operating system

Question

Suppose the following jobs are to be executed in a uniprocessor system.

Multilevel Feedback Queue(MLQ) is used with queues numbered 1-10, quantum = 2ⁱ, where i is thequeue level number and processes are initially placed in the first queue (i.e., level 1). In this schedulingpolicy, each process executes at a particular level for one quantum and then moves down a level; processesnever move up a level. The average process turnaround time, the normalized turnaround time for process2, and the processor efficiency using MLQ is,

1.

11.4, 2, 83.3%

2. 

18, 4.25, 71.4%

3.

18.6, 3.375, 71.4%

4.

11.6, 3.75, 80.6%

 

Need good explanation :( (answer given is b.)

Comments

but answer not matching exactly

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@sahil_malik where are the following jobs or burst times mentioned in the question ?  It is not showing up especially the burst times. Don't u think after the first line in ur question there would have been the scheduling table mentioned especially the burst times ?  

 

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@user2525

do u know, which test series ques. is it? I am having doubt, if there was some picture, which is missing now

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@srestha ......... it is @sahil_malik who had posted the question. He might know.

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I took Context switch overhead = 1 as it is given in the question “each process executes at a particular level for one quantum”. I hope this is right :)

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urshi ,

why P1 executed at 10 sec in queue 2 ? p5 and p4 should be executed in queue1 before , right ?

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@sarthak_02

p1 is executed at 10 because at time 9 sec p4,p5 are not available as time 9-10 is used for context switch so at 9 it is necessary for a process to be present to get scheduled.

hope this helps.

Answer 1

In multilevel queue scheduling , if one level time quantum exits, it goes to next lower level 

ML Queue Scheduling uses RR scheduling

Here it is given time quantum of 1st level queue is 2 ms

2nd level of queue is 4 ms

and 3rd level queue is 8 ms

Now, solve accordingly

$P_{1}$

$P_{2}$

$P_{3}$

$P_{1}$

$P_{2}$

$P_{4}$

$P_{5}$

$P_{4}$

$P_{5}$

$P_{2}$

$0.$          $2$          $4$.         $6$.            $8$.         $12$        $14$.        $16$.        $20$.       $23$.        $25$

$\underbrace{.{\color{Magenta}{1st- level-queue } } }$$\underbrace{.{\color{blue}{2nd-- level } } }$$\underbrace{.{\color{Magenta}{1st-- level } } }$$\underbrace{.{\color{blue}{2nd-level } } }$$\underbrace{.{\color{red}{3rd- level } } }$

Now, Throughput for all processes $\frac{8+24+3+10+11}{5}=11.2 ms$

Efficiency $\frac{25}{\left ( 2\times 5 \right )+\left ( 4\times 4 \right )+8}\times 100=73.5 ms$

https://www.youtube.com/watch?v=cApdURNWJPc

https://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/5_CPU_Scheduling.html

Comments

@srestha before checking the links, have u checked the question posted above ?

In the first line, Sahil has written : 

Suppose the following jobs are to be executed in a uniprocessor system.

 

Check the question first and I am sure if the question has been a complete one, the correct answer would definitely come.

It's clearly written in the first line  "SUPPOSE THE FOLLOWING JOBS "

 

My question is where are the following jobs and their details ?

THE SECOND PARAGRAPH OF THE QUESTION DEALS ONLY WITH QUEUES AND THEIR QUANTUM TOWARDS THE PROCESSES BUT WHERE ARE THE PROCESSES AND THEIR CONCERNED INFORMATION ?? 

 I am very much sure there must be something like the scheduling table between the first line and the second paragraph. The sentence should follow some information when the word following is already written in the first lyn.

Moreover, the question is asking find the turnaround time and other things regarding PROCESS 2. We dont even know what are the processes .

See, as we all know the queues perform a particular scheduling algo. Here, queue at level 1 is performing Round robin algo with quantum 2, queue 2 is performing Round robin algo with quantum 4 and so on. It is visualized like that only.

Even in Round Robin algo. problems we are given burst times in addition to tym quantum. But here only tym quantum is given. Dnt u think the question is incomplete ?

@sahil_malik @Manish Singh 4 @sathvikchiramana and Srestha , kindly reply if I am wrong with the said words.

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process	Arrival Time	burst
1	0	4
2	1	8
3	3	2
4	10	6
5	12	5

this is the table along with the question

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Good 👍