21-sol - 21 C H A P T E R The Linux System Practice...

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Unformatted text preview: 21 C H A P T E R The Linux System Practice Exercises 21.1 Dynamically loadable kernel modules give flexibility when drivers are added to a system, but do they have disadvantages too? Under what circumstances would a kernel be compiled into a single binary file, and when would it be better to keep it split into modules? Explain your answer. Answer: There are two principal drawbacks with the use of modules. The first is size: module management consumes unpageable kernel memory, and a basic kernel with a number of modules loaded will consume more memory than an equivalent kernel with the drivers compiled into the kernel image itself. This can be a very significant issue on machines with limited physical memory. The second drawback is that modules can increase the complexity of the kernel bootstrap process. It is hard to load up a set of modules from disk if the driver needed to access that disk itself a module that needs to be loaded. As a result, managing the kernel bootstrap with modules can require extra work on the part of the administrator: the modules required to bootstrap need to be placed into a ramdisk image that is loaded alongside the initial kernel image when the system is initialized. In certain cases it is better to use a modular kernel, and in other cases it is better to use a kernel with its device drivers prelinked. Where minimizing the size of the kernel is important, the choice will depend on how often the various device drivers are used. If they are in con- stant use, then modules are unsuitable. This is especially true where drivers are needed for the boot process itself. On the other hand, if some drivers are not always needed, then the module mechanism al- 71 72 Chapter 21 The Linux System lows those drivers to be loaded and unloaded on demand, potentially offering a net saving in physical memory. Where a kernel is to be built that must be usable on a large variety of very different machines, then building it with modules is clearly preferable to using a single kernel with dozens of unnecessary drivers consuming memory. This is particularly the case for commercially dis- tributed kernels, where supporting the widest variety of hardware in the simplest manner possible is a priority. However, if a kernel is being built for a single machine whose configuration is known in advance, then compiling and using modules may simply be an unnecessary complexity. In cases like this, the use of modules may well be a matter of taste. 21.2 Multithreading is a commonly used programming technique. Describe three different ways that threads could be implemented. Explain how these ways compare to the Linux clone mechanism. When might each alternative mechanism be better or worse than using clones?...
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This note was uploaded on 11/03/2009 for the course IT OS taught by Professor Dr.stephan during the Winter '09 term at Abu Dhabi University.

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21-sol - 21 C H A P T E R The Linux System Practice...

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