Traditionally, FreeBSD has had what is called a “monolithic” kernel. This
means that the kernel was one large program, supported a fixed list of devices, and if
you wanted to change the kernel's behavior then you had to compile a new kernel, and then
reboot your computer with the new kernel.
Today, FreeBSD is rapidly moving to a model where much of the kernel's functionality
is contained in modules which can be dynamically loaded and unloaded from the kernel as
necessary. This allows the kernel to adapt to new hardware suddenly becoming available
(such as PCMCIA cards in a laptop), or for new functionality to be brought into the
kernel that was not necessary when the kernel was originally compiled. This is known as a
modular kernel.
Despite this, it is still necessary to carry out some static kernel configuration. In
some cases this is because the functionality is so tied to the kernel that it can not be
made dynamically loadable. In others it may simply be because no one has yet taken the
time to write a dynamic loadable kernel module for that functionality.
Building a custom kernel is one of the most important rites of passage nearly every
BSD user must endure. This process, while time consuming, will provide many benefits to
your FreeBSD system. Unlike the GENERIC kernel, which must
support a wide range of hardware, a custom kernel only contains support for your PC's hardware. This has a number of
benefits, such as:
-
Faster boot time. Since the kernel will only probe the hardware you have on your
system, the time it takes your system to boot can decrease dramatically.
-
Lower memory usage. A custom kernel often uses less memory than the GENERIC kernel, which is important because the kernel must always
be present in real memory. For this reason, a custom kernel is especially useful on a
system with a small amount of RAM.
-
Additional hardware support. A custom kernel allows you to add in support for devices
which are not present in the GENERIC kernel, such as sound
cards.
