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## 系统结构学习中容易弄错的问题1

(2011-06-14 22:00:30)

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Pitfall: Expecting the improvement of one aspect of a computer to increase overall performance by an amount proportional to the size of the improvement.

This pitfall has visited designers of both hardware and software. A simple design problem illustrates it well. Suppose a program runs in 100 seconds on a computer, with multiply operation responsible for 80 seconds of this time. How much do I have to improve the speed of multiplication if I want my program to run five times faster?
The execution time of the program after making the improvement is given by the following simple equation known as Amdahl's law:

For this problem:

Since we want the performance to be five times faster, the new execution time should be 20 seconds, giving

That is, there is no amount by which we can enhance-multiply to achieve a fivefold increase in performance, if multiply accounts for only 80% of the workload. The performance enhancement possible with a given improvement is limited by the amount that the improved feature is used. This concept also yields what we call the law of diminishing returns in everyday life.

We can use Amdahl's law to estimate performance improvements when we know the time consumed for some function and its potential speedup. Amdahl's law, together with the CPU performance equation, is a handy tool for evaluating potential enhancement.

A common theme in hardware design is a corollary of Amdahl's law: Make the common case fast. This simple guideline reminds us that in many case the frequency with which one event occurs may be much higher than the frequency of another.

Amdahl's law reminds us that the opportunity for improvement is affected by how much time the event consumes. Thus, making the common case fast will tend to enhance performance better than optimizing the rare case. Ironically, the common case is often simpler than the rare case and hence is often easier to enhance.

Amdahl's law is also used to argue for practical limits to the number of parallel processors.

From “Computer Organization and Design – The Hardware/software Interface”

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