Pipeline

Question

Consider a 5 stage pipeline with IF, ID, EX, MEM and WB latencies 8, 6, 4, 6 and 4 respectively (in ns). If IF stage is made 50% faster, the percentage it will improve the performance CPU is __________.

Comments

is it 33%

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Which is the correct answer? 33% or 25% .. Can u explain please

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Performance is based on clock cycles. 

in first case since its a non-uniform , the clock cycle will be fixed with the highest latency i.e C = 8 ns

in 2nd case due to 50% increment latency of IF = 4ms. In this case the highest latency becomes C = 6ns

Thus, Decrement is from 8ns to 6ns is (6-8)/8 *100 = 25% 

In Case if you are confused about the percentage calculation check this : http://www.calculatorcat.com/free_calculators/percent_gain.phtml

Answer 1

In case of pipeline we have to consider maximum stage delay as uniform delay so in normal case time taken to execute an instruction will be '8ns' as it is highest time taken by IF.

Now as per question IF is becoming 50% faster so new delay for IF stage will be '4ns'. So average time taken by an instruction in second case is max(4,6,4,6,4). which is '6ns'

%performance increased = (8-6/8)*100 = 25%