GATE CSE 2006 | Question: 06, ISRO2009-14

Question

Consider three CPU-intensive processes, which require $10$, $20$ and $30$ time units and arrive at times $0$, $2$ and $6$, respectively. How many context switches are needed if the operating system implements a shortest remaining time first scheduling algorithm? Do not count the context switches at time zero and at the end.

• A. $1$
• B. $2$
• C. $3$
• D. $4$

Comments

option c

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Refer https://gateoverflow.in/885/gate2006-06-isro2009-14

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@MiNiPanda

Please answer this comment : 

https://gateoverflow.in/885/gate2006-06-isro2009-14?show=220079#a220079

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Explanation 

Answer 1

Processes execute as per the following Gantt chart 

So, here only $2$ switching possible (when we did not consider the starting and ending switching )

Now here might be confusion that at $t= 2 \;\;p_{1}$ is preempted and scheduler checks that the newly arrived process has shorter job time or not, and does not find one, so it should not be consider as context switching (same happened at $t=6$).

Reference: http://stackoverflow.com/questions/8997616/does-a-context-switch-occur-in-a-system-whose-ready-queue-has-only-one-process-a (thanks to anurag_s)

Answer is (B)

Comments

Meghna, that is why initial and last CS is ignored.

So only 2

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But after termination of the process ,PCB of that process gets deleted.

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THANKS I HAVE A MISSCONCEPTION NOW RESOLVE

 

Answer 2

Only two switches wil be required after process 1 and 2 complete.Option B.

Answer 3

So ans is B, only two context switching is needed.

Answer 4

BEST ANSWER

Answer 5

Since SRTF is used, processess execute in order:switching to P1 included(given in question)P1-- 10 units; then switch to P2 -- 20 units; then switch to P3 -- 30 units. (Here arrival time need not be considered)

Total context switch= 3

Comments

where is given switching to p1 included context switch ?

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It's clearly mentioned that not to take first "Do not count the context switch at time zero and at the end".so it can't be 3

Actual answer is 2

Answer 6

0

when execution of one process is completed and other process is scheduled , Is it called Context switch ?

I think , It is not Context switch, as according to wiki

In computing, a context switch is the process of storing the state of a process or of a thread, so that it can be restored and execution resumed from the same point later. This allows multiple processes to share a single CPU, and is an essential feature of a multitasking operating system.

As we are not storing results od earlier process for later execution. If there are any results produced by some operation within process or program , It is responsibility of program (or process when it running) to store them back.

For example : when C program to add two number gets converted to Assembly language code , It also has instruction to store result of addition back into memory.

Hence answer to this previous year gate question must be 0

Comments

@Headshot I also had this doubt while solving but none of the options matched :/

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Say if round robin would be used here with time slice equals to 1 , then those switches will be considered as context switches, right?

Because here we are saving the burst time of a particular job in it’s PCB and restoring it after processor processes the same job again. So, here context switches would occur according to what I have understood. Please correct me if I am wrong.

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@iamavx yeah it's  true .And context switch means process switch and it's acceptable .But if there's no process in the cpu then also same thing happens that some process will be pulled out from ready queue and all the register values from pcb will be restored and starts execution. 

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actually context switch happens when we schedule P2  form P1 but it is different from regular context switch when P1 is terminated 

hope it helps.
https://cs.stackexchange.com/questions/74049/does-a-context-switch-happen-when-a-process-has-terminated