Assuming an Instruction cache miss rate for a program is 1% and a Data cache mis

Question

Assuming an Instruction cache miss rate for a program is 1% and a Data cache miss rate is 5%. If a processor bas a ideal CPI of 2 without any memory stalls, and the miss penalty is 100 cycles for all misses. Assume that (i) 30% of the instructions are load & stores, (ii) 50% of the instructions are ALUs and (ii) 20% of the instructions are controls. i) What is actual CPI with the possible cache misses? (ii) Assume that we have perfect data and instruction cache (no cache misses), what is the performance speed up of the CPU with the ideal perfect cache vs. the CPU with actual cache. What if the processor is faster, but memory system is not. Suppose we speed up the ideal CPI without any memory stalls from 2 to 1 without changing the clock rate (i) What is the CPI of the system with cache misses? (ii) What is the performance speed up of the CPU with the ideal perfect cache vs. the CPU with actual cache? What is your observation on the effect of the memory on the system performance?

Explanation / Answer

5)

Instruction cache miss rate --> 1%
Data cache miss rate --> 5%
Ideal CPI --> 2 (without any memory stalls)
Miss penalty --> 100 cycles

Instruction

load and stores --> 30%
ALU's --> 50%
Controls --> 20%

Miss cycles per instruction

* Instruction cache = I * 0.01 * 100 = 1I ( Here, I is Instructions)

* Data cache = I * (0.30) * (0.50) * (0.20) * 0.05 * 100 = 0.15I

5.1)

i) Actual CPI = (CPI(Ideal) + I cache + D cache) I
= (2 + 1 + 0.15) I
= 3.15 I

(ii) CPU Performance = Actual CPI / Ideal CPI
= 3.15 / 2
= 1.575 faster

5.2)

Ideal CPI --> 1 (without any memory stalls)

i) Actual CPI = (CPI(Ideal) + I cache + D cache) I
= (1 + 1 + 0.15) I
= 2.15 I

(ii) CPU Performance = Actual CPI / Ideal CPI
= 2.15 / 1
= 2.15 faster

5.3)

case 1: Ideal CPI --> 2 (without any memory stalls) the CPU performance speed up is 1.575.
case 2: Ideal CPI --> 1 (without any memory stalls) the CPU performance speed up is 2.15.

Case 2 is more faster. So, If Ideal CPI without any memory stalls decreases then system performance increases.

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