sysctl(8) is an interface that allows you to make changes to a running FreeBSD system. This includes many advanced options of the TCP/IP stack and virtual memory system that can dramatically improve performance for an experienced system administrator. Over five hundred system variables can be read and set using sysctl(8).
At its core, sysctl(8) serves two functions: to read and to modify system settings.
To view all readable variables:
% sysctl -a
To read a particular variable, for example, kern.maxproc:
% sysctl kern.maxproc kern.maxproc: 1044
To set a particular variable, use the intuitive variable=value syntax:
# sysctl kern.maxfiles=5000 kern.maxfiles: 2088 -> 5000
Settings of sysctl variables are usually either strings, numbers, or booleans (a boolean being 1 for yes or a 0 for no).
If you want to automatically set some variables each time the machine boots, add them to /etc/sysctl.conf. For more information see the sysctl.conf(5) manual page and Section 12.11.3.
In some cases it may be desirable to modify read-only sysctl(8) values. While this is sometimes unavoidable, it can only be done on (re)boot.
For instance on some laptop models the cardbus(4) device will not probe memory ranges, and fail with errors which look similar to:
cbb0: Could not map register memory device_probe_and_attach: cbb0 attach returned 12
Cases like the one above usually require the modification of some default sysctl(8) settings which are set read only. To overcome these situations a user can put sysctl(8) “OIDs” in their local /boot/loader.conf. Default settings are located in /boot/defaults/loader.conf.
Fixing the problem mentioned above would require a user to set hw.pci.allow_unsupported_io_range=1 in the aforementioned
file. Now cardbus(4) will work
properly.