sysctl 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.
At its core, sysctl 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
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.