Consider three different processors P1, P2, and P3 executing the same instruction set.

P1 has a 3 GHz clock rate and a CPI of 1.5.

P2 has a 2.5 GHz clock rate and a CPI of 1.0.

P3 has a 4.0 GHz clock rate and has a CPI of 2.2.

Which processor has the highest performance expressed in instructions per second?

P2 has the highest performance expressed in instructions per second i. e. 2.5 * 10^9

Explanation:

Given:

Clock rate of P1 = 3 GHz

Clock rate of P2 = 2.5 GHz

Clock rate of P3 = 4.0 GHz

Cycles Per Instruction CPI of P1 = 1.5

Cycles Per Instruction CPI of P2 = 1.0

Cycles Per Instruction CPI of P3 = 2.2

To find:

Highest Performance expressed in instruction per second.

Solution:

Performance = clock rate / CPI

The performance of processor depends on instructions count and CPU time.

As we know that

CPU time = Instructions * Cycles Per Instruction / clock rate

Instructions per second = Instruction count / CPU time

As:

CPU time = I * CPI / clock rate

I/CPU time = clock rate/CPI

So IPS = clock rate / CPI

Hence

Performance = Clock rate / Cycles per Instruction = clock rate CPI

Performance for P1 = IPS = clock rate / CPI = 3 GHz / 1.5 = 2

As we know that 1 GHz = 10 ^ 9 Hz. So:

Performance of P1 expressed in instructions per second is 2 * 10^9

Performance for P2 = IPS = clock rate / CPI = 2.5 / 1.0 = 2.5

Performance of P2 expressed in instructions per second is 2.5 * 10^9

Performance for P3 = IPS = clock rate / CPI = 4 GHz / 2.2 = 1.82

Performance of P3 expressed in instructions per second is 1.82 * 10^9

Hence P2 has the highest performance expressed in instructions per second.