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Round-Robin (RR) Scheduling

Overview:

Round-Robin (RR) Scheduling is a pre-emptive scheduling algorithm used in operating systems to allocate CPU time to processes. Each process is assigned a fixed time slot, known as a time quantum, and processes are executed in a cyclic order.

Ensures fairness by giving each process an equal share of the CPU.
Reduces starvation since every process gets a chance to execute.
Ideal for time-sharing systems where response time is crucial.

Time Quantum

Definition:

The time quantum is the fixed time period that a process is allowed to run before being interrupted by the scheduler.

Too short a time quantum increases context switching overhead.
Too long a time quantum can lead to poor response times.

Context Switching

Definition:

Context switching is the process of storing the state of a process so that it can be resumed from the same point later. It occurs when the CPU switches from executing one process to another.

Involves saving and loading process registers, memory maps, etc.
Frequent context switching can degrade system performance.

Example 1: Basic Round-Robin Scheduling

Scenario:

Consider three processes P1, P2, and P3 with burst times of 4, 5, and 7 units respectively. The time quantum is 2 units.


    Process | Burst Time | Completion Order
    P1      | 4         | P1, P2, P3, P1, P2, P3, P3
    P2      | 5         |
    P3      | 7         |

Explanation:

Each process is given 2 units of CPU time in a round-robin fashion. The scheduler cycles through the processes until all are complete.

Example 2: Impact of Time Quantum Size

Scenario:

Using the same processes as Example 1, change the time quantum to 4 units.


    Process | Burst Time | Completion Order
    P1      | 4         | P1, P2, P3, P2, P3
    P2      | 5         |
    P3      | 7         |

Explanation:

With a larger time quantum, the number of context switches decreases, potentially improving performance but increasing response time.

Example 3: Handling Long Processes

Scenario:

Consider a long process P1 with a burst time of 20 units and two short processes P2 and P3 with burst times of 3 units each. Time quantum is 4 units.


    Process | Burst Time | Completion Order
    P1      | 20        | P2, P3, P1, P1, P1, P1, P1
    P2      | 3         |
    P3      | 3         |

Explanation:

Short processes complete quickly, while the long process continues to receive CPU time in cycles, reducing the wait time for short processes.

Example 4: Equal Burst Times

Scenario:

Three processes P1, P2, and P3 each have a burst time of 6 units. The time quantum is 2 units.


    Process | Burst Time | Completion Order
    P1      | 6         | P1, P2, P3, P1, P2, P3, P1, P2, P3
    P2      | 6         |
    P3      | 6         |

Explanation:

Each process receives equal CPU time, demonstrating the fairness of the round-robin approach.

Example 5: Varying Arrival Times

Scenario:

Three processes P1, P2, and P3 with burst times of 4, 5, and 7 units arrive at different times. The time quantum is 3 units.


    Process | Burst Time | Arrival Time | Completion Order
    P1      | 4         | 0            | P1, P2, P3, P2, P3
    P2      | 5         | 1            |
    P3      | 7         | 2            |

Explanation:

Processes are scheduled based on their arrival times, with each receiving CPU time in a round-robin manner after arrival.