1. Consider three different processors P1, P2, P3 executing the same set of inst
ID: 3742992 • Letter: 1
Question
1. Consider three different processors P1, P2, P3 executing the same set of instructions. P1 has a 3 GHz clock rate and a CPI of 1.5. P2 runs at 2 GHz and has a CPI of 1.2. P3 has a 4 GHz clock rate and a CPI of 2.4 a. Which processor has the highest performance in terms of instructions per second? b. If each processor executes a program in 5 second, find the number of cycles and the number of instructions for each processor. c. We would like to reduce the execution time by 20%. This will, however, increase the CPI by 25%. What clock rate do we need to obtain this amount of time reduction?Explanation / Answer
SOLUTION:-
(1) P1: 3GHz / 1.5 = 2 * 10^9 instructions per second
P2: 2GHz / 1.2 = 1.67 * 10^9 instructions per second
P3: 4GHz / 2.4 = 1.67 * 10^9 instructions per second
So P1 will have the highest performance.
(2) Cycles:
P1: 3GHz * 5 = 3 * 5*10^9 cycles
P2: 2GHz * 5 = 2 * 5*10^9 cycles
P3: 4GHz * 5 = 4 * 5*10^9 cycles
Number of instructions:
P1: 3GHz * 5 / 1.5 = 2 * 5*10^9 instructions
P2: 2GHz * 5 / 1.2 = 1.67 *5* 10^9 instructions
P3: 4GHz *5 / 2.4 = 1.67 * 5*10^9 instructions
(3) Execution time = (Num of instructions * CPI) / (Clock rate)
So if we reduce the execution time by 20%, and CPI increases by 25%, so we have:
Execution time * 0.8 = (Num of instructions * CPI * 1.25) / (New Clock rate)
New Clock rate = Clock rate * 1.25 / 0.8 = 1.56 * Clock rate
New Clock rate for each processor:
P1: 3GHz * 1.56 = 4.68 GHz
P2: 2GHz * 1.56 = 3.12 GHz
P3: 4GHz * 1.56 = 6.24 GHz
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