Question

Page tables are stored in memory , which has access time of 100 ns. The TLB holding 8 page table entries, has an access time of 10 ns. Using execution of process , it is found that 85 % of the time, a required page table entry exist in TLB and only 2 % of the total references causes page fault. Page replacement time is 2 ms . Calculate the effective memory access time , assuming page memory access requires 2 memory accesses and TLB requires one memory access.

A) 38120 ns B) 40000 ns C) 40120 ns D) None

Comments

what if page fault occured in case of TLB hit also. Should we not consider that in this?

Answer 1

Page table access time=$100 ns$

TLB has 8 PTE and access time of a PTE is $10 ns$, that means TLB access time $10 ns$

Page Table are in TLB i.e. TLB  hit time$=85$%

Page Fault is for $=2$%

TLB miss time $=13$%

For TLB hit =$1$ memory access

For TLB miss=$2$ memory access

So, Effective Memory Access Time$=0.85\left (100+10 \right )+0.13\left ( 100+100 \right )+\frac{2}{100}\times 2000000$

$=40119.50ns\simeq 40120ns.$

Ans $C)$

https://www.inf.ed.ac.uk/teaching/courses/os/slides/10-paging16.pdf

Similar ques from book https://gateoverflow.in/211302/operating-system-by-harris

Comments

@Shaik

 EMAT = ( TLBaccess time ) + TLBmiss ( Page table Fetch ) + Page_Fault ( Page_service ) + ( Page_Fetch )

how tlb hit not require any main memory  access? It needs 1 access, explicitely given in question too 

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how page table not require any main memory  access?

i considered it when TLB miss, Read my comment clearly mam

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in case of tlb miss we must add tlb access time also.. 0.85(100+10)+.13(100+100+10)+ .02×2 milliseconds

Answer 2

D

0.85(10+110)+0.15(13/15(10+100+100)+2/15(10+100+2000000+100)=40125

85% time there will be a TLB hit so it will take (10+100) ns.

15% of the time two things can happen:

• X% of the time page fault will occur that is it will take (10+100+2000000+100) ns.
• (1-X)% of the time no page fault so it will take (10+100+100) ns.

Page fault occurs 2% of the total memory accesses so X% of 15 should be equal to 2.

i.e. 15 * X/100 = 2        X=200/15.

Comments

@Viral Kapoor 

how 13/15 comes in your equation? 

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can u explain how you got those accesses time and what all are your considerations?