cycles per instruction

Question

Assume a program requires the execution of 50 x 106 FP instructions, 110 x 106 INT

instructions, 80 x 106 L/S instructions, and 16 x 106 branch instructions. The CPI for

each type of instruction is 1, 1, 4, and 2, respectively. Assume that the processor has a

2 GHz clock rate. By how much must we improve the CPI of FP instructions if we

want the program to run two times faster?

Comments

This question requires additional information like no. of cores of cpu for parallelization. Also only improving the cpi of floating point nos. would never speedup the program execution by two times as its only 50*10^6 instructions. Let me show the minimum percentage of program whose cpi should be changed for a 10 core cpu according to amdahl's law is:

2=1/((1-x)+x/10)   =>x=56% . Since 50/256 is much less than 0.56 or 56%.

So atleast 56% instructions must be paralleled in a 10 core cpu to get 2 times speedup.

Answer 1

plz correct me.......

Comments

i think execution time of program should be sum of execution times of fp, int and l/s instructions.

Answer 2

Average CPI_old = Σ Ci x Ii

= CPIfp × No. FP instr. + CPIint × No. INT instr. + CPIl/s × No. L/S instr. + CPIbranch × No. branch

= 50 x 10⁶ x 1 +110 x 10⁶ x 1 +80 x 10⁶ x 4 +16 x 10⁶ x 2 =512 x10⁶

As we want the program to run two times faster,

To half the number of clock cycles by improving the CPI of FP instructions:

Average CPI_old=2 x Average CPI_new

Average CPI_new= 0.5 x Average CPI_old

Average CPI_new =CPI_{improved fp} × No. FP instr. + CPIint × No. INT instr. + CPIl/s × No. L/S instr. +

                              CPIbranch × No. branch instr.

0.5 x Average CPI_old= CPI_{improved fp} × 50 x 10⁶  +110 x 10⁶ x 1 +80 x 10⁶ x 4 +16 x 10⁶ x 2

256= CPI_{improved fp} × 50 +110  +320 +32

CPI_{improved fp} × 50 =-206

CPI_{improved fp}  =-4.12

Hence quedtion is wrong...If CPI of FP becomes 0 still program execution not to be 2 times faster.

But we can't reduce the clock from 1 to any becoz that is minimum already..of they have asked clk time then the question is ok

Comments

Calculation mistake in 3rd last line. It should be -206 . But its not valid , see the comments of this question