[Qgis-user] File Descriptor Limits in Linux

[Patrick Dunford]

Good day

For many years it has been known that applications such as Qgis running 
under the Linux operating system have been subject to configuration of 
maximum number of file descriptors for open files. Generally the default 
is set to something like 1024 and will affect Qgis projects with more 
than 1024 layers.

There are currently two limits in Linux that may affect users of Qgis 
with a large number of layers in their project. These can be checked 
using the ulimit -n command which will display the maximum number of 
open files permitted in the current configuration.

1. PAM session limits are configured in the file /etc/security/limits.conf

2. On some distros, the limits also have to be separately configured for 
systemd.

Ubuntu up to and including 22.04 requires only step 1 to be completed.

Debian 12 on the other hand requires step 2 to be completed. It is not 
clear if this is a requirement on any other releases of Debian. However 
experience suggests this step has not been a requirement on most earlier 
Debian releases.

The specifics as far as I can determine for step 2 suggest that a .conf 
file is placed in the following path:

/etc/systemd/user.conf.d

This file can be any name chosen by a user (e.g. limits.conf). The file 
was copied from the user.conf file placed in /etc/systemd and it is also 
the case that editing this user.conf file in its default path would have 
the same effect, however the subdirectory usage is recommended.

This file must at a minimum contain the following:

[Manager]

DefaultLimitNOFILE=xxxxxx


This is a per-user limit and it does not apply to the root user. There 
is a different procedure for the root user, which I do not have 
documentation for. The result of ulimit -n run under a root shell will 
display a different result from running it as a non-root user.