Weighted Fair Queueing

1. Weighted Fair Queueing GPS PGPS SCFQ Implementation

2. Round-robin • problems • packets lengths are not the same  not fair a a b e b e c d c d (a)round-robin (b) weight round-robin

3. Generalized Processor Sharing (GPS): (A. Parekh, 1992) • assumptions • traffic is infinitely divisible. • serve multiple sessions simultaneously based on the ratio of pre-defined weights. • properties • fairness • minimum throughput guarantee • analyzable with leaky bucket admission control

4. notations: • an example of GPS

5. minimum throughput guarantee • Summing over all session j, service rate=r:

6. e.g. Fp=packet p's departure time under GPS packet i session 1 GPS t Fi packet j session 2 Fj ideally, serve packet j first. But at time t, there is only packet, i, in the system. Have to serve i first PGPS : (A. Parakh, 1992) • PGPS (packet-by-packet scheme) : an approximation to GPS for operating in real world • ideally, PGPS wants to serve packets in the increasing departure order of GPS • an example of different between GPS and PGPS

7. Lmax(N-1)/r Lmax/r ^ ^ Fp Fp Fp (PGPS) (GPS) (PGPS) exact low bound not exist F1=F2=F3 GPS ^ ^ ^ F1 F2 F3 Lmax/r PGPS ^ F1-F1=Lmax(3-1)/r • properties of PGPS • departure time of the two scheme

8. properties (continued)

9. F1 a1 a1 a2 • Hard to keep track of departure time (GPS) • implementation of PGPS (virtual time) • use hypothetical ideal fluid flow model as reference • use virtual time, v(t), to represent the progress of work in the reference system. • example:

10. the same departure order Real system reference system virtual time, v(t) real time, t 2 6 8 4 8 12 s1 s2 V(s1) V(s2) v(t) 12 8 4 t 2 6 8

11. PGPS algorithm • virtual time finishing times can be determined at the packet arrival time • packets served in order of virtual time finishing time • problems • not feasible • way of assigning weight is not defined

12. picking up

13. Self Clocked Fair Queueing: J. Golestani(1994) • Motivation: eliminate the need for the hypothetical fluid-flow reference system. • Approach: different notion of virtual time, which depends on the progress of work in the actual packet-based system.

14. SCFQ algorithm • each arriving packet is tagged with a service tag. • service tags are iteratively computed as • virtual time is defined equal to the service tag of the packet receiving service. • transmit packets in the increasing order of service tags.

15. Properties of SCFQ • Implementable • fairness property • The end-to-end session delay bounds are comparable to that of PGPS with leaky bucket admission policy. • Problems • Way of determining value is still not defined.

16. Efficient FQ Architectures(info 96) • Motivation: • develop efficient scheduling algorithms for high-speed ATM network • Contents: • sorting scheme • hierarchical approach for wide range of services

17. SCFQ • To simplify implementation, calculate the service tag until the cell reaches the head of its connection queue.

18. Observation: • for session k,

19. 0 ... ... 1 sorting bins connection FIFOs • Architecture • if the ith bin is receiving service, put the new head-of line cell to the th bin • logic to transmit a cell and locate the next non-empty sorting bin is required. • problem : • a large number of sorting bins are required to handle a wide range of bandwidth parameters

20. SCFQ SCFQ SCFQ SCFQ • Hierarchical Fair Queueing (two-level scheduler) • first stage handle connections with similar rates • second stage serves a small number of groups • Group weights