GATE IT 2006 | Question: 12

Question

In the working-set strategy, which of the following is done by the operating system to prevent thrashing?

1. It initiates another process if there are enough extra frames.
2. It selects a process to suspend if the sum of the sizes of the working-sets exceeds the total number of available frames.

• A. I only
• B. II only
• C. Neither I nor II
• D. Both I and II

Comments

gate 2006 is a different gravy, evertime someting new and spicy.

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Simple Rule :

Enough frames?
     ↓
   YES → Can start another process

Not enough frames?
     ↓
   YES → Suspend a process

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yeah especially from gate 2006,2013 ..

Answer 1

Extract from Galvin "If there are enough extra frames, another process can be initiated. If the sum of the working-set sizes increases, exceeding the total number of available frames,the operating system selects a process to suspend. The process’s pages are written out (swapped), and its frames are reallocated to other processes. The suspended process can be restarted later."

So Option (D)

Comments

Arjun sir noone is clearing this since years in this discussion that why is $I$ directly reducing thrashing directly. Please! Help. 

edit: It's prolly cause if $I$ is not followed then the algo would have initiated another process even when enough number of frames r not available and hence it would still thrash.

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In the next paragraph of Galvin, it is written that “This working-set strategy prevents thrashing while keeping the degree of
multiprogramming as high as possible. Thus, it optimizes CPU utilization.”

Thus, I think I statement is for “keeping the degree of multiprogramming as high as possible” and II statement is to “Prevent Thrashing”.

Can anyone confirm about this, please?

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why both option a and b ...why a ??
see here they clearly told that in working set strategy prevent thrashing
in option a increse if enough frames it doesnt cause thrashing as there are enough frames so indirelty it prevent thrashing in working set model view
and option b u know it is correct

so both are correct

Answer 2

ANS - D

Thrashing is a condition or a situation when the system is spending a major portion of its time in servicing the page faults, but the actual processing done is very negligible.

I is true because to prevent thrashing we must provide processes with as many frames as they really need “right now”.If there are enough extra frames, another process can be initiated.

2 is true because to avoid thrashing process must be suspended.

Comments

Yes, you are right but in conclusion I, it’s written that we already have extra frames and not we must provide extra frames. So, I think conclusion is correct in it’s own place but doesn’t helps in reducing thrashing. If anyone knows what was the answer by GATE organizers then please let me know. Thank you.

Answer 3

ans D)
 

http://courses.mpi-sws.org/os-ss11/lectures/mem4.pdf

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

Answer 4

In layman terms working set is used to predict locality of a process, so that we can predict how many and what pages a process may be needed for execution. If we do this we can reduce no of page faults and hence thrashing can be reduced.

Wss i = working set required by an i_th process

∑ Wss i = sum of the sizes of the working-sets

D = total number of available frames.

Case 1 : D > ∑ Wss i

Before the start of referring pages for a process, with the help of pre-paging we bring all the pages required by a process into the available frames. We can do this for every process because we have enough frames since D > ∑ Wss i . 

"It initiates another process if there are enough extra frames". This line means we can pre-fetch the pages required for a process into available frames and start its execution.If we do this chance of thrashing are very less because very less page faults may occur.

Case 2 : D < ∑ Wss i

We have less total frames than total pages required by all processes, then the chance of page fault is very high.To prevent page fault we suspend a victim process ( i.e move it secondary memory). When we suspend the process then '∑ Wss i' term decreases then chances of page fault decrease, hence chance of thrashing decreases