report memory in proc.mem[] item. For example:

  1. $ ps -ef | grep kthreadd
  2.        root         2     0  0 09:51 ?        00:00:00 [kthreadd]
  4.        $ zabbix_get -s localhost -k 'proc.num[kthreadd]'
  5.        1
  7.        $ zabbix_get -s localhost -k 'proc.mem[kthreadd]'
  8.        ZBX_NOTSUPPORTED: Cannot get amount of "VmSize" memory.

But what happens if there is a user process with the same name as a kernel thread ? Then it could look like this:

  1. $ ps -ef | grep kthreadd
  2.        root         2     0  0 09:51 ?        00:00:00 [kthreadd]
  3.        zabbix    9611  6133  0 17:58 pts/1    00:00:00 ./kthreadd
  5.        $ zabbix_get -s localhost -k 'proc.num[kthreadd]'
  6.        2
  8.        $ zabbix_get -s localhost -k 'proc.mem[kthreadd]'
  9.        4157440

proc.num[] counted both the kernel thread and the user process. proc.mem[] reports memory for the user process only and counts the kernel thread memory as if it was 0. This is different from the case above when ZBX_NOTSUPPORTED was reported.

Be careful when using proc.mem[] and proc.num[] items if the program name happens to match one of the thread.

Before putting parameters into proc.mem[] and proc.num[] items, you may want to test the parameters using proc.num[] item and ps command.