Pipelining Speedup

Question

Consider a non-pipelined processor design which has a cycle time of 15ns and average CPI of 1.6. The maximum speedup pipelined processor can get by pipelining it into 5 stages and each stage takes 3ns is______________?

1.   5
2.   6
3.  10
4.   7

Answer 1

We know :

a) Speedup   =   Time_{non pipelined} /  Time_pipelined 

                   =   (CPI_{non pipelined} * Cycle Time_{non pipelined} )/  (CPI_pipelined *  Cycle time_pipelined)

b) Maximum speedup achievable  =   Number of stages in the pipeline(Also known as pipeline depth)

c) CPI_{ideal pipeline}  =  1

So we find theoretical speedup       =        (CPI_{non pipelined} * Cycle Time_{non pipelined} )/  (CPI_pipelined *  Cycle time_pipelined)

                                                   =         (1.6 * 15) / (1 * 3)

                                                   =         8 

But this is exceeding the maximum speedup   =   Number of stages   =  5 which is impractical.

Hence maximum achievable speedup      =      5

Hence 1) should be correct  option.

Comments

Superb explanation!!