Rebuilding the kernel sounds like a pastime for hackers, but it is an important skill for any system administrator. Rebuilding the kernel on your system to eliminate the device drivers you don't need is one good reason to do so. This reduces the amount of memory used by the kernel itself, as described in "Managing Swap Space" in Chapter 10. The kernel is always present in memory, and the memory it uses cannot be reclaimed for use by programs if necessary.
It should be noted here that most distributions today ship with modularized kernels . This means that the kernel they install by default contains only the minimum functionality needed to bring up the system; everything else is then contained in modules that add any additionally needed functionality on demand. We will talk about modules in much greater detail later. But even with these stripped-down kernels, distributions have to ship several versions, for example, in order to provide support for both single-processor and multiprocessor machines, as this is something so central that it cannot be moved into a module. The installers that come with distributions are typically smart enough to figure out which kernel you need and install the right one, however.
Why is the ability to select features a win for you? All kernel code and data are "locked down" in memory; that is, they cannot be swapped out to disk. For example, if you use a kernel image with support for hardware you do not have or use, the memory consumed by the support for that hardware cannot be reclaimed for use by user applications. Customizing the kernel allows you to trim it for your needs.
You also need to occasionally upgrade your kernel to a newer version. As with any piece of your system, if you know of important bug fixes or new features in a kernel release, you may want to upgrade to pick them up. Those people who are actively developing kernel code will also need to keep their kernel up-to-date in case changes are made to the code they are working on. Sometimes, it is necessary to upgrade your kernel to use a new version of the compiler or libraries. Some applications (such as the VMware emulator) require a certain kernel version to run.
You can find out what kernel version you are running through the command uname -a. This should produce something like the following:
rutabaga$ uname -a Linux pooh 184.108.40.206-21.7-default #1 Thu Jun 2 14:23:14 UTC 2005 i686 i686 i386 GNU/Linux
Here, we see a machine running Version 220.127.116.11 of the kernel, which was last compiled on June 2, 2005. We see other information as well, such as the hostname of the machine, the number of times this kernel has been compiled (once), and the fact that the machine is a Pentium Pro or better (as denoted by i686). The manpage for uname(1) can tell you more.