Книга: Fedora™ Unleashed, 2008 edition
Types of Kernels
Types of Kernels
In the early days of Linux, kernels were a single block of code containing all the instructions for the processor, the motherboard, and the other hardware. If you changed hard ware, you were required to recompile the kernel code to include what you needed and discard what you did not. Including extra, unneeded code carried a penalty because the kernel became larger and occupied more memory. On older systems that had only 4MB-8MB of memory, wasting precious memory for unnecessary code was considered unacceptable. Kernel compiling was something of a black art as early Linux users attempted to wring the most performance from their computers. These kernels compiled as a single block of code are called monolithic kernels.
As the kernel code grew larger and the number of devices that could be added to a computer increased, the requirement to recompile became onerous. A new method of building the kernel was developed to make the task of compiling easier. The part of the kernel's source code that composed the code for the device drivers could be optionally compiled as a module that could be loaded and unloaded into the kernel as required. This is known as the modular approach to building the kernel. Now, all the kernel code could be compiled at once — with most of the code compiled into these modules. Only the required modules would be loaded; the kernel could be kept smaller, and adding hardware was much simpler.
The typical Fedora kernel has some drivers compiled as part of the kernel itself (called in-line drivers) and others compiled as modules. Only device drivers compiled in-line are available to the kernel during the boot process; modular drivers are available only after the system has been booted.
NOTE
As a common example, drivers for SCSI disk drives must be available to the kernel if you intend to boot from SCSI disks. If the kernel is not compiled with those drivers in-line, the system will not boot because it will not be able to access the disks.
A way around this problem for modular kernels is to use an initial RAM disk (initrd) discussed later in section "Creating an Initial RAM Disk Image." The initrd loads a small kernel and the appropriate device driver, which then can access the device to load the actual kernel you want to run.
Some code can be only one or the other (for technical reasons unimportant to the average user), but most code can be compiled either as modular or in-line. Depending on the application, some system administrators prefer one way over the other, but with fast modern processors and abundant system memory, the performance differences are of little concern to all but the most ardent Linux hackers.
When compiling a kernel, the step in which you make the selection of modular or in-line is part of the make config step that we detail later in this chapter. Unless you have a specific reason to do otherwise, we suggest that you select the modular option when given a choice. The process of managing modules is addressed in the next section because you will be managing them more frequently than you will be compiling a kernel.
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