Consider three different processors P1, P2, and P3 executing the same instructio
ID: 3744837 • Letter: C
Question
Consider three different processors P1, P2, and P3 executing the same instruction set: . P1 has a 3.5 GHz clock rate and a CPI of 1.5 . P2 has a 2.0 GHz clock rate and a CPI of 1.0 P3 has a 4.0 GHz clock rate and a CPI of 2.5 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 for each processor. C) We wish to reduce the execution time to 8 seconds, but this leads to a 25% increase in CPI. What clock rate would we need for each processor to get this time reduction?Explanation / Answer
Ans a)
Performance of P1 (instruction/Sec) = 3.5 * 109/1.5 = 2.3 * 109
Performance of P2 (instruction/Sec) = 2.0 * 109/1 = 2.0 * 109
Performance of P3 (instruction/Sec) = 4.0 * 109/2.5 = 1.6 * 109
So P1 has the highest performance
Ans B) Number of cycles = time * clock rate
Cycles(P1) = 10 * 3.5 * 109 = 35 * 109
Cycles(P2) = 10 * 2 * 109 = 20 * 109
Cycles(P3) = 10 * 4 * 109 = 40 * 109
Number of instructions = No. of Cycles/CPI
Instructions (P1) = 35 * 109 / 1.5 = 23.3 * 109
Instructions (P2) = 20 * 109 / 1.0 = 20 * 109
Instructions (P3) = 40 * 109 / 2.5 = 16.0 * 109
Ans C)
CPI NEW = CPI OLD * 1.25
CPI NEW(P1) = 1.5*1.25 = 1.875
CPI NEW(P2) = 1*1.25 = 1.25
CPI NEW(P3) = 2.5*1.25 = 3.125
New time = 8 Seconds
F= No. of instructions * CPI/time
F(P1) = 23.3 * 109 * 1.875/8 = 5.46 GHz
F(P2) = 20 * 109 * 1.25/8 = 3.125 GHz
F(P3) = 16 * 109 * 3.125/8 = 6.25 GHz