Scalability of delays in input queued switches

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1 Scalability of delays in input queued switches Paolo Giaccone Notes for the class on Router and Switch Architectures Politecnico di Torino December 2011

2 Scalability of delays N N switch Key question How does the average delay W scale with N, when N? Assumptions Bernoulli i.i.d. arrivals with rate λij [0, 1] cell/slot at input i for output j Admissible traffic: ρ (0, 1) and λ kj ρ j k λ ik ρ i k P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

3 Output Queued (OQ) switch Assume uniform traffic: λ ij = ρ/n Delay of an OQ switch ( W OQ = ) ρ N 2(1 ρ) 1 + ρ = O(1) (1) 2(1 ρ) Proof: each output queue is a slotted M/D/1 queue with binomial (N, ρ/n) arrivals per slot and service time equal to one slot P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

4 Scheduling in Input Queued (IQ) switches Queue-independent policies Arrival rates are known Frame scheduling Random Frame scheduler Periodic Frame scheduler Queue-aware policies Arrival rates are unknown Slot-by-slot schedulers e.g.: MWM, islip Queue-aware frame scheduler P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

5 Random Frame scheduling Assume λ ij = ρµ ij for some ρ < 1 where [µ ij ] is double-stochastic matrix: k µ ik = k µ kj = 1 Thanks to BvN Theorem, µ ij = k p kmij k with p k 0, k p k = 1 and M k = [Mij k ] be one matching matrix (1 k N!). At each timeslot, the scheduler selects M k at random with probability p k Delay of Random Frame (RF) scheduler W RF = N 1 1 ρ = O(N) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

6 Proof - I For a slotted Geom/Geom/1 queue with arrival probability λ and service probability µ, the average delay is W Geom/Geom/1 = η λ(1 η) with η = λ(1 µ) µ(1 λ) (2) In the random frame scheduler, VOQ ij is a Geom/Geom/1 queue with service probability 1 ( 1 pk M k ) ( ij 1 1 ) p k Mij k = p k Mij k = µ ij k k k and arrival probability λ ij = ρµ ij. P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

7 Proof - II Now Recalling (2) The total arrival traffic is η = ρ 1 µ ij 1 ρµ ij 1 η = 1 ρ 1 ρµ ij W ij = 1 ρ (1 µ ij) (1 ρµ ij ) = 1 µ ij ρµ ij (1 ρµ ij ) (1 ρ) µ ij (1 ρ) and the overall average delay is λ tot = i,j λ ij = ρn W RF = 1 λ ij W ij = 1 λ ij (1 µ ij ) λ tot Nρ µ ij (1 ρ) = i,j i,j 1 Nρ i,j ρ(1 µ ij ) (1 ρ) = 1 Nρ i,j ρ λ ij 1 ρ = N2 ρ Nρ Nρ(1 ρ) = N 1 1 ρ P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

8 Periodic Frame scheduling Assume uniform traffic: λ ij = ρ/n The scheduler serves each VOQ exactly every N timeslots fixed frame of N timeslots during timeslot t, input i is connected to output (i + t) mod N e.g. for N = 3: frame is (M1, M 2, M 3 ) M 1 = M 2 = M 3 = Delay of Periodic Frame (PF) scheduling W PF = 1 + N 2(1 ρ) = O(N) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

9 Proof - I Each VOQ is a slotted, single server queue with arrival probability ρ/n, service duration N slots and server vacation of N 1 slots. Now sample the state of the queue every N slots, in correspondence of each service opportunity. The sampling period is N slots. The VOQ appears as a slotted M/D/1 queue with binomial (N, ρ/n) arrivals and service equal to one sampling period. For such queue, we know (see (1)): ρ W M/D/ (1 ρ) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

10 Proof - II The average delay in the original VOQ will be N times the M/D/1 delay. In addition, the delay must include the average waiting time before being served and must be reduced since the service time is just 1 slot and not N slots as in the considered sub-sampled system. W PF = N 2 + NW M/D/1 (N 1) = N 2 + N + Nρ 2(1 ρ) N + 1 = N Nρ 1 + N 2 ( 1 + ρ 1 ρ 2(1 ρ) = ) = 1 + N 2 ( 1 1 ρ ) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

11 Queue-independent Frame Scheduling For the previous queue-independent frame schedulers W RF = O(N) W PF = O(N) General property: Delay of a Queue-Independent frame scheduler For any scheduling algorithm that operates independently of the queue size W queue independent = O(N) (3) proved in [1] M.J. Neely, E. Modiano, Y.S. Cheng, Logarithmic Delay for N N Packet Switches Under the Crossbar Constraint, IEEE Transaction on Networking, Vol. 15, N. 3, June 2007 by comparing with (3) with (1), queue-independent frame scheduling appears inefficient in terms of delays P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

12 Generic Frame Scheduling Occupancy matrix L = [L ij ] where L ij is the size of VOQ ij BvN theorem T = max i=1,...,n is the minimum clearance time for L { N } N L ik, L ki k=1 k=1 Minimum clearance time and maximal size matchings Any arbitrary sequence of maximal size matchings will be able to serve all packets of L in 2T 1 timeslots. Proof: A given packet can be delayed by at most T 1 packets on the same input and by at most T 1 packets on the same output. In total, each packet can be delayed by at most 2T 2 other packets. P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

