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深入理解计算机系统

重点导读

A Tour of Computer Systems

1.9 Important Themes

1.9.1 Amdahl’s Law

The main idea is that when we speed up one part of a system, the effect on the overall system performance depends on both how significant this part was and how much it sped up. Consider a system in which executing some application requires time $T_{old}$ . Suppose some part of the system requires a fraction α of this time, and that we improve its performance by a factor of $k$.

The overall execution time would thus be

$$T_{new}=(1-\alpha )T_{old}+\alpha T_{old}/k=T_{old}[(1-\alpha )+\alpha /k]x$$

From this, we can compute the speedup as $S=T_{old}/T_{new}=\frac{1}{(1-\alpha )+\alpha /k}$.

This is the major insight of Amdahl’s law—to significantly speed up the entire system, we must improve the speed of a very large fraction of the overall system.

2 Representing and Manipulating Information