Wednesday, 13 January 2016

Implication of Amdahl's law

We have discussed the theory and some examples based on Amdahl's theory in previous posts. Here, we will learn about the implication of Amdahl's law that guide the engineers working round the clock to improve the performance of the processor.

For convenience, we will specify the Amdahl's law here.



Frac(unused) is the fraction of time (not instruction) that the improvement is not in use.

Frac(used) is the fraction of the time that improvement is in use and speedup(used) is the speedup that occurs when the improvement is used. 

Let's consider two scenarios in which processor's performance is enhanced.


Enhancement 1: Speedup of 20 on 10% of time

Here, Frac_used=0.1 
         speedup_used=20

Frac_unused= 1- Frac_used=0.9




Hence, speedup in the first scenario is 1.15


Enhancement 2: Speedup of 1.6 on 80% of time

Here, Frac_used=0.8
         speedup_used=1.6

Frac_unused= 1- Frac_used=0.2







Hence, speedup in the second scenario is 1.43

We observed that speedup in the second case is more than the first one.

Implication of Amdahl's law:

From this, it is concluded that it is better to have small speedup on the large percentage of execution time than large speedup on the small percentage of execution time.

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