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The Linux Kernel

What Does the Kernel Do?

You've probably heard people talking about compiling the kernel or building a kernel, but what exactly is the kernel and what does it do? The kernel is the center of your computer. It is the foundation for the entire operating system. The kernel acts as a bridge between the hardware and the applications. This means that the kernel is (usually) the sole piece of software responsible for ordering around the hardware components of your computer. It is the kernel that instructs the hard drive to search for a certain data stream. It is the kernel that instructs your network card to transmit rapid changes in voltage. The kernel also listens to hardware as well. When the network card detects a remote computer sending information, it forwards that information to the kernel. This makes the kernel both the single most important piece of software on your computer and the most complex.

Working with Modules

The complexity of a modern linux kernel is staggering. The source code for the kernel weighs in at nearly 400MB uncompressed. There are thousands of developers, hundreds of options, and if everything were built together, the kernel would soon pass 100MB in size itself. In order to keep the size of the kernel down (as well as the amount of RAM needed for the kernel), most of the kernel options are built as modules. You can think of these modules as device drivers which can be inserted or removed from a running kernel at will. In truth, many of them aren't device drivers at all, but contain support for things such as network protocols, security measures, and even filesystems. In short, nearly any piece of the linux kernel can be built as a loadable module.

It's important to realize that Slackware will automatically handle loading most modules for you. When your system boots, udevd(8) is started and begins to probe your system's hardware. For each device it finds, it loads the proper module and created a device node in /dev. This usually means that you will not need to load any modules in order to use your computer, but occasionally this is necessary.

So what modules are currently loaded on your computer and how do we load and unload them? Fortunately we have a full suite of tools for handling this. As you might have guessed, the tool for listing modules is lsmod(8).

darkstar:~# lsmod
Module                  Size  Used by
nls_utf8                1952  1 
cifs                  240600  2 
i915                  168584  2 
drm                   168128  3 i915
i2c_algo_bit            6468  1 i915
tun                    12740  1 
... many more lines ommitted ...

In addition to showing you what modules are loaded, it displays the size of each module and tells you what other modules are using it.

There are two applications for loading modules: insmod(8) and modprobe(8). Both will load modules and report any errors (such as loading a module for a device that isn't present in your system), but modprobe is preferred because it can load any module dependencies. Using either is straight-forward.

darkstar:~# insmod ext3
darkstar:~# modprobe ext4
darkstar:~# lsmod | grep ext
ext4                  239928  1 
jbd2                   59088  1 ext4
crc16                   1984  1 ext4
ext3                  139408  0 
jbd                    48520  1 ext3
mbcache                 8068  2 ext4,ext3

Removing modules can be a tricky process, and once again we have two programs for removing them: rmmod(8) and modprobe. In order to remove a module with modprobe, you'll need to use the -r argument.

darkstar:~# rmmod ext3
darkstar:~# modprobe -r ext4
darkstar:~# lsmod | grep ext

Compiling A Kernel and Why to do So

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  • Originally written by Alan Hicks, Chris Lumens, David Cantrell, Logan Johnson

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