Consider a program comprising 25% arithmetic instructions, 32% load instruction,

Question

Consider a program comprising 25% arithmetic instructions, 32% load instruction, 13% store instructions, and 30% branch instructions. Assume the CPI for arithmetic instructions is 1, for load and store instructions it is 4, and branches have a CPI of 2. The CPU clock rate is 3.4 GHz. We have a referenceprogram with 1,102,000 instructions.

a. What is the run time of the program given?

b. Consider running the program on a machine with a large graphics card.When we run the program on this machine, the arithmetic instructions only can be run in parallel on the card, everything else is run sequentially. As the number of stream processors on the GPU goes toward infinity, what is the maximum speedup obtainable on this program?

c. Consider running the program on a version of the CPU with a clock rate of 2.4 GHz, but which has a CPI of 3 for load and store instructions (all other’s CPIs

remain the same). Which CPU performs better on the program and by how much ?

Explanation / Answer

Solution:

a)

The runtime will be calculated as

First Average CPI = 0.25*1 + 0.32 * 4 + 0.13 * 4 + 0.3 * 2= 2.65

Execution time= Instruction count*CPU/Clock frequency = 1102000 * 2.65/(3.4 * 10^9) = 0.00085891176 = 0.85891176 ms

b)

The new CPI will be 0.32 * 4 + 0.13 * 4 + 0.3 * 2= 2.4

Speedup = 2.65/2.4 = 1.10416666667

c)

The new average CPI will be

0.25*3+ 0.32 * 4 + 0.13 * 4 + 0.3 * 2 = 3.15

CPU time = 1102000 * 3.15/(2.4 * 10^9) = 0.001446375 = 1.446375 ms

which is slower

so the previous implementation is fasteer.

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