Another pitfall cited in Section 1.10 is expecting to improve the overall performance of a computer by improving only one aspect of the computer. Consider a computer running a program that requires 250 s, with 70 s spent executing FP instructions, 85 s executed L/S instructions, and 40 s spent executing branch instructions. 1.13.1 [5] <§1.10> By how much is the total time reduced if the time for FP operations is reduced by 20%? 1.13.2 [5] <§1.10> By how much is the time for INT operations reduced if the total time is reduced by 20%? 1.13.3 [5] <§1.10> Can the total time can be reduced by 20% by reducing only the time for branch instructions?

1) 236 s

2) 91%

3) no

Explanation:

solution:

The new time required to run FP operations is

0.8 x 70 s = 56 s

So, the new time required to run the program is

250 — (70 — 56) = 236 s (because we reduced the time of execution by 70 — 56 = 14 seconds).  

The new total time of execution is  

0.8 x 250 = 200 s  

If we assume that we changed only the time needed to execute INT operations, the new time needed to execute INT operations is

200 — 70 — 85 — 40 = 5 s

Since the old time was 250 — 70 — 85 — 40 = 55 seconds, and  

5/55=0.09

this is the decrease of a whooping 91%!

Let's assume that we completely avoid using branch operations. Then the time of execution is  

55 + 70 + 85 = 205

This is the decrease of 18%, since 205/250 = 0.82 This means that we cannot decrease the total time by 20% by just decreasing the time of branch operations.