One of my least favorite activities as a support engineer was trying to sift through the debris pile that is the
ps command. Inevitably, a Java process would error out and get stuck on a port that I needed open to restart a service. The service would show as up, but there was no functionality. This situation was confusing to junior techs and was something that I was asked to correct many, many times by finding the process that was stuck and killing it. There are also times where you may need to see if a particular process is running or find out if a process has spawned child processes. These scenarios are where understanding control groups will help tremendously. Control groups, at a basic level, organize processes based on the parent and then organize processes into a hierarchy.
Here, we will look at two ways to improve on the standard
ps command that most people use. I know many people pair
grep, and like pecan pie and Noah's Mill, I fully endorse this practice. I also encourage you to check into the following two methods, as they can make understanding process hierarchies a bit easier.
The first method is a standard
ps command with the process tree enabled. When you run this command:
[root@fed31 ~]# ps xawf -eo pid,user,cgroup,args
the following output might be the result:
This output has been shortened as it is very verbose. However, you receive a large amount of information with this command. From left to right, we have the PID, USER, CGROUP, and COMMAND.
I find that this output is a great way of seeing exactly which process belongs to which parent job. It's also in a fairly user-friendly format. The command itself can be a lot to type out on a regular basis, so I recommend setting up a shell alias. For more information on how to create an alias, see my article on the subject here. For now, something like this would be appropriate:
alias pscgroup='ps xawf -eo pid,user,cgroup,args'
The next option we will look at is a
systemd utility. This method is an even better way, in my humble opinion, to see which job belongs to which parent process or owner. When you type this:
[root@fed31 ~]# systemd-cgls
the result might look like this:
Again, the screenshot has been shortened due to the length of the output. In this version, you can more easily see which process belongs to which user, plus each process shows its child processes as subsets of information. This method is a super simple way to see the organization of processes and find ownership quickly.
These two methods of viewing processes helped me to get a grasp of what owns what. Hopefully, you will find these helpful as well!
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