Tutorial 9 Cont

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.

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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.

This page is from WIKIPEDIA
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
• Rwi / (w1 w2 wN)
• WFQ explicitly considers data queue, and by
  regulating the weights dynamically, WFQ can be
  utilized for controlling the QoS.

This page is from WIKIPEDIA
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