operating system - How to Calculate Effective Access Time -


assume tlb hit ratio 90%, physical memory access takes 100ns, tlb access takes 20 ns, compute effective access time processor uses 2 level page tables, , parallel tlb , page table indexing.

this formula: 

(h)(tlb access time + mem access time) + (1-h)(tlb access + pt access + mem access)

this calculation:

eat = [(.90) * (100ns + 20ns) + (1-.90)(20ns + 100ns*2 + 100ns)] = 130ns

i understand 2* 100ns because of two-level page table, confusion when professor said tlb access time 20ns should not included within miss calculation.

could please explain reasoning behind this?

the tlb , pt indexing assumed "parallel". lookup tlb , pt simultaneous.

upon memory lookup, processor launches both pt , tlb query. if tlb has answer, answer first; that's hit case. if not, query continue run until pt answers; that's non-hit case.

the thing of importance pt has started answer during time tlb determines cannot answer. tlb lookup time should not taken account upon cache miss.


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