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 set automatically some variables each time the machine boots, add them
to the /etc/sysctl.conf file. For more information see the sysctl.conf(5) manual
page and the Section
11.10.4.
Contributed by Tom Rhodes.
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 the /boot/defaults/loader.conf
file.
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.
