A program repeatedly executes a loop that has 120 iterations. Each iteration tak

Question

A program repeatedly executes a loop that has 120 iterations. Each iteration takes 10,000
cycles. On a multiprocessor system, 50,000 cycles are required to synchronize the processors
once all iterations of the loop have been completed:
a. What is the execution time of each loop on a uniprocessor system?
b. What is the execution time of each loop on a 2-processor system, and what is the speedup
over the uniprocessor system?
c. What is the execution time of each loop on a 4-processor system, and what is the speedup
over the uniprocessor system?

Explanation / Answer

Given no. of iterations = 120

Let no. of instruction count is I and clock rate is 'n'.

We know, execution time = CPI * instruction count * clock rate

a,) For a uni-processor system, no. of cycles required = 120*10,000 = 12,00,000

Execution time = 12,00,000 * I * n

b.) For 2-processor system, no. of cycles required = 12,00,000 + 50,000 = 12,50,000

Execution time = (12,50,000/2) * I *n

Speedup = (Execution time for uni-processor)/(Execution time for 2-processor)

= (12,00,000 * I *n) / {(12,50,000/2) * I *n}

= 24,00,000/ 12,50,000 = 1.92

c.) For 2-processor system, no. of cycles required = 12,00,000 + 50,000 = 12,50,000

Execution time = (12,50,000/4) * I *n

Speedup = (Execution time for uni-processor)/(Execution time for 4-processor)

= (12,00,000 * I *n) / {(12,50,000/4) * I *n}

= 48,00,000/ 12,50,000 = 3.84

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