CPU Execution Time / Performance

computer-architecturecpuperformance

I'm trying to solve a question concerning performance. I have the final answer (24 * 10^9)but I can't determine how to get there.

I know the formula for performance is

Execution time: CPI * I * 1/CR
CPI = Cycles Per Instruction
I = Instructions

Question:
Determine the number of instructions for P2 that reduces its execution time to that of P3.

performance

The first step should be to find out the cycles per Instruction for P3. The equation would be:

  8 = 1 / (2.5) * CPI * 40      
  8 = 1/ 2.5 * CPI * 40
  8 = .4 * CPI * 40
  8 = 160 CPI
  1 =                80 / 16

I know the answer here should be 0.008 CPI, but I don't understand how to get there. 2.5 * 40 would be 100, but it doesn't make mathematical sense to me. What am I missing? Thanks.

Best Answer

Since the clock rate is fixed, the number of instructions needed for the "new" P2 is 8/10 * 30 x 10^9, or 24 x 10^9. That is, if you reduce the number of instructions to 80% of the original, the execution time drops to 80% as well.

Note that the question made no mention of cycles per instruction. It just asked how many instructions are needed to produce the desired execution time.

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