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Packet Scheduling: SCFQ, STFQ, WF2Q

Packet Scheduling: SCFQ, STFQ, WF2Q. Yongho Seok. Contents. Review: GPS, PGPS SCFQ( Self-clocked fair queuing ) STFQ( Start time fair queuing ) WF2Q( Worst-case fair weighted fair queuing ) Conclusion. GPS.

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Packet Scheduling: SCFQ, STFQ, WF2Q

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  1. Packet Scheduling:SCFQ, STFQ, WF2Q Yongho Seok

  2. Contents • Review: GPS, PGPS • SCFQ( Self-clocked fair queuing ) • STFQ( Start time fair queuing ) • WF2Q( Worst-case fair weighted fair queuing ) • Conclusion

  3. GPS • An idealized policy that can split bandwidth among multiple sessions simultaneously • Each session I has a queue and a weight • At time t, GPS serves all non-empty queues simultaneously in proportion to • Property

  4. GPS(Cont’) • Observation : • Guaranteed service rate for session j whenever it becomes backlogged • Another View • Weighted round robin with infinitely small service amount

  5. PGPS • Intuition • Compute the time a packet would complete service had we been serving packets with a GPS server, then serve packets in order of these finishing times • emulates GPS “on the side” and uses the results of this simulation to determine service order • Three virtual times • Not real time, but time for representing the amount of service • Virtual Star time, Virtual Finish time : each flow • Virtual System time : system-wide time

  6. Virtual Start Time Virtual Finish Time Virtual Time Implementation of WFQ PGPS(Cont’)

  7. PGPS(Cont’)

  8. WFQ : Scheduling Example • Situation • Three sessions : A, B, and C • Time 0 : packets of size 1(A), 2(B), and 2(C) arrives • Time 4 : a packet of size 2(A) arrives • Assumption • Weight are all same • Link Capacity C = 1

  9. Result comparison(GPS, WFQ) GPS 0 3 5.5 6 7 4 WFQ 0 1 3 5 7

  10. A 3.5 B,C 3 V(t) 2.5 Virtual Time F(0) = Max(0,0) + 2 = 2 2 1.5 F(0) = Max(0,0) + 1 = 1 1 0.5 Slope = C / weighted sum of backlogged flows = 1/3 0 1 2 3 4 5 6 7 8 Real Time(At time 0)

  11. A 3.5 B,C 3 V(t) 2.5 Virtual Time 2 1.5 Flow A is unbacklogged in WFQ at time 1 1 Flow A is unbacklogged in GPS at time 3 0.5 0 1 2 3 4 5 6 7 8 Real Time(AT time 3)

  12. 2nd packet of size 2 arrives A 3.5 F(4) = Max(1.5,1) + 2 = 3.5 B,C 3 V(t) 2.5 Virtual Time 2 1.5 Slope = 1/2 1 0.5 0 1 2 3 4 5 6 7 8 Real Time(At time 4)

  13. A 3.5 B,C 3 V(t) 2.5 Virtual Time Flow B or C is unbacklogged in GPS at time 5.5 2 Slope = 1/3 1.5 1 0.5 0 1 2 3 4 5 6 7 8 Real Time(At time 5.5)

  14. A 3.5 B,C 3 V(t) Slope = 1 2.5 Virtual Time 2 1.5 1 0.5 0 1 2 3 4 5 6 7 8 Real Time(At time 7)

  15. Fairness of WFQ • The difference between GPS and WFQ • Cannot fall behind GPS by one packet • One Packet difference means • Problem • Cannot fall behind GPS by one packet, however, can fall ahead GPS by infinite amount • Motivation of WF2Q

  16. Self-Clocked Fair Queuing (SCFQ) • Same as WFQ except • CF : the virtual finish time of the packet currently in service • Easier to implement than WFQ

  17. Relative Fairness & Absolute Fairness • The service rate allocated to connection I at the kth switch on its path from source to its destination • Relative Fairness • Absolute Fairness

  18. Fairness of SCFQ • The author has shown that the relative fairness bound for SCFQ is • But, the absolute fairness bounded for SCFQ is currently unknown • Although the SCFQ round number update rule is easy to implement, it can be unfair over short term scales

  19. Fairness of SCFQ(Cont’) • Worst-case latency for SCFQ is compared to for WFQ • STFQ improve a large worst case delay and short-term unfairness

  20. Start-time Fair Queuing (STFQ) • Same as WFQ except • SF : the virtual start time of the packet currently in service • Serves the packet having smallest virtual start time • Easier to implement than WFQ • Note

  21. Worst-case Fair Weighted Fair Queuing (WF2Q) • Conceptually, • S(t) : the total amount of service received by flow I • V(t) : the total amount of service , which would be received in GPS • WF2Q algorithm • WFQ scheduling + eligibility test • eligible test • Among packets that have started service under GPS, pick the packet having the smallest virtual finish time

  22. WF2Q : Scheduling Example

  23. Fairness of WF2Q • Cannot fall behind GPS by one packet, however, can not fall ahead GPS by one packet • This means

  24. Conclusion • WFQ provides fairness and end-to-end delay bound, but has a heavy implementation complexity • SCFQ, STFQ, WF2Q are easy to implement • Issues • Delay & bandwidth requirement is coupled

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