If there is no page fault, effective access time is effective memory acces time is
eatNO-PF = emat
If there is a page fault, we have
eatPF = pfst + emat ≅ pfst
where pfst is page fault service time. Since emat is very small compared to pfst it can be ignored.
Now, let p be the probability for a page fault (0 ≤ p ≤ 1) to occur. Then, the effective access time must be calculated as follows:
eat = p * eat PF + (1 – p) * eatNO-PF ≅ p * pfst + (1 – p) * emat
Example :
Effective memory access time for a system is given as 1 microseconds, and the average page fault service time is given as 10 milliseconds. Let p=0.001. Then,
effective access time is
eat = 0.001 * 10 msec +(1 – 0.001) * 1 µsec = 0.001 * 10 000 µsec + 0.999 * 1 µsec ≅ 10 µsec + 1 µsec = 11 µsec
