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Consider a system using demand paging architecture

  • it takes $3ms.$ to service page fault if either empty frame is available or replaced page is not to be modified
  • takes $10ms.$ if replaced page is modified.
  • Main Memory access time is $1ms.$
  • and page to be replaced is modified $60$% of time.

The maximum acceptable page fault rate to get effective memory access time not more than $4ms.$ is_________(%)

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Let the page fault rate be $p$, then

$ \Rightarrow p \times [0.4 \times 3 ms + 0.6 \times 10 ms] + (1-p) \times 1 ms \leq 4 ms$

$\Rightarrow 6.2p = 3$

$\Rightarrow p = 48 \% $

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why don't you add the memory access time at the time of page fault?

b'coz in the time of page first we service the page and the access main memory.

correct me if I am wrong
From the time when page fault occurs to handling of the page fault is called page fault service time. That's why here no need to add memory access time

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