How to Set Limits on User Running Processes in Linux

One of the Linux’s beauties is that you can control almost everything about it. This gives a system administrator a great control over his system and better utilization of the system resources.
While some might have never thought about doing this, it is important to know that in Linux you can limit how much resource a single user may use and for how long.

In this short topic, we will show you how to limit the number of processes started by user and how to check the current limits and modify them.

Before we go any further there are two things we need to point:

1. You need root access to your system to modify the user limits
2. You must be extremely careful if you plan to modify these limits

To setup user limits, we will need to edit the following file:

/etc/security/limits.conf

This file is used to apply ulimit created by the pam_module. 

The file has the following syntax:

<domain> <type> <item> <value>

Here we will stop to discuss each of the options:

• Domain – this includes usernames, groups, guid ranges etc
• Type – soft and hard limits
• Item – the item that will be limited – core size, file size,  nproc etc
• Value – this is the value for the given limit

A good sample for a limit is:

@student          hard           nproc                20

The above line sets a hard limit of maximum 20 processes on the "student" group.

If you want to see the limits of a certain process has you can simply “cat” the limits file like this:

# cat /proc/PID/limits

Where PID is the actual process ID, you can find out process id by using ps command.
So here is an example:

# cat /proc/2497/limits

Sample Output

Limit                     Soft Limit           Hard Limit           Units     

Max cpu time              unlimited            unlimited            seconds   

Max file size             unlimited            unlimited            bytes     

Max data size             unlimited            unlimited            bytes     

Max stack size            8388608              unlimited            bytes     

Max core file size        0                    unlimited            bytes     

Max resident set          unlimited            unlimited            bytes     

Max processes             32042                32042                processes 

Max open files            1024                 4096                 files     

Max locked memory         65536                65536                bytes     

Max address space         unlimited            unlimited            bytes     

Max file locks            unlimited            unlimited            locks     

Max pending signals       32042                32042                signals   

Max msgqueue size         819200               819200               bytes     

Max nice priority         0                    0                    

Max realtime priority     0                    0                    

Max realtime timeout      unlimited            unlimited            us  

All of the lines are pretty much self-explanatory. However if you want to find more the settings you can input in limits.conf file.
If you have any questions or comments, please do not hesitate to submit them in the comment section below.