Practice question · Select all that apply
Select every statement that genuinely follows from the CPU performance equation and Amdahl's law.
Hints
- Two of the options generalise from one factor while quietly holding nothing else fixed.
- For the Amdahl option, ask what the run time approaches as the parallel part shrinks to nothing.
Show the answer
- A. A program that is 50 percent sequential can never be sped up more than 2 times, however many cores are added
- B. Reducing the number of instructions executed reduces the run time, all else being equal
- E. Halving the cycles per instruction halves the run time, at a fixed clock and instruction count
Why
The equation is a product of three factors, so halving one of them halves the time when the others are fixed, which validates options 2 and 5 and destroys option 3, since a faster clock says nothing about CPI. Amdahl's law caps the speedup at 1 divided by the sequential fraction, so a half-sequential program stops at 2 times, and option 4 is false for exactly that reason.
Practise Performance Trade-offs
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