Cannot Increase open file limit past 4096 (Ubuntu)

OK, I finally figured this out. The limits I was setting in /etc/security/limits.conf were being applied, but they were not being applied to the graphical login. This can be verified like this from a terminal window:

$ ulimit -n
4096
$ su mkasberg
Password:
$ ulimit -n
65535

More research led me to this bug report, which got me pointed in the right direction. In order to modify the limit that is used by the login shell, we need to add the following line to /etc/systemd/user.conf:

DefaultLimitNOFILE=65535

That change works, but only affects the soft limit. (Leaving us capped with a hard limit of 4096 still.) In order to affect the hard limit also, we must modify /etc/systemd/system.conf with the same change.

The changes I made in /etc/pam.d were not necessary. At least on Ubuntu, this is already working. Also, it was not necessary to change settings for root and * in limits.conf. Changing limits for mkasberg was sufficient, at least for my use case.

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In Summary

If you want to increase the limit shown by ulimit -n, you should:

• Modify /etc/systemd/user.conf and /etc/systemd/system.conf with the following line (this takes care of graphical login):

  DefaultLimitNOFILE=65535
  

• Modify /etc/security/limits.conf with the following lines (this takes care of non-GUI login):

  mkasberg hard nofile 65535
  mkasberg soft nofile 65535
  

• Reboot your computer for changes to take effect.

No need to change anything in the /etc/security/limits.conf file, it is ignored if you are using systemd.

(reproducing a modified answer to another question on the network...)

An alternative for those who prefer not to edit the default /etc/systemd/system.conf and /etc/systemd/user/conf files:

1. create a new file /etc/systemd/system.conf.d/limits.conf with these contents:

   [Manager]
   DefaultLimitNOFILE=65535
   

2. run systemctl daemon-reexec as root

3. logout and login again

4. check your new limit with ulimit -n.

Refer to the systemd-system.conf manpage for details.

TL;DR I felt the need to concentrate the answers, so they're easier to find. Took me ages to get all the pieces together to make it work correctly ...

There are 2 locations to be considered.

1. GUI session

   $ grep DefaultLimitNOFILE /etc/systemd/system.conf
   DefaultLimitNOFILE=65535
   

   or better here:

   $ grep NOFILE /etc/systemd/system.conf.d/limits.conf
   DefaultLimitNOFILE=65535
   

2. shell environment

   $ grep nofile /etc/security/limits.conf
   user soft nofile 65535
   user hard nofile 65535`
   

   or better here:

   $ grep nofile /etc/security/limits.d/user.conf
   user soft nofile 65535
   user hard nofile 65535
   

3. After altering the settings in the above files, reboot and then check the limits with: ulimit -n -Hn -Sn