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# Assignment No. 02 Semester: Fall 2015 CS604 –Operating System..........Total Marks: 20 Due date: 09-12-2015

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#### 6.3.4 Round Robin Scheduling

• Round robin scheduling is similar to FCFS scheduling, except that CPU bursts are assigned with limits called time quantum.
• When a process is given the CPU, a timer is set for whatever value has been set for a time quantum.
• If the process finishes its burst before the time quantum timer expires, then it is swapped out of the CPU just like the normal FCFS algorithm.
• If the timer goes off first, then the process is swapped out of the CPU and moved to the back end of the ready queue.
• The ready queue is maintained as a circular queue, so when all processes have had a turn, then the scheduler gives the first process another turn, and so on.
• RR scheduling can give the effect of all processors sharing the CPU equally, although the average wait time can be longer than with other scheduling algorithms. In the following example the average wait time is 5.66 ms.
Process Burst Time
P1
24
P2
3
P3
3

• The performance of RR is sensitive to the time quantum selected. If the quantum is large enough, then RR reduces to the FCFS algorithm; If it is very small, then each process gets 1/nth of the processor time and share the CPU equally.
• BUT, a real system invokes overhead for every context switch, and the smaller the time quantum the more context switches there are. ( See Figure 6.4 below. ) Most modern systems use time quantum between 10 and 100 milliseconds, and context switch times on the order of 10 microseconds, so the overhead is small relative to the time quantum.

Figure 6.4 - The way in which a smaller time quantum increases context switches.

• Turn around time also varies with quantum time, in a non-apparent manner. Consider, for example the processes shown in Figure 6.5:

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Assignment No. 02
Semester: Fall 2015

CS604 –Operating System

Total Marks: 20

Due date: 09-12-2015

Assignment Objective:

• Implementation of CPU scheduling algorithm
• To know algorithm behind different scheduling mechanisms
• Comparisons of CPU scheduling algorithms
• How to find average waiting time

• Your assignment must be in .doc format (Any other formats like scan images, PDF, bmp, etc will not be accepted).
• No assignment will be accepted through email.

Rules for Marking:

It should be clear that your assignment will not get any credit if:

• The assignment is submitted after due date.
• The submitted assignment does not open or file is corrupted.
• Your assignment is copied from internet, handouts or from any other student

(Strict disciplinary action will be taken in this case).

Assignment

Question

Three processes P1, P2, P3 reaches at the same time for processing with burst time as given in the following Gantt chart.

 Process P1 P2 P3 Burst Time(ms) 11 3 6

Consider time slice of 4 milliseconds for both algorithms.

Give Gantt chart for each of the following CPU scheduling algorithms and calculate average waiting time and average turnaround times.

1. Round Robin
2. Shortest Remaining Time First

Note: No need to write theory. Just give Gantt chart, Waiting Time and Turnaround Time.

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Average turnaround time for SRTF & round robin ? How will calculate?

How can we find the arrival time for SRTF algorithm given in the assignment?

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