Performance Analysis

What idea does Gustafson's law express?

  1. AWhen adding processors, also enlarging the problem increases the parallel fraction, yielding large speedups
  2. BNo matter how many processors are added, speedup is always capped at 2×
  3. CThe parallel fraction is always constant regardless of problem size
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Answer: A. When adding processors, also enlarging the problem increases the parallel fraction, yielding large speedups

Amdahl's law considers adding processors to a fixed-size problem, so the serial part caps the speedup. Gustafson's law instead assumes that with more processors one solves a larger problem in the same time. Enlarging the problem, for example with a finer simulation grid, grows the parallel part and shrinks the serial fraction, giving gains nearly proportional to processor count.

  • A. Correct. This is why parallelism works for large-scale computing.
  • B. There is no such cap.
  • C. Larger problems often have larger parallel fractions.
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More Performance Analysis questions

  1. What is the difference between strong scaling and weak scaling in evaluating parallel computing?
  2. What is the most appropriate approach when evaluating computer performance with benchmarks?
  3. Which describes a standard benchmark such as SPEC CPU?
  4. When combining ratios of speed against a reference machine across several benchmarks, why is the geometric mean often used?
  5. Two programs with equal instruction counts run back to back, one at 1000 million instructions per second and the other at 500 million. What is the overall average rate?