13 Queue-aware frame scheduling Fix the frame duration T Let I k = [Tk,..., T (k + 1) 1] the slots corresponding to the kth frame At the beginning of kth frame, i.e. at slot Tk, the scheduler computes all the matchings for the future slots in I k based on just the arrivals in I k 1 Overflow packets are packets that arrived in I k 1 and were not served in I k We assume (for now) that overflow packets are dropped Key Idea 1 Choose T large enough to (almost) avoid overflow packets 2 Delays are O(T ) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

14 Queue-aware frame scheduling Let A(T ) = [a ij (T )] be the cumulative number of arrived packets during the kth frame I k By BvN theorem, it is possible to serve all the packets and avoid overflow packets iff k a ik T and k a kj T We will show that if T = θ(log(n)) Pr(frame overflow) can become negligible delays become O(log(N)) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

15 Chernoff Bound Theorem Let X 1, X 2,..., X n be independent binary random variables, with p i = Pr(X i = 1). Let X = n i=1 X i and µ = E[X ]. For any δ > 0: ( e δ ) µ Pr(X > (1 + δ)µ) < (1 + δ) (1+δ) (4) 1 P(X>(1+δ)µ) e-06 µ=1 µ=10 µ=100 µ=1000 µ= e-08 1e δ P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

16 Proof from Wikipedia From Pr[X > (1 + δ)µ)] inf t>0 = inf t>0 = inf t>0 E [ n i=1 exp(tx i)] exp(t(1 + δ)µ) n i=1 E[exp(tX i)] exp(t(1 + δ)µ) n i=1 [p i exp(t) + (1 p i )] exp(t(1 + δ)µ) The third line above follows because e tx i takes the value e t with probability p i and the value 1 with probability 1 p i. Rewriting p i e t + (1 p i ) as p i (e t 1) + 1 and recalling that 1 + x e x (with strict inequality if x > 0), we set x = p i (e t 1). P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

17 Proof from Wikipedia Thus, n i=1 Pr[X > (1 + δ)µ] < exp(p i(e t 1)) exp(t(1 + δ)µ) = exp ((et 1) n i=1 p i) = exp((et 1)µ) exp(t(1 + δ)µ) exp(t(1 + δ)µ). If we simply set t = log(1 + δ) so that t > 0 for δ > 0, we can substitute and find exp((e t [ ] 1)µ) exp((1 + δ 1)µ) exp(δ) µ = exp(t(1 + δ)µ) (1 + δ) (1+δ)µ = (1 + δ) (1+δ) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

18 Minimum frame size to avoid overflow Frame size and overflow Let γ = ρe 1 ρ. If then Pr(frame overflow) ɛ. T log(n/ɛ) log(1/γ) γ ρ P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

19 Minimum frame size to avoid overflow Frame size and overflow Let γ = ρe 1 ρ. If then Pr(frame overflow) ɛ. T log(n/ɛ) log(1/γ) Minimum frame size for N=16 ε=0.1 ε=0.01 ε=0.001 ε= Minimum frame size for N=1024 ε=0.1 ε=0.01 ε=0.001 ε= T T ρ ρ P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

20 Proof - I Consider a generic output j. Let C(T ) be the number of packets arrived during the frame and destined to j: C(T ) = N a ij (T ) i=1 Pr(overflow for output j) = Pr(C(T ) > T ) C(T ) = N i=1 T t=1 X it where X it = 1 with probability λ ij and all X it are independent random variables. We can use Chernoff Bound: µ = E[C(T )] = N i=1 t=1 having defined ρ = N i=1 λ ij ρ. T E[X it ] = T N λ ij = T ρ i=1 P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

21 Proof - II Using (4) Pr(C(T ) > T ) = Pr (C(T ) > µρ ) ( e δ < (1 + δ) (1+δ) being 1 + δ = 1/ρ and δ = 1/ρ 1 ( e 1/ρ 1 Pr(C(T ) > T ) < (1/ρ ) 1/ρ ) ρ T since function γ is increasing with respect to ρ. By the union bound: Pr(overflow for any output) j = ( ) µ ) T e 1 ρ 1/ρ = (ρ e 1 ρ ) T γ T Pr(overflow for output j) Nγ T Now we can set Nγ T ɛ and obtain T log(γ) log(ɛ/n) T log(n/ɛ) log(1/γ) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

22 Queue-Aware Frame scheduling Assume ɛ enough small to experience negligible frame overflows. Then all packets are served with a delay 2T. Delay for Queue-Aware Frame scheduling W QAF 2 log(n/ɛ) log(1/γ) = O(log N) Note that the [1] proves formally the property for the average delay W QAF by considering also the delays for the overflow packets, which are not dropped as assumed here. P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

23 Conclusions Take home messages Output-Queued switch W OQ = O(1) Queue-Independent Frame scheduler W QIF = O(N) Random Frame scheduler W RF = O(N) Periodic Frame scheduler W PF = O(N) Queue-Aware Frame scheduler W QAF = O(log N) P. Giaccone (Politecnico di Torino) Delay and frame scheduling Dec / 23

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