modify and apply limits.conf without reboot
I have added a line in /etc/security/limits.conf to increase the number of open files.
* hard nofile 4096
root hard nofile 16384
However when I run ulimit -n it says 1024 which is the default value. I did a logout and login but still see 1024. How can I apply the change?
If you're using bash, ulimit -n will only display the soft limit. To get the hard limit, you need to do ulimit -Hn.
On my system, I see this:
$ ulimit -n
1024
$ ulimit -Hn
4096
Changes made by ulimit command will be applied immediately without reboot (for new processes)
$ ulimit -n 4096 # set soft limit
$ ulimit -Hn 16384 # set hard limit
Also prlimit command (from util-linux package on Debian) can be used to check (or modify) limit value (for current shell):
$ prlimit
RESOURCE DESCRIPTION SOFT HARD UNITS
AS address space limit unlimited unlimited bytes
CORE max core file size 0 unlimited bytes
...
Limits can be configured per-process, you can check it using:
cat /proc/<PID>/limits # where <PID> is replaced by actual process ID
prlimit -p 4562 # or using prlimit
For modifying the limit is syntax is following:
prlimit -p <PID> --<resource>=<soft>:<hard>
set maximum number of opened files for process 4561:
prlimit --pid 4561 --nofile=128051:256102
In order to make such changes permanent, you have to modify /etc/security/limits.conf by adding your limits:
* soft nofile 4096
* hard nofile 16384
However, wildcard * won't apply for root user. In order to do so, you have to state it explicitly:
* soft nofile 4096
* hard nofile 16384
root soft nofile 4096
root hard nofile 16384
These limits will be applied after creating a new session (of course after reboot). Switch to another user using su or login via ssh and check the updated limits:
su - myuser
ulimit -Sa
ulimit -Ha
NOTE: Usually some service is running out of resources, be sure to check limits also on process level (having largest possible value for root user is not a solution). If value configured in /etc/security/limits.conf is too large (not supported on your kernel), the default value will be used instead.
On systemd systems there are /etc/systemd/user.conf (/etc/systemd/user.conf.d/*.conf for user-specific values) config where limits per user can be overriden:
[Manager]
DefaultLimitAS=4G:16G
DefaultLimitNOFILE=1048576
Also there /etc/systemd/system.conf for system-wide configuration.
Some older distributions had issues with applying limits immediately, where modifying /etc/pam.d/common-session or /etc/pam.d/login would help. This is usually not necessary:
session required pam_limits.so
Make sure to check /var/log/auth.log first, when you're having issues with applying limits.