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GATE CSE 2020 Q63: Effective memory access time with TLB, page faults and dirty pages

Question

Consider a paging system that uses 1-level page table residing in main memory and a TLB for address translation. Each main memory access takes 100 ns and TLB lookup takes 20 ns. Each page transfer to/from the disk takes 5000 ns. Assume that the TLB hit ratio is 95%, page fault rate is 10%. Assume that for 20% of the total page faults, a dirty page has to be written back to disk before the required page is read in from disk. TLB update time is negligible. The average memory access time in ns (round off to 1 decimal places) is _______.

Try it: numerical answer: type the value

Numerical answer. No negative marking.
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Answer: 154.5 to 155.5 (official GATE 2020 final answer key)

Concept used: Weight each path (TLB hit, TLB miss with page in memory, TLB miss with page fault, clean or dirty) by its probability.

First step: A TLB hit costs 20+10020 + 100 ns; a TLB miss adds a 100 ns page-table access, and a page fault adds 5000 ns (or 10000 ns if dirty) before the access.

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Source: official GATE 2020 CS question paper and final answer key, organised by IIT Delhi. Question paper · Answer key

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