ISRO CSE 2008 | Question: 51

Question

With Round-Robin CPU scheduling in a time shared system

• A. using very large time slices (quantas) degenerates into First-Come First served (FCFS) algorithm.
• B. using extremely small time slices improves performance
• C. using very small time slices degenerates into Last-In First-Out (LIFO) algorithm.
• D. using medium sized times slices leads to shortest Request time First (SRTF) algorithm

Answer 1

Answer : A

Using very large time slices (quantas) degenerates into First-Come First served (FCFS) algorithm. And all the processes will be able to get completed  very fast.

Comments

Degeneration means decline /not possible

 

If we increase the time quantum of Round Robin Algorithm then the  scheduling will acts like First come First serve (FCFS)

 

How you should say it will degenerates FCFS

The option A is wrong

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@SABAREESH V  not possible nahi balki indistinguishable , 
degeneration means that the behavior of the Round-Robin (RR) CPU scheduling algorithm becomes indistinguishable from that of the First-Come, First-Served (FCFS) scheduling algorithm under certain conditions.

Explanation:

1. Round-Robin Scheduling:

   • In RR scheduling, every process gets a fixed time slice (quantum) to execute.
   • If a process does not finish during its quantum, it is preempted and moved to the back of the ready queue, allowing the next process to execute.

2. When Quantum Becomes Very Large:

   • If the time slice (quantum) is set to a very large value, it becomes long enough for any process to finish its execution in one go without being preempted.
   • In this case, processes are not preempted, and they execute in the order they arrive in the queue.

3. Resemblance to FCFS:

   • In FCFS, processes execute in the order they arrive, without preemption.
   • With a very large quantum, RR essentially mimics this behavior because each process runs to completion before the next process gets CPU time.

4. Why It’s Called Degeneration:

   • Degeneration here means the loss of the distinct feature of RR, which is frequent preemption to ensure fair CPU sharing.
   • As the quantum grows excessively large, RR loses this characteristic and becomes similar to FCFS, an entirely different scheduling strategy.

Example:

• Processes:
  P1 (Burst: 5ms), P2 (Burst: 8ms), P3 (Burst: 12ms).
  Quantum:
  • Very Small Quantum (2ms): Processes are frequently preempted, and CPU time is evenly distributed among all processes.
  • Very Large Quantum (e.g., 100ms): Each process completes entirely in the order it arrives, just like FCFS.

Conclusion:

Degeneration in this case refers to the scenario where the core behavior of an algorithm (fair preemption in RR) is lost, and it starts behaving like another algorithm (FCFS) under specific conditions (large quantum).

Answer 2

Ans A)using very large time slices (quantas) degenerates into First-Come First served (FCFS) algorithm

Comments

Can you explain option B?

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it increases context switching there by increasing cpu overhead