Amdahl's Law - Speedup in A Sequential Program

Speedup in A Sequential Program

The maximum speedup in an improved sequential program, where some part was sped up times is limited by inequality

where is the fraction of time (before the improvement) spent in the part that was not improved. For example (see picture on right):

• If part B is made five times faster, and, then

• If part A is made to run twice as fast, and, then

Therefore, making A twice as fast is better than making B five times faster. The percentage improvement in speed can be calculated as

• Improving part A by a factor of two will increase overall program speed by a factor of 1.6, which makes it 37.5% faster than the original computation.
• However, improving part B by a factor of five, which presumably requires more effort, will only achieve an overall speedup factor of 1.25, which makes it 20% faster.