self doubt

Question

A demand paging system takes 250 time units to service a page fault and 450 time units to replace a dirty page. Memory access time is 10 time units. The probability of page faults is 0.3. In case of a page fault, the probability of page being dirty is 0.06. The average access time is ________ time units (Upto 1 decimal place)

 

for this question what should be correct answer??

88.3 or 88.7 or 85.6??

Comments

abhishek.....her u are using (450+250) for page service time for dirty bit ...why i think according to question it should be only 450 for DP

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@ hs

AMAT=.7*10 +.3*(0.06*450+.94*250)  =85.7    this is right or wrong??

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Effective PF time = (Prob of Dirty).(Dirty service time) + (1 - Prob. of Dirty)(PF service time)

                              =(0.06)(450) + (0.94)(250) = 27 + 235 = 262 time units

AMAT = (Prob of PF)(Effective PF time) + (1 - Prob of PF)(m)

           =(0.3)(262) + (0.7)(10) = 78.6 + 7 = 85.6 time units

 

I feel i have brought this answer

(I'm confused as to include mem access time in PF prob part or nor, as after PF we still have to access the mem)

Correct me if I'm wrong somewhere