Fair Scheduling With Tunable Latency : A RoundRobin Approach

1
Fair Scheduling With Tunable Latency A
Round-Robin Approach

• AuthorHemant M. Chaskar and Upamanyu Madhow,
  Senior Member, IEEE
• SourceNetworking, IEEE/ACM Transactions on
  , Volume 11 , Issue 4 , Aug. 2003 Pages592 -
  601
• Reporter Tsang-Yuan Tsai
• Date2004/8/7

2
Outline

• Introduction
• Multiclass Weighted Round Robin(WRR)
• Comparisons with worst-case fairness and latency
  tuning characteristics
• Conclusion and Comments

3
Introduction

• Different Weighted Fair Queueing based schemes
• Packet Generalized Processor Sharing (PGPS)
• Self-Clock Fair Queueing (SCFQ)
• Worst-case Weighted Fair Queueing (WF2Q)

4
Example of related scheduling

• First source get 0.5. Other 10 sources get 0.05
  each. First source
• sends 11 cells. 2-11 send one each at t 0.

5
GPS

• Each cell of the first source takes 2 units of
  times. Sources 2-11
• take 20 units each.

6
PGPS
7
SCFQ

• Tag packet length/service rate max tag of
  the previous packet

8
WF2Q

• difference in packet service time bounded by max.
  packet size.

9
Multiclass WRR (1/3)

• Provided two classes C1 and C2
• N1 sessions in C1 and N2 sessions in C2
• Assumed that D2nD1 for n gt 1 and the following
  feasibility conditions hold
• Session i in Ck class only visits once during Dk
  minicycles
• The fraction of total link BW Fi 1/Dk (fixed)

10
Multiclass WRR(2/3)
11
Multiclass WRR(3/3)

• C1A,D12, C2B,C,D24
• Condition 1/2 2/4 1
• gt 2/4 2/4 1

D1 minicycle
D2/D1 minicycle
12
Multiclass WRR Control

• Control 1 Measuring the Length of Minicycle in
  Terms of Offered Service Opportunities Rather
  Than Transmissions
• Control 2 Not Starting the New Round-Robin
  Cycle Too Early

13
Control 1

• C1 A,B , D1 4 and C2 C,D,E,F, D2 8
• Condition 2/4 4/8 4/8 4/8 (no visit
  wasted)
• means some session is not backlogged

14
Control 2

• C1 A , D1 2 and C2 B, C , D2 8
• Condition 1/2 2/8 4/8 2/8 (two visits
  wasted)
• D2/D1 4 means that visits in class C1 C2 41

15
Example of Multiclass WRR

• C1A,B,D15, C2C,D,E, D210, C3F,G,H,I,J,
  D320
• Condition 2/5 3/10 5/20 1
• gt 8/20 6/20 5/20 1
• gt 19/20 1

16
Comparisons with Worst Case Fairness(1/2)

• Worst Case Fairness
• Supposed a packet of session i Ck arrives at
  time ta

Qi(ta)
9
Depart time
td ta Qi(ta) Dk Dk ta (Qi(t)/Fi)
(1/ Fi )
Where 1/Fi is independent of the total number of
session
17
Comparisons with Worst Case Fairness(2/2)

• Provided FA 1/2 , CA A , FB 1/10, CB
  B1,B2,B3,B4,B5 and all these sessions are
  backlogged at all time (DA 2, DB 10)
• Worst-Case Fair
• W2FQ A B1 A B2 A B3 A B4 A B5
• Multiclass WRR A B1 A B2 A B3 A B4 A B5
• Worst-Case unfair
• PGPS A A A A A B1 B2 B3 B4 B5

18
Comparisons with Latency tuning
characteristics(1/2)

• Latency Turning Characteristics
• A packet of session i Ck that starts the
  backlogged period for i departs before a maximum
  Dk transmission slots.
• The latency of session i is Ti Dk

19
Comparisons with Latency tuning
characteristics(2/2)

• Provided FA 1/2 , CA A , FB 1/10, CB
  B1,B2,B3,B4,B5 and B1-B5 sessions are
  backlogged at all time but A becomes backlogged
  from the fourth time slot (DA 2, DB 10)
• Efficient latency tuning
• PGPS B1 B2 B3 A A B4 B5 A A A A A B1 B2 B3 B4
  B5
• WF2Q B1 B2 B3 A A B4 B5 A B1 A B2 A B3 A B4 A
  B5
• Multiclass WRR B1 B2 B3 A B4 A B5 A B1 A B2 A
  B3 A B4
• Inefficient latency tuning
• SCFQ B1 B2 B3 B4 B5 A A A A A

20
Conclusion and Comments

• Lower complexity than WFQ-based schemes
• Multiclass WRR scheme is described for fixed
  packet length.
• Similar to proportional sharing, but waste visits
• Every Round Robin looked as every time slot. And
  every visit session looked as simultaneous
  transmitted sources
• Credit-based scheme may applied for variable
  length packet and long term scheduling.