Explain the Resource limits on UNIX systems

On UNIX systems, the ulimit command controls the limits on system resource, such as process data size, process virtual memory, and process file size. Specifically.

On UNIX systems, each user can either inherit resource limits from the root user or have specific limits defined. When setting resource limits for a process, it is important to know that the limits that apply are those that are in effect for the parent process and not the limits for the user under which the process runs. For example, the IBM Directory server runs under the ldap user account that was created at install time. However, the IBM Directory server is typically started while logged in as the root user. Starting while logged in as the root user means that any limits that are in effect for the ldap user have no effect on the IBM Directory server process unless the IBM Directory server process is started while logged in as the ldap user.

To display the current user’s resource limits, use the ulimit command (see the following example):

# ulimit -Ha
time(seconds) unlimited
file(blocks) 2097151
data(kbytes) unlimited
stack(kbytes) unlimited
memory(kbytes) unlimited
coredump(blocks) unlimited
nofiles(descriptors) unlimited

# ulimit -Sa
time(seconds) unlimited
file(blocks) 2097151
data(kbytes) 131072
stack(kbytes) 32768
memory(kbytes) 32768
coredump(blocks) 2097151
nofiles(descriptors) 2000

 -H option instructs the command to display hard resource limits.
 -S option instructs the command to display soft resource limits.

The hard resource limit values are set by the root user using the chuser command for each user. The soft resource limit values can be relaxed by the individual user using the ulimit command, as long as the values are smaller than the hard resource limit values.
Increasing process memory size limit

Enter the following command to check the current process data size and virtual memory size limits:

ulimit -d
ulimit -m

It is recommended that the process data size and virtual memory size be set to unlimited. Setting to unlimited can be done by modifying the following lines in the /etc/security/limits file:

default:
data = -1
rss = -1

For changes to the /etc/security/limits file to take effect, the user must log out of the current login session and log back in.

At minimum, set these size limits to 256 MB, which is the value of 256000 in the /etc/security/limits file. Increase these limits when a larger-than-default IBM Directory server cache is to be used. For more information, see the IBM Directory Server documentation.

In addition to the /etc/security/limits file, the process virtual memory size is limited by the number of segments that a process can use. By default, a process can only use one memory segment, which limits a process to 128 MB. AIX support a large memory model that is enabled through the LDR_CNTRLenvironment variable.
Increase file size limit

Enter the following command to check the current file size limits:

ulimit -f

It is recommended that the file size limit be set to unlimited. Setting to unlimited can be done by modifying the following lines in the /etc/security/limits file:

default:
fsize = -1

Create file systems with large file support

The standard file system on AIX has a 2 GB file size limit, regardless of the ulimit setting. One way to enable files larger than the 2 GB limit is to create the file system with the Large File Enabled option. This option can be found through the Add a Journaled File System option of the smit menu. Refer to AIX documentation for additional information and file system options.
Edit/Change the Ulimit  Values for uid:

    Edit the limits file under /etc/security/limits (takes effect after reboot)
    Use the chuser command to change individual user settings (logout and login required)

Here are a few of the flags that can be set:

chuser rss=-1 username
chuser fsize=-1 username
chuser data=-1 username
chuser nofiles=4000 username
chuser “stack=8388608? username