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How to roll your own Linux
By Varun Singh <Varun_Singh@jasubhai.com>
So what's a kernel? Here's how the Linux Kernel How-To describes it: "The Unix kernel acts as a mediator for your programs and your hardware. First, it does (or arranges for) the memory management for all of the running programs (processes), and makes sure that they all get a fair (or unfair, if you please) share of the processor's cycles. In addition, it provides a nice, fairly portable interface for programs to talk to your hardware." A kernel is simply the piece of code that builds the base on which a whole OS can be designed. Applications must go through the kernel to perform tasks and use processing power, memory, devices and disk access. This makes the kernel the most important part of a Unix system. Linux became the most popular of the Unix systems because it was open-sourced. Everyone could get the Linux source code and use, modify and customize it--to be recompiled to suit individual needs. And these were all for free! So, hordes of people pitched in to develop the Linux kernel, helping the OS incorporate many new features and device drivers. The Linux kernel now supports thousands of devices. And the number is increasing every day. With support for so many devices, the kernel's performance is bound to take a beating. If not properly implemented, the kernel can add to the size of the code and end up consuming more memory in loading unnecessary drivers. That's how the idea of kernel modules came from. Kernel modules are small attachments to the base kernel which allow it to incorporate support for many devices, without programming them straight into the kernel. This helps the kernel remain compact and still be customizable to support many devices. The default kernels bundled with many Linux distributions are neither optimized, nor do they support SCSI emulation that's needed for CD writing while using a non-SCSI CD Writer. Instead, they include support for many common devices and interfaces that aren't used in all systems--parallel ports, various sound cards, unused file systems and devices, PCMCIA ports, network cards and so on. You can always recompile the Linux kernel for a particular system and hence add functionality, or tweak it to get optimal performance. The fun part is that you don't need to be a programmer to do this: there are automated configuration scripts to help you out. Three kinds of configuration scripts are available: The standard sequential 'make config' that asks you hundreds of questions about
almost every kernel option that can be tweaked; But, before you set about exploring the exciting world of kernel modules, make sure you have some necessary things with you. The first is the kernel source code, itself. Almost all Linux distributions come with kernel source code packages. You can install them using either the GnoRPM or KDE Package managers, or manually, with the standard RPM tool. Just switch to the directory where the kernel source RPMs exist and do an 'rpm -ivh kernel-source*.rpm'. This should install the kernel source code in the /usr/src/linux-x.x.xx directory, where x.x.xx is the version of the kernel source. Change to the /usr/src/linux-x.x.xx directory and run 'make menuconfig'. This makes it easier to understand what each option means before you use them. If you're running X, just type 'make xconfig' and you'll get a GUI-based system to do the same work, while, in the 'menuconfig' mode, you can just type '?' to access specific comments and help about a particular option. Make sure you read what a particular service can deliver before you plan to disable it. The same can be done in 'xconfig' mode by using the 'Help' button. To ensure the kernel re-mains small in size, load most options as kernel modules rather than compiling them straight into the kernel. Do this in the 'menuconfig' mode. A* in front of the option means it's selected; an M means it's to be loaded as a module; and a blank means the option is not selected. In the 'xconfig' mode, this is indicated by y (="yes"), m (="module") and n (="no"). If you want to configure Linux for a server, you may want to eliminate support for things like sound cards, PCMCIA slots, unused file systems (everything except file systems like EXT2, DOS, ISO9660, Joliet and VFAT which are needed most of the time), IrDA support, Video for Linux, joysticks and amateur radio support. This makes the kernel much smaller in size, without the risk of missing out on some devices. If you feel you know what your system needs and fancy adventure, go into your computer's network configuration to remove the extra network card drivers and tweak the TCP/IP options. That will make a difference, however small, in the kernel size and performance. You can, of course, always select the processor type of your machine for compiling a kernel optimized for that particular processor class. Once you've made the selections, save your configuration to an alternate file for using it later, if needed. If using 'menuconfig', just select Exit and it will ask you to save the configuration. Say Yes, and it saves the configuration to the 'makefile'. If you're using 'xconfig', just choose Save and exit. Now ensure that you're in the /usr/src/linux-x.x.xx and give the command 'make dep'. This command ensures that all files needed for compiling the kernel are in their proper place. Once this process is finished, type 'make bzImage' (note the 'I' is in the upper case). This begins the kernel-compile process. In earlier kernel versions, the command was 'make zImage'--an option still available--which shows an error message saying the kernel image is too large because it can't handle the large-size code. The kernel takes time to compile. Be patient! Once the process is complete and you don't see any errors, you'll have a file called 'bzImage' in the /usr/src/linux-x.x.xx/arch/i386/boot/ directory. This is your new kernel. Now copy this file into the /boot directory and rename it to 'vmlinuz-test'.
Then open /etc/lilo.conf in your favorite text editor. It will already have
an entry used to boot your existing kernel that looks something like this: Clone this entry, as it stands, just below the last line in the file, and replace the 'image=/boot/vmlinuz-x.x.xx-xx' with 'image=/ boot/vmlinuz-test'. Now change the 'label=linux' to 'label=lintest'. Save the file and exit the editor. Now type 'lilo' at the command prompt. This whole process adds your newly compiled kernel to the boot list of lilo, the Linux loader, without removing your existing Linux kernel. This ensures that even if the newly compiled kernel doesn't work as expected, you can still boot your machine with the older kernel. Now reboot your machine. And, the next time 'lilo' prompts to load an OS, just type 'lintest' to make a new kernel boot-up. If this doesn't work, don't panic. Just reboot your machine and use the default Linux kernel you used to boot it in the first instance and you'll be back where you started. If, however, this succeeds, and everything functions just fine, swap the 'label= lintest' line in the fresh entry you made with the default entry's name (in this case, the name will be 'label= linux' in the /etc/lilo.conf). Then change the default entry of the old Linux kernel to 'label=linback' and run 'lilo'. This way, you make the new kernel your default kernel and preserve your older kernel in the boot menu -- just in case you ever need it.
Other articles by Varun Singh
Current Rating: [ 8.06 / 10 ]
Number of Times Rated: [ 100 ]
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