In a particular system, a CPU is used for 78% of the time and a disk drive for 22% of the time. A designer has two options:

a. improve the disc performance by 40% and the CPU performance by 20%
or
b. improve the disc performance by 10% and the CPU performance by 80%
Which is the better option, and why?

a. The fraction of time used by the CPU is 0.78. If the CPU speedup is 20%, the new time take by the CPU will be 0.78 × 0.80 = 0.624. Similarly, the time taken by the disk will be 0.22 × 0.60 = 0.132. The total time is 0.624 + 0.132 = 0.756.

b. If the CPU speedup is 80%, the new time take by the CPU will be 0.78 × 0.20 = 0.156. Similarly, the time taken by the disk will be 0.22 × 0.90 = 0.198. The total time is 0.156 + 0.198 = 0.354.

Option b gives the better overall performance.