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Tutorial 9 Cont

Round-Robin. Round-robin is one of the simplest scheduling algorithms for processes in an operating system, which assigns time slides to each process in equal portions and in order, handling all processes without priority.Round-robin scheduling is both simple and easy to implement, and starvation-f

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Tutorial 9 Cont

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    1. Tutorial 9 (Cont)

    2. Round-Robin Round-robin is one of the simplest scheduling algorithms for processes in an operating system, which assigns time slides to each process in equal portions and in order, handling all processes without priority. Round-robin scheduling is both simple and easy to implement, and starvation-free. The name of the algorithm comes from the round-robin principle where each person takes an equal share of something in turn.

    3. Round-Robin Example 3 service flows, the Round-robin will schedule as follows: 1 2 3 1 2 3 1 2 3 .

    4. Weighted Round-Robin Weighted Round-robin (WRR) is a best-effort connection scheduling discipline. WRR serves a number of packets for each nonempty connection. Number = normalized ( weighted / mean packet size) Drawbacks: Mean packet size must be known; In a scale of one round WRR doesnt provide fair link sharing.

    5. WRR Example The mean packet sizes of class A, B, C are 2, 3, 4 The weights of A, B, C are 0.5, 0.3, 0.2 The normalized weights of A, B and C are: A: 0.5/2 = 0.25 B: 0.3/3 = 0.1 C: 0.2/4 = 0.05 Minimal normalized weight is 0.05

    6. WRR Example (Cont) The number of packets serviced for A, B and C are: A: 0.25/0.05 = 5 B: 0.1/0.05 = 2 C: 0.05/0.05 = 1 A possible schedule is: A A A A A B B C .

    7. Weighted Fair Queuing Weighted Fair Queuing (WFQ) is a data packet scheduling technique allowing different scheduling priorities to statistically multiplexed data flows. Link data rate is R, weights of N data flows are w1,w2,,wN, the ith data flow will achieve an average data rate of R*wi / (w1 + w2 + + wN) WFQ explicitly considers data queue, and by regulating the weights dynamically, WFQ can be utilized for controlling the QoS.

    8. WFQ Example Link data rate R = 1 kbps Weights of class A, B and C are 0.5, 0.3 and 0.2 Thus, the average date rate of A, B, C are: A: 0.5 kbps B: 0.3 kbps C: 0.2 kbps

    9. WFQ Example (cont) Suppose the packet sizes of A, B and C are the same: 128 bytes/packet = 1 kb/packet We can calculate that the number of packets serviced for A, B and C in every 10 seconds are: A: 5 packets B: 3 packets C: 2 packets A possible schedule is A A A B B A A B C C .

    10. WFQ Example (cont) The data rate control can be provided by Token Bucket

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