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I got some doubt while solving previous year questions:-

Since While calculating EMAT and question involves page fault service time,we use formulae:-

p*s+(1-p)*m ,taken from Galvin

p is page fault rate

s is page fault service time

m is main memory access time

Why don't we include 2m instead of m in case if it is not a page fault?With the (1-p) as probability i will not get a page fault so for these i will goto page table and then to main memory.Why to use m instead of 2m?

Ref question:-https://gateoverflow.in/2122/gate2011-20-ugcnet-june2013-ii-48

Where as in questions where page fault service time is not there we use 2m

Ref question;- https://gateoverflow.in/2067/gate2014-3-33 which does make sense.

So why in first reference/doubt we are not using 2m instead of m in case of NOT a page fault?

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2 Answers

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10 votes

According to Galvin EMAT = (1-p) ma + p* page fault service time

Page fault service time (if nothing specific is given ) includes time taken to access secondary memory to find the page + time taken to access the main memory after the page is loaded.

Suppose we want a page called Page X which is not in the Main memory, the following things will happen -

1) Page Table (assuming there is one) will search for Page X and wont find any, this will lead to Page Fault.

2) This will begin the Page Fault Service Time where the it will access the Secondary memory for Page X and once found will load it into the Primary memory and update the page table.

3) Now we will access the Primary memory knowing it is loaded into the Primary memory successfully.

So , " The time required to access page in secondary memory is 100 ns. " ,

then the Page Fault Service Time would be 100 + 10 = 110 ns. where Time required to access a page in primary memory is 10 ns.

And 100 ns is time require to access page in secondary memory .

 we can say , Accessing the page in secondary memory + Loading the page in main memory + Accessing the page in Main memory = 100 ns where service the page fault time given is 100 ns .

see , page fault service time = page fault overhead + swap page out + swap page in + restart overhead

so all these is included there in service time ...

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0 votes

i think :- here  S-> stands for page fault service time and m->main memory access time

rahul sharma

if there is no page fault....(consider single level paging)

MAT=m(for page table) only bcoz it will provide the frame no.(main memory) where the real page exits...(now once the cpu find the effective address of the data it will  start execution)....

​​​​​​​1.here we calculate the time taken to find out effective address... once the real address of the data(page ) is found then memory access time is over...

????

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