Performance Analysis

A part taking 50% of total time is made 4× faster. What is the overall speedup (Amdahl's law)?

  1. A2×
  2. B1.6×
  3. C4×
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Answer: B. 1.6×

The new time is the unchanged 50% (0.5) plus the accelerated 50% (0.5 ÷ 4 = 0.125), totaling 0.625. The speedup is 1 ÷ 0.625 = 1.6×. No matter how much the improved part is accelerated, the other 50% remains, so the overall speedup can never exceed 2×. Large overall gains require finding and improving the parts that take most of the time.

  • A. Even eliminating that part entirely caps the speedup at 2×.
  • B. Correct. 1 ÷ (0.5 + 0.5 ÷ 4) = 1.6.
  • C. Only half of the total was sped up.
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More Performance Analysis questions

  1. You want to make an entire task 2× faster by speeding up only the part that takes 60% of the time. How many times faster must that part become?
  2. Which lesson about how to approach performance improvement follows best from Amdahl's law?
  3. What idea does Gustafson's law express?
  4. What is the difference between strong scaling and weak scaling in evaluating parallel computing?
  5. What is the most appropriate approach when evaluating computer performance with benchmarks?