9.2 Why Build a Custom Kernel?

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 for advanced BSD users. 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: