locality

Computer Systems A Programmer's Perspective Second Edition

Well-written computer programs tend to exhibit good

locality

. That is, they tend

to reference data items that are near other recently referenced data items, or

that were recently referenced themselves. This tendency, known as the

principle

of locality

, is an enduring concept that has enormous impact on the design and

performance of hardware and software systems.

 

 

Section 6.2 Locality

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Locality is typically described as having two distinct forms:

temporal locality

and

spatial locality

. In a program with good temporal locality, a memory location

that is referenced once is likely to be referenced again multiple times in the near

future. In a program with good spatial locality, if a memory location is referenced

once, then the program is likely to reference a nearby memory location in the near

future.

 

Programmers should understand the principle of locality because, in general,

programs with good locality run faster than programs with poor locality

. All levels

of modern computer systems, from the hardware, to the operating system, to

application programs, are designed to exploit locality. At the hardware level, the

principle of locality allows computer designers to speed up main memory accesses

by introducing small fast memories known as

cache memories

that hold blocks of

the most recently referenced instructions and data items. At the operating system

level, the principle of locality allows the system to use the main memory as a cache

of the most recently referenced chunks of the virtual address space. Similarly, the

operating system uses main memory to cache the most recently used disk blocks in

the disk file system. The principle of locality also plays a crucial role in the design

of application programs. For example, Web browsers exploit temporal locality by

caching recently referenced documents on a local disk. High-volume Web servers

hold recently requested documents in front-end disk caches that satisfy requests

for these documents without requiring any intervention from the serve.