Consider three different processors P1, P2, and P3 executing the same instructio
ID: 3876099 • Letter: C
Question
Consider three different processors P1, P2, and P3 executing the same instruction set. P1 has a 4GHz clock rate and a CPI of 2. P2 has a 2.5GHz clock rate and a CPI of 1.0, P3 has a 3GHz and a CPI of 1.2. a) Which processor has the highest performance expressed in instructions per second? b) If the processors each execute a program in 10 seconds, find the number of cycles and the number of instructions. c) We are trying to reduce the execution time by 20% but this leads to an increase of 15% in the CPI. What clock rate should we have to get this time reduction?
Explanation / Answer
1
Max performance is max instruction per sec which can be easily calculated by dividing frequency by CPI of the processor
for P1:
Instructions per second: 4G/2 = 2G
for P2:
Instructions per second: 2.5G/1 = 2.5G
for P3:
Instructions per second: 3G/1.2 = 2.5G
So P1 and P2 have max instruction theoritically
b
This is quite easy. Since we have instructions per second data with us, we can simply multiply that with 10 to get number of instructions executed in a 10 secs
so for P1:
2G * 10 = 20G of instructions
so for P2:
2.5G * 10 = 25G of instructions
so for P3:
2.5G * 10 = 25G of instructions
c
This is the toughest of the lot. To understand this let us give a blunt example
let us say a set of instruction needs 100time units and Instructions per unit time is x, so the set contains 100x instructions
now the requirement is time 80unit time to solve 100x instuction
so instruction per time is 1.25x
So we need 25% more clocks to reduce the execution time by 20% (given CPI remains same)