Localized Asynchronous Packet Scheduling for Buffered Crossbar Switches

1. Localized Asynchronous Packet Scheduling forBuffered Crossbar Switches Deng Pan and Yuanyuan Yang State University of New York Stony Brook

2. Outline • Introduction • Related work • Localized asynchronous packet scheduling • Simulation results • Conclusions

3. Introduction • Crossbar switches have long been the preferred structures for high speed switches and routers: • Provide non-blocking capability. • Overcome the bandwidth limitation of bus-based switches. • Packet forwarding is simple.

4. Introduction • For a crossbar switch, packets may be buffered at either • Output ports • Input ports • Crosspoints

5. Introduction • Output queued (OQ) switches only have buffer space at the output side. • Achieve 100% throughput. • Require speedup of N for an NxN switch. • Input queued (IQ) switches only have buffer space at the input side. • Require no speedup. • Have to work with high time complexity algorithms in order to achieve 100% throughput.

6. Introduction • Combined input-output queued (CIOQ) switches make a trade-off between the crossbar speedup and the complexity of the scheduling algorithms. • Have small fixed speedup of two. • Achieve 100% throughput with any iterative maximal matching algorithms. • Emulate OQ switches.

7. Introduction • Buffered crossbar switches are a special type of CIOQ switches. • Each crosspoint of the crossbar has a small buffer. • Crosspoint buffers eliminate the input and output contention. • Buffered crossbar switches can directly schedule and switch variable length packets.

8. Introduction • Previous scheduling algorithms for crossbar switches mainly focused on fixed length packet scheduling or cell scheduling. • At input ports, new packets are segmented into fixed length cells. • The cells are used as the scheduling units and transmitted across the switching fabric. • At output ports, the cells are reassembled into original packets.

9. Introduction • Variable length packet scheduling, or packet scheduling, improves the switch efficiency by avoiding the segmentation-and-reassemble (SAR) process. • Higher throughput. • Shorter packet latency. • Lower hardware cost.

10. Introduction • [Turner Infocom’06] proposed two packet scheduling algorithms for buffered crossbar switches. • They can provide work-conserving guarantees, or emulate scheduling algorithms for OQ switches. • They schedule packets by imposing an order on buffered packets. • Each crosspoint needs 2L or more buffer space, where L is the maximum packet length.

11. Introduction • We consider the other side of the problem, low time complexity and easy to implement packet scheduling algorithms. • We present the Localized Asynchronous Packet Scheduling (LAPS) algorithm and analyze its performance. • Local info based • No comparison • Crosspoint buffer size of L

12. Outline • Introduction • Related work • Localized asynchronous packet scheduling • Simulation results • Conclusions

13. Related work • Scheduling algorithms in the literature for buffered crossbar switches are generally designed with two possible objectives: • To achieve high throughput. • To emulate scheduling algorithms for OQ switches. • The latter is a stronger requirement, but the implementation of the former can be simpler.

14. Related work • Cell scheduling algorithms for high throughput • CIXB-1, CIXOB-k, MCBF, SCBF… • Cell scheduling algorithms to emulate scheduling algorithms for OQ switches • GBVOQ_OCF, GBFG_SP, MCAF-LTF… • Packet scheduling schemes • Packet VOQ, Packet LOOFA, DPFQ…

15. Outline • Introduction • Related work • Localized asynchronous packet scheduling • Simulation results • Conclusions

16. Localized asynchronous packet scheduling • Structure of a buffered crossbar switch • Ini: input port • Outj: output port • Bij: crosspoint buffer • Qij: virtual queue • The crossbar has speedup of two.

17. Localized asynchronous packet scheduling • Based on the locations of the packets to be scheduled, there are three types of scheduling involved in a buffered crossbar switch. • Input scheduling • Crossbar scheduling • Output scheduling

18. Localized asynchronous packet scheduling • Output scheduling has been well studied, and various scheduling algorithms are proposed. • Output scheduling usually does not affect the throughput performance as long as they are work-conserving. • We use a simple FIFO algorithm for output scheduling, which is work-conserving.

19. Localized asynchronous packet scheduling • For input scheduling, • Select a backlogged virtual queue whose crosspoint buffer is empty, and send its head packet to the crosspoint buffer. • When there are multiple eligible virtual queues, different arbitration rules can be used. • Since the crossbar has speedup of two, the packet is sent to the crosspoint buffer with bandwidth 2R. • Crossbar scheduling is similar.

20. Localized asynchronous packet scheduling • In order to reduce the packet latency, cut-through switching can be used on the crossbar. • Similarly, cut-through switching can be used at output ports.

21. Localized asynchronous packet scheduling

22. Localized asynchronous packet scheduling • In input scheduling, the scheduling candidates of an input port are only the virtual queues whose crosspoint buffers are empty. • This restriction simplify the implementation by enabling one bit to represent the status of the crosspoint buffer.

23. Localized asynchronous packet scheduling • With speedup of two, LAPS achieves 100% throughput for any admissible traffic. • Define Zij(t)=Qij(t)+Bij(t) • If Bij is not empty at time t, ∑kZkj(t) has a negative derivative. • If Qij is not empty at time t, ∑kQik(t) +∑kZkj(t) has a negative or zero derivative.

24. Localized asynchronous packet scheduling • Assume that the traffic arrives according to a Poisson process and the packet length follows an exponential distribution with mean M. • Ini can be approximately modeled as an M/M/1 system, and accordingly

25. Localized asynchronous packet scheduling • Hardware implementation • Only local info is necessary, and it is suitable for distributed implementation and highly scalable. • Since no comparison is necessary, the arbiters can implemented by priority encoders, which can make fast decisions in hardware. • Since each crosspoint buffer needs only L buffer space, it minimize the cost for the switch.

26. Outline • Introduction • Related work • Localized asynchronous packet scheduling • Simulation results • Conclusions

27. Simulation results • We have conducted simulations to verify the 100% throughput of LAPS and to measure its delay and buffer requirement. • We consider five different LAPS implementations: • Fixed priority (FP) • Random (RD) • Round-robin (RR) • Oldest packet first (OPF) • Longest queue first (LQF)

28. Simulation results • In order to reflect the burst nature of real network traffic, we emulate the incoming traffic by a Markov modulated Poisson process.

29. Simulation results • We considered both uniform traffic and non-uniform traffic. • The packet length in the simulation is uniformly distributed between [50, 1500] bytes. • We consider a 16×16 switch, and each input port or output port has bandwidth of 1G bps.

30. Simulation results • Throughput

31. Simulation results • Average delay

32. Simulation results • Maximum queue length

33. Outline • Introduction • Related work • Localized asynchronous packet scheduling • Simulation results • Conclusions

34. Conclusions • Due to the introduction of crosspoint buffers, buffered crossbar switches can directly schedule and transmit variable length packets. • Packet scheduling algorithms avoid SAR and are more efficient than cell scheduling algorithms. • Higher throughput • Shorter latency • Lower hardware cost

35. Conclusions • We presented the Localized Asynchronous Packet Scheduling (LAPS) scheme. • Local info based • No comparison • Crosspoint buffer of size L • We theoretically proved that LAPS achieves 100% throughput with speedup of two, and conducted simulations to verify the results.

36. Thank you! • Questions?