Option one: You did not set the softlimit higher aswell.
Possible solution:
in /etc/security/limits.conf add
* soft nofile 2048
test with
ulimit -n 2048
Option two: You are logged in as user and in some "config" file (profile, bashrc, something like this) the soft limit is set to a lower value.
Possible solution f.e. grep for ulimit in your etc folder and/or home folder.
Warning: Depending on the amount of files/directorys you have in there you might want to consider only specific directorys/files
ps: there are a lot of similiar question here you might want to read up.
Specially Hard vs Soft Limit
Read here for possible other solution which go more into detail Too Many Open Files
There is a bug in Debian. To increase ulimit you need to add this into the /etc/pam.d/common-session file:
session required pam_limits.so
and in /etc/security/limits.conf add:
* soft nofile 65535
* hard nofile 65535
Then reboot the system.
On more modern systemd-based distros, limits are often set by systemd. See man systemd-system.conf.
So for example, to raise the file descriptor limit for root, this is what I would do:
/etc/systemd/system.conf.d/10-filelimit.conf:
[Manager]
DefaultLimitNOFILE=16384:524288
Make sure to read the manual for details.
As an aside, the limits within containers are usually further restricted by the container engine. This needs to be configured separately; see this answer.
In my case of Apache on Debian 10 the above didn't help though this worked:
echo "APACHE_ULIMIT_MAX_FILES='ulimit -n 16384'" >> /etc/apache2/envvars
service apache2 restart
I checked with:
cat /proc/$pid/limits
..where $pid is the process ID of one of the Apache processes.
