UGC NET CSE | January 2017 | Part 3 | Question: 2

Question

Which of the following is incorrect for virtual memory?

• A. Large programs can be written
• B. More I/O is required
• C. More addressable memory available
• D. Faster and easy swapping of process

Comments

I think its C because memory remains same..

Answer 1

Virtual Memory

• The ability to load only the portions of processes that were actually needed ( and only when they were needed ) has several benefits:
  • Programs could be written for a much larger address space ( virtual memory space ) than physically exists on the computer.
  • Because each process is only using a fraction of their total address space, there is more memory left for other programs, improving CPU utilization and system throughput.
  • Less I/O is needed for swapping processes in and out of RAM, speeding things up.

so B is the answer

refer:https://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/9_VirtualMemory.html 

Answer 2

incorrect for virtual memory 

1)Large programs can be written..True

2)More I/O is required..false

3)More addressable memory available True

4)Faster and easy swapping of process true
option B...

Comments

I think due to page fault and replacements I/O increases so 2) should be true and 3) should be false bcoz main memory does not increase or decrase.

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if u were not using virtual memory and u have a process p1 in the memory and suddenly a need for P2 arise..then u would have to swap P1 and bring P2..there we have more I/O....but in virtual memory this is not hte ase we can have many processes simultaneously

without virtual memory, we use main memory or RAM...with virtual memory, we use part of secondary memory also as main memory(just an halucination)...so addressable memory increased

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how 4 is true?

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^ because only few pages of a process reside in the memory ...so to swap a whole process we need not swap all its pages...just swap whatever is inside the memory..so that will obviously be faster in comparison to without virtual memory

but the whole procedure of this virtual memory concept is quite slow..as eveytime we get some page fault we need to load that page...also thrashing is quite frequent in virtual memory

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But demand paging can also be done even without virtual memory but you need contigous mem locations.

 

This is even more easier to implement but will accomodate less processes.

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demand paging is different from virtual memory?

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What is virtual memoy? You can have pages not continous but at different locations. Thats it.

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https://en.wikipedia.org/wiki/Demand_paging

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I think 4th Option should be incorrect

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@Sudsho. If we having fixe sized pages and if we allocate contigous memory to each process then we need to keep track of last address. By this, we can also bring limited pages inside.

Answer 3

Answer is option 3

option 1-Large programs can be written.

True because entire program need not be in main memory at a time.

option 2-More I/O is needed.

True because we are not bringing entire program into memory at a time .i.e, bring when needed.Hence more i/o is needed.

option 3-More addressable memory available.This is false because the amount of memory addressable by C.P.U depends on size of address bus

Option 4--Faster and easy swapping of process.This is true because we aren't bringing entire program .instead we bring it in parts i.e the amount of data to be brought to main memory and data to be written to disk is less.

Hence less time for swapping

Comments

Isn’t less I/O required with virtual memory? If no virtual memory, complete program will have to be in memory which will require more I/O as compared to bringing in only the part of program needed.

Answer 4

i think 4 is answer because ...how virtual memory can make FAST and easy swapping..???

in VM we simply load only required pages of process...but it doesn't increase speed of swaping ....