12.5 One problem with contiguous allocation is that the user must preallocate enough space for each ?le. If the ?le grows to be larger than the
space allocated for it, special actions must be taken. One solution to this
problem is to de?ne a ?le structure consisting of an initial contiguous
area (of a speci?ed size). If this area is ?lled, the operating system
automatically de?nes an over?ow area that is linked to the initial
contiguous area. If the over?ow area is ?lled, another over?ow area
is allocated. Compare this implementation of a ?le with the standard
contiguous and linked implementations. Answer:
This method requires more overhead then the standard contiguous allocation. It requires less overheadthan the standard linked allocation.
12.6 How do caches help improve performance? Why do systems not use
more or larger caches if they are so useful? Answer:
Caches allow components of differing speeds to communicate more ef?ciently by storing data from the slower device, temporarily, in
a faster device (the cache). Caches are, almost by de?nition, more
expensive than the device they are caching for, so increasing the number
or size of caches would increase system cost.
12.7 Why is it advantageous for the user for an operating system to
dynamically allocate its internal tables? What are the penalties to the
operating system for doing so? Answer:
Dynamic tables allow more ?exibility in system use growth — tables
are never exceeded, avoiding arti?cial use limits. Unfortunately, kernel
structures and code are more complicated, so there is more potential
for bugs. The use of one resource can take away more system resources
(by growing to accommodate the requests) than with static tables.Practice Exercises 45
12.8 Explain how the VFS layer allows an operating system to support
multiple types of ?le systems easily. Answer:
VFS introduces a layer of indirection in the ?le system implementation.
In many ways, it is similar to object-oriented programming techniques.
System calls can be made generically (independent of ?le system type).
Each ?le system type provides its function calls and data structures
to the VFS layer. A system call is translated into the proper speci?c
functions for the target ?le system at the VFS layer. The calling program
has no ?le-system-speci?c code, and the upper levels of the system call
structures likewise are ?le system-independent. The translation at the
VFS layer turns these generic calls into ?le-system-speci?c operations.
