Chapter 16
Page and Buffer Cache
Performance and efficiency are two factors to which great importance is attached during kernel development. The kernel relies not only on a sophisticated overall concept of interaction between its individual components, but also on an extensive framework of buffers and caches designed to boost system speed.
Buffering and caching make use of parts of system RAM to ensure that the most important and the most frequently used data of block devices can be manipulated not on the slow devices themselves but in main memory. RAM memory is also used to store the data read in from block devices so that the data can be subsequently accessed directly in fast RAM when it is needed again rather than fetching it from external devices.
Of course, this is done transparently so that the applications do not and cannot notice any difference as to from where the data originate.
Data are not written back after each change but after a specific interval whose length depends on a variety of factors such as the free RAM capacity, the frequency of usage of the data held in RAM, and so on. Individual write requests are bundled and collectively take less time to perform. Consequently, delaying write operations improves system performance as a whole.
However, caching has its downside and must be employed judiciously by the kernel:
- Usually there is far less RAM capacity than block device capacity so that only carefully selected data may be cached.
- The memory areas used for caching ...
