Virtual memory question

Question

TLB lookup time ($t_h$) = 20 ns

TLB hit ratio ($h_t$) = 99%

Memory access time ($t_s$) = 100ns

Page fault rate ($h_p$) = 0.05%

Swap page time (in or out) ($t_p$) = 5000,000 ns

What is the effective access time (EAT) if we assume that all pages currently in main memory are dirty?

1. 5118.91 ns

2. 51.21 ns

3. 5120.9 ns

4. None of the above.

Comments

@Manu thakur

u gave this formula

E.M.A.T =
=TLB Hit*( TLB A.T + M/M A.T) + (1 - TLB Hit)( TLB A.T + NoPageFault( M/M A.T + M/M A.T) +
P*(Page Fault Service Time)).

means if there is a page fault there u r taking only page fault service time. That is right.

But I have a query, if there is a page fault, is there no TLB hit?

page fault means page is not accessable, that doesnot mean page is not available in main memory

https://gateoverflow.in/73973/%23operating-system

So, can u tell me if anything wrong in my concept?

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@srestha I am not denying the possibility of occurring page fault in case of TLB hit. that is definitely possible but such special cases should be mentioned in the question. Can you please show me any gate question when they considered page fault with tlb hit?

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ok, right ,I need to chk it

thanks :)

Answer 1

EAT=No PageFault(TLB hit+TLB miss)+Page Fault(TLB hit+TLB miss+Page swap time)

     $=0.9995\left ( 0.99\left ( 100+20 \right )+0.01\left ( 100+100+20 \right ) \right )+

0.0005\left (  0.99\left ( 100+20 \right )+0.01\left ( 100+100+20 \right )+5000000 \right )$

=2621ns

Comments

on page fault why you look in TLB ????

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When we got TLB hit then our page is in the memory. (Here we don't get page fault).
The confusing part is when we got TLB miss. I'm not able to get it.

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if there is TLB hit, still there can be page fault

https://gateoverflow.in/73973/%23operating-system

https://gateoverflow.in/153152/true-false

so, similarly we can say if there is page fault, still there can be TLB hit

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Hmm you are using formula in different way.