Question

Problem 1 Assume that a processor with a clock rate of 1 GHz clock runs a benchmark program w mix of instructions given in the table below. Instruction class CPI FP INT Load/Store (L/S) Branch Instruction count 50 x 106 110 x 106 50 x 106 10 x 106 By how much must the CPI of L/S instructions be improved if the program is to run two times faster?

Answer 1

Solution for the problem is provided below, please comment if any doubts:

Problem 1:

The clock rate of the processor = 1 GHz

The total Cycles of the benchmark program= sum of CPI of each instruction multiplied by instruction count

Total cycles=( 2 x50 x10⁶)+ ( 1 x110 x10⁶)+ ( 8 x50 x10⁶)+ ( 2 x10 x10⁶)

=(100+110+400+20) x10⁶ = 630 x10⁶ cycles

If the program needs to be run two times faster, the total cycles needs to be reduced to half.

That is the Cycles should be = 315 x10⁶ cycles

The CPI of L/S needs to be varied to achieve this:

Thus the new cycles = 315 x10⁶ =( 2 x50 x10⁶)+ ( 1 x110 x10⁶)+ ( newCPI x50 x10⁶)+ ( 2 x10 x10⁶)

315 x10⁶ =130 x10⁶+( newCPI x50 x10⁶)

185= newCPI x50

newCPI= 3.7

Thus in order to run the program two times faster, the new CPI of L/S should be improved to 3.7