Computer Abstractions and Techno

1.10 Fallacies and Pitfalls

每一章的最后，都会安排这样一个小节，来说最常见的问题。这一章有几个点需要提示，第一个是：

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

具体来说：

The great idea of making the common case fast has a demoralizing corollary that has plagued designers of both hardware and software. It reminds us that the opportunity for improvement is affected by how much time the event consumes.

对应的是一个叫做Amdahl’s Law：

具体案例：

Suppose a program runs in 100 seconds on a computer, with multiply operations 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?

自己计算一下，也能知道，这个结果是无法达成的。

另外：

Amdahl’s Law is also used to argue for practical limits to the number of parallel processors. We examine this argument in the Fallacies and Pitfalls section of Chapter 6.