12.9. Tuning with sysctl(8)

sysctl(8) is used 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, specify its name:

% sysctl kern.maxproc
kern.maxproc: 1044

To set a particular variable, use the variable=value syntax:

# sysctl kern.maxfiles=5000
kern.maxfiles: 2088 -> 5000

Settings of sysctl variables are usually either strings, numbers, or booleans, where a boolean is 1 for yes or 0 for no.

To automatically set some variables each time the machine boots, add them to /etc/sysctl.conf. For more information, refer to sysctl.conf(5) and Section 12.9.1, “sysctl.conf.

12.9.1. sysctl.conf

The configuration file for sysctl(8), /etc/sysctl.conf, looks much like /etc/rc.conf. Values are set in a variable=value form. The specified values are set after the system goes into multi-user mode. Not all variables are settable in this mode.

For example, to turn off logging of fatal signal exits and prevent users from seeing processes started by other users, the following tunables can be set in /etc/sysctl.conf:

# Do not log fatal signal exits (e.g., sig 11)
kern.logsigexit=0

# Prevent users from seeing information about processes that
# are being run under another UID.
security.bsd.see_other_uids=0

12.9.2. sysctl(8) Read-only

Contributed by Tom Rhodes.

In some cases it may be desirable to modify read-only sysctl(8) values, which will require a reboot of the system.

For instance, on some laptop models the cardbus(4) device will not probe memory ranges and will fail with errors similar to:

cbb0: Could not map register memory
device_probe_and_attach: cbb0 attach returned 12

The fix requires the modification of a read-only sysctl(8) setting. Add hw.pci.allow_unsupported_io_range=1 to /boot/loader.conf and reboot. Now cardbus(4) should work properly.

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