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Scheduling Algorithms in High Speed Network

Scheduling Algorithms in High Speed Network. 2000-20731 양우진. Outline. Introduction Goals Difficulties PFQ Algorithms Grouping architecture LBT / GBT For fixed length cells For variable length packets Timestamp Implementation Delay Result. Goals.

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Scheduling Algorithms in High Speed Network

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  1. Scheduling Algorithms in High Speed Network 2000-20731 양우진

  2. Outline • Introduction • Goals • Difficulties • PFQ Algorithms • Grouping architecture • LBT / GBT • For fixed length cells • For variable length packets • Timestamp Implementation • Delay Result

  3. Goals • Support a large number of sessions with diverse bandwidth requirement • Operate at very high speed • Maintain important properties of GPS • Guarantee an end-to-end delay to a leaky-bucket-constrained session, regardless of other sessions • Ensure fair allocation of bandwidth

  4. Difficulties • Key difficulty with PFQ algorithms • require buffering on a per session basis and non-trivial service arbitration among all sessions -> Not scailable • Arbitration needed(per session) • Computation of the system virtual time • Management of a priority queue • Management of another priority queue to regulate

  5. PFQ Algorithms(1) • Complexity of calculating virtual time • Accuracy and complexity trade-offs

  6. PFQ Algorithms(2) • Packet selection policy • SFF(Smallest virtual Finish time First) • SSF(Smallest virtual Start time First) • SEFF(Smallest Eligible virtual Finish time First) • Maintain a priority queue • Number of entities in the priority queue is the number of active sessions • Implementation complexity • on-chip / off-chip

  7. Outline • Introduction • Goals • Difficulties • PFQ Algorithms • Grouping architecture • LBT / GBT • For fixed length cells • For variable length packets • Timestamp Implementation • Delay Result

  8. LBT / GBT • LBT • Bounds the differences of virtual start times between two sessions in the same group • GBT • Bounds the differences between the system virtual time and virtual start time

  9. Grouping Architecture for ATM Network(1)

  10. Grouping Architecture for ATM Network(2) • Restriction : only a fixed number of guaranteed rates are supported by the server • Sort by start time in the group • Finish time is calculated • Fi = Si + L/ri

  11. Grouping Architecture for ATM NetworkChoose Next Eligible Cell(1) • Consider only the packets in the scheduler(head) • SSF: smallest S(.) in the scheduler -> smallest S(.) among all packet • SFF: packet in the scheduler -> smallest F(.) within the group -> smallest S(.) within the group (because of same rate) • SEFF: eligible packet exist in the group -> head packet is also eligible

  12. Grouping Architecture for ATM NetworkChoose Next Eligible Cell(2)

  13. Grouping Architecture for ATM NetworkMaintaining Within Groups • Maintain a priority queue with a simple linked list • Local timestamp bound(LTB) : Stail – Shead < L/r • Situations when insertions into the list is needed (Re-calculate start time, finish time) • After finishing services head session • A new session joins • Idle session becomes active

  14. Grouping Architecture for Packet Network • Problem: variable packet size • Fi - Si = L/ri (L is not determined) • Smallest Si --X-> Smallest Fi • 2-D sort is needed • By Fi – Si (=service interval Φi) • By Fi • Grouping by service interval • Same service interval -> same Fi-Si • Large number of unused group • Sessions change groups due to change of packet length

  15. Grouping Architecture for Packet NetworkDiscretization of Service Interval • Determine group group

  16. Grouping Architecture for Packet NetworkProblem with Discretization • The maximum gap between time to be eligible and computed virtual start time ( )

  17. Grouping Architecture for Packet NetworkSorting Within The Group • Aproximated finish time

  18. Outline • Introduction • Goals • Difficulties • PFQ Algorithms • Grouping architecture • LBT / GBT • For fixed length cells • For variable length packets • Timestamp Implementation • Delay Result

  19. Timestamp Implementation(1) • Size of timestamp determines… • The range of supportable rates • The accuracy • Memory requirements (bandwidth / storage space) • Size of timestamp • 1-bit larger than the largest service interval of the largest rate in the system • Pf) With globally bounded timestamp property, largest gap of timestamps < L/r (=service interval) (n+1 bit) needed for (2n) difference

  20. Timestamp Implementation(2) • Representations of timestamp • V(t)=429496678410 , high rate session i, low rate session j • Fixed point • Floating point

  21. Timestamp Implementation(3) • Compressed timestamp • Sj = V(t) + Φj • We only store CSj = Sj[M+ej+1:ej]

  22. Delay Result • ATM scheduling architecture(Proposed in this paper) • A cell of session i will miss its deadline by no more than (transmit time of a ATM cell + service interval of session I) • Packet network architecture • A packet of session j will miss its deadline by no more than (transmit time of maximum length packet + the maximum gap between time to be eligible and computed virtual start time + the maximum inflation of virtual finish time)

  23. Summary • Scalability problems with PFQ Algorithms • Per session management • Propose grouping architecture • For fixed length cell: grouping rate • For variable length packet: grouping rate and length • Propose time stamp optimization • Calculate minimum bit • Propose compressed timestamp • Delay Result

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