The right way to increase file descriptors limit on alarm?

[IsoaSFlus]

I have tried edit DefaultLimitNOFILE in /etc/systemd/user.conf and system.conf, but it didn't take effect. So what is the right way to change fd limit on alarm?

[train]

Haven't tried it yet, but you're on the right track. For just one process, you could try to set the limits via ulimits and then start that process
$this->bbcode_second_pass_code('', '
~ ulimit -n
1024
~ ulimit -n 2048
~ ulimit -n
2048
')

To makeit permanent, you may need to set it also in /etc/security/limits.conf. Check
https://wiki.archlinux.org/index.php/Limits.conf on archlinux wiki.

[ufo6000]

I would not propose to increase the default for the whole system.
On which program do you have issues, what is the error message ?

The better way would be to increase ulimit only for this process.

Please post some more details, which program or systemd unit and how you start it.