We propose a modular star-coupler-based switch architecture along with a scalable multicast scheduling algorithm to enable all-optical multicasting among data center nodes. With broadcast domain partitioning in a 126-port switch, we achieve up to 24% improvement in the maximum achievable throughput

Agrell, Erik

Keykhosravi, Kamran

Rastegarfar, Houman

English

Chalmers Research

Scalable Interconnection Scheme for Data Center MulticastApplicationsDownloaded from: https://research.chalmers.se, 2020-01-17 16:13 UTCCitation for the original published paper (version of record):Keykhosravi, K., Rastegarfar, H., Agrell, E. (2018)Scalable Interconnection Scheme for Data Center Multicast ApplicationsPhotonics in Switching and Computinghttp://dx.doi.org/10.1109/PS.2018.8751384N.B. When citing this work, cite the original published paper.research.chalmers.se offers the possibility of retrieving research publications produced at Chalmers University of Technology.It covers all kind of research output: articles, dissertations, conference papers, reports etc. since 2004.research.chalmers.se is administrated and maintained by Chalmers Library(article starts on next page)Scalable Interconnection Scheme forData Center Multicast ApplicationsKamran KeykhosraviDept. of Electrical EngineeringChalmers University of TechnologyGothenburg, Swedenkamrank@chalmers.seHouman RastegarfarCollege of Optical SciencesThe University of ArizonaTucson, AZ, USAhouman@optics.arizona.eduErik AgrellDept. of Electrical EngineeringChalmers University of TechnologyGothenburg, Swedenagrell@chalmers.seAbstract—We propose a modular star-coupler-based switcharchitecture along with a scalable multicast scheduling algorithmto enable all-optical multicasting among data center nodes. Withbroadcast domain partitioning in a 126-port switch, we achieveup to 24% improvement in the maximum achievable throughput.Index Terms—Data center, optical multicasting, scheduling,star coupler, switch architecture.I. INTRODUCTIONOptical interconnects have received much attention for sup-porting a diverse set of data center traffic patterns in a flexibleand bit-rate-transparent fashion. Today, a significant portion ofdata center jobs depend on the simultaneous transmission ofthe same information copy to a multitude of recipients (i.e.,multicasting). However, the existing electronically switcheddeployments fall short of an efficient mechanism for handlingthis requirement. As such, recently several efforts have beendevoted to developing data center multicast solutions directlyin the optical domain [1], [2]. Due to their energy efficiencygains, multicast-enabled switches are also being proposed toreplace the electronic top-of-rack switches [3].The key enabler of traffic scheduling flexibility in theoptical domain is the passive optical coupler. A 1×N powersplitter allows the traffic of one sender to be delivered toall N receivers. An N × N star coupler, on the other hand,makes it feasible for N nodes to simultaneously participatein unicast, multicast, or broadcast sessions, provided thatthey are equipped with wavelength-tunable transceivers. Thelimited port count of star couplers as well as the tuningrange of optical transceivers and filters make scalable opticalmulticasting a significant challenge [4]. Efforts have beenmade to overcome these shortfalls by interconnecting couplersin a nontransparent fashion [5]; however, performing frequentoptoelectronic conversions is not desirable within data centers.In this paper, we propose an all-optical multicast switcharchitecture for large-scale data center applications. Specif-ically, using K star couplers with port count N , our pro-posed solution can interconnect K × (N − K + 1) nodes.The multicast structure benefits from interconnecting disjointbroadcast domains, and can be used for either packet or circuitswitching. Without loss of generality, here we focus on thepacket-switching scheduling problem to quantify the delay–throughput performance. An optical packet comprises a burstof IP packets that are destined to the same place and is longenough to compensate for the hardware reconfiguration andscheduling overheads.II. MODULAR MULTICAST INTERCONNECTARCHITECTURETo illustrate our star-coupler interconnection mechanism,we give an example with two N × N couplers that canlead to a multicast structure supporting 2(N − 1) computingnodes. Fig 1(a) depicts the architecture of the interconnectedswitch. It is assumed that all servers (or computing nodes)are connected to a software-defined networking (SDN) con-troller, which performs the scheduling tasks. The first N − 1input/output ports of each coupler are connected to the serversand the last ports are employed to interconnect the two cou-plers using static filters (SFs) that act as optical gates. An SFcould flexibly be realized through programming a wavelength-selective switch (WSS) [2]. Each server is equipped witha tunable transmitter and a tunable receiver and performsinput buffering to resolve contentions. We assume that thetransceivers are able to tune to W different wavelengths. Theset of W wavelengths is partitioned into three disjoint subsets,namely WS1, WS2, and WS3. The wavelengths in WS1 areused for handling the traffic internal to each star coupler,whereas the wavelengths in WS2 (WS3) are used to carrythe interdomain traffic from coupler 1 (2) to coupler 2 (1).As an example, server 1 is able to multicast its traffic to anysubset of the servers connected to coupler 1 (coupler 2) usinga single wavelength in WS1 (WS2). Deploying SFs in theinterconnection scheme ensures that no collisions take placebetween the intra- and interdomain traffic demands.It is straightforward to generalize the aforementionedscheme for interconnecting K > 2 star couplers of size N×Nto support K × (N −K + 1) nodes in an all-optical multicaststructure. To this end, the last K − 1 input (output) ports ofeach coupler are connected to the output (input) ports of theremaining K − 1 couplers via 2(K − 1) optical fibers that aregated by SFs. In order to ensure disjoint broadcast domainsin terms of interference, in each coupler the sets of wave-lengths used for intradomain and interdomain communication. . .. . .. . .. . .SF2RxRxRxTxTxTx#1#2#N-1N ×NCoupler#2WS3SF1RxRxRxTxTxTx#1#2#N-1N ×NCoupler#1WS2Coupler# 2WS1Coupler# 1WS1Coupler# 4WS1Coupler# 3WS1WS2WS3WS4WS5WS6WS7(a) (b)Fig. 1: Multicast switching structure deploying multiple couplers. The set of available wavelengths is partitioned into multiple subsets denoted by WSi. (a)The architecture of a 2(N − 1) × 2(N − 1) switch based on two N ×N star couplers. (b) The schematic of a switch constructed by interconnecting fourstar couplers along with the wavelength allocation pattern.should be disjoint. Hence, we partition the set of availablewavelengths into 2K − 1 subsets WSi, 1 ≤ i ≤ 2K − 1.The intradomain traffic is always serviced by the wavelengthsin WS1, and each SF only passes the wavelengths in onesubset WSi, 2 ≤ i ≤ 2K − 1, such that all fibers connectedto any arbitrary coupler carry different sets of wavelengths.Fig. 1(b) illustrates this wavelength partitioning strategy in aninterconnection among four couplers, where the 7 wavelengthsubsets are highlighted using different colors. In general, bