Weighted deficit earliest departure first scheduling

1
Weighted deficit earliest departure first
scheduling

• Source Computer Communications, vol. 28, Issue
  15,
• September 15, 2005, pp. 1711-1720
• Author Teck Meng Lim, Bu Sung Lee and Chai Kiat
  Yeo
• SCI 40.3 (23/57, Impact Factor 0.574)
• Report Chao-Yu Kuo (???)
• Date 2005/10/13

2
Key words

• QoS (Quality-of-Service)
• WRR (Weighted Round Robin)
• DRR (Deficit Round Robin)
• WDRR (Weighted Deficit Round Robin)
• WFQ (Weighted Fair Queueing)
• EDF (Earliest Departure First)

3
Outline

• Introduction
• WDEDF (Weighted Deficit Earliest Departure First)
  
• Simulations
• Conclusions
• Comments

4
Introduction

• Link scheduling is an important mechanism to
  achieve Quality-of-Service (QoS)
• Scheduling provide the differentiated bandwidth
  fairness and delay among queue on each router

5
Introduction

• The Weighted Round Robin (WRR)
• It is the pioneer that shows differentiated
  fairness among queues

Weight 3
Weight 2
Weight 1
6
Introduction

• The Deficit Round Robin (DRR)
• To provides near-perfect throughput fairness

i-st Round
(i1)-st Round
Packet Length 500 byte Quantum Size 800 byte
Deficit 800
Deficit 300
Deficit 1100
Deficit 1200
Deficit 200
Deficit 1000
Deficit 800
Deficit 300
Deficit 1100
7
Introduction

• The Weighted Deficit Round Robin (WDRR)
• To provide unfair bandwidth allocation to
  different queues in the scheduler

8
Introduction

• The Weighted Fair Queueing (WFQ)
• To guarantees both differentiated fairness and
  delay among its queues

Packet Length 10 bits
Round Number20
Round Number25
Queue 1
20
35
Queue 2
25
35
Queue 3
30
9
Introduction

• The Earliest Departure First (EDF)
• It is one of the best-known algorithms for
  Real-Time processing
• Scheduler tags packets with a deadline, which is
  the sum of the expected arrival time

10
Introduction

• The Weighted Deficit Earliest Departure First
  (WDEDF) scheduler
• To maintain the weighted share of bandwidth among
  queues
• Reducing the number of deadline misses in the
  delivery of packets

11
WDEDF (Weighted Deficit Earliest Departure First)

• WDEDF
• To allocate a weighted quantum using a
  proportional weighted function
• To forward the packet at the head of the queue
  with the earliest departure time
• It adopts a quantum-borrowing scheme to allow
  preemption of packets in the Blocked List

12
WDEDF (Weighted Deficit Earliest Departure First)

• Deficit
• Each queue is allocated a weighted quantum

Wi the quantum size of the queue i wMIN the
minimum quantum size of WDEDF RMIN min(Rj)
where R is the set of reservation rates P.S.
wMIN gt MTU (Maximum Transmission Unit) At least
one packet from each queue can be transmitted in
a service round
13
WDEDF (Weighted Deficit Earliest Departure First)

• Earliest Departure First
• The departure time (TD) is the time that a packet
  needs to leave the scheduler

14
WDEDF (Weighted Deficit Earliest Departure First)

• Earliest Departure First
• The expected departure time (TD) of each arrived
  packet

TDTC-L/Cl
TA the arrival time of a packet L the packet
size R the reservation rate of the packet Cl
the service rate of the server
15
WDEDF (Weighted Deficit Earliest Departure First)

• The quantum-borrowing scheme
• To maintain worst-case delay differentiation
• It reserves a percentage of the total quantum
  specified by network administrator

16
WDEDF (Weighted Deficit Earliest Departure First)

• The quantum-borrowing scheme
• Queues in the Blocked List are allowed to utilise
  this preemptive quantum
• If the first packet of the queue in the Blocked
  List has a TD that is earlier than the packet
  from the next queue in the Active List

17
WDEDF (Weighted Deficit Earliest Departure First)

• WDEDF Scheduling

18
WDEDF (Weighted Deficit Earliest Departure First)

• WDEDF Scheduling

19
WDEDF (Weighted Deficit Earliest Departure First)

• WDEDF Scheduling

20
WDEDF (Weighted Deficit Earliest Departure First)

• Example

Link Capacity 10 Mbps, WMIN 1 Mbps
Arrival Time 100
Arrival Time 100
Arrival Time 130
100400/2M-400/10M260
100500/3M-500/10M217
130500/3M-500/10M247
Queue A
260
Queue B
217
247
247
Time Unit µs
Deficit A 500
PQ 10000
PQ 9500
Deficit B 700
Deficit B 200
PDC B 700
PDC B 200
21
WDEDF (Weighted Deficit Earliest Departure First)

• State diagram of a WDEDF Queue

22
Simulations - a single hop
Link capacity 4 Mbps 20 flows Sending rate 200
kbps Packet arrival Poisson Distribution Packet
size 64500 bytes Sending rate of Flow 9 300
kbps
23
Simulations - a single hop
Throughput of each VBR flow when the router
implements the WFQ, DRR, EDF and WDEDF
schedulers. All flows generate traffic at the
same rate except for the malicious Flow 9. (a)
200 kbps for each flow
24
Simulations - a single hop
Throughput of each VBR flow when the router
implements the WFQ, DRR, EDF and WDEDF
schedulers. All flows generate traffic at the
same rate except for the malicious Flow 9. (b) f
kbps for flow f
25
Simulations - a single hop

• Flow 1-10
• CBR (Constant Bit Rate) traffic
• Packet size 500 bytes
• Sending rate 1-10 Mbps
• Flow 11-10
• VBR (Variable Bit Rate) traffic
• Poisson packet arrivals
• Packet size 64500 bytes
• Sending rate 1-10 Mbps

26
Simulations - a single hop
Average and worst-case delay of each flow when
router implements the WFQ, EDF, WDRR and WDEDF
schedulers. Flows 110 are CBR flows, and flows
1120 are VBR flows. (a) Average delay of each
flow
27
Simulations - a single hop
Average and worst-case delay of each flow when
router implements the WFQ, EDF, WDRR and WDEDF
schedulers. Flows 110 are CBR flows, and flows
1120 are VBR flows. (b) Worst-case delay of
each flow
28
Simulations deadline misses
Percentage () of packets delivered within their
specified deadlines
29
Conclusions

• WDEDF scheduler
• It is to provide separate guarantees for rate and
  delay
• It can maintain its weighted share of bandwidth
• It has a higher algorithmic complexity
• To sort by the deadlines of the packets

30
Conclusions

• WDEDF scheduler
• A worst-case packet delay bound is statistically
  less than the WDRR scheduler
• The throughput fairness is comparable to the WDRR
  and WFQ schedulers

31
Comments

• WDEDF needs revising in DiffServ
• Think of the solution