Consider a paging hardware with a TLB. Assume that the entire page table and…
2014
Consider a paging hardware with a TLB. Assume that the entire page table and all the pages are in the physical memory. It takes 10 milliseconds to search the TLB and 80 milliseconds to access the physical memory. If the TLB hit ratio is 0.6, the effective memory access time (in milliseconds) is _________.
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Correct answer: 122
Key idea: On a TLB hit you pay the TLB lookup time plus one memory access. On a TLB miss you pay the TLB lookup time, one memory access to read the page table entry, and another memory access to fetch the data.
Given: TLB lookup = 10 ms, memory access = 80 ms, TLB hit ratio = 0.6.
Time on a TLB hit = 10 ms (TLB) + 80 ms (memory) = 90 ms.
Time on a TLB miss = 10 ms (TLB) + 80 ms (page table in memory) + 80 ms (data in memory) = 170 ms.
Effective memory access time = 0.6 × 90 ms + 0.4 × 170 ms = 54 ms + 68 ms = 122 ms.
Answer: 122 ms
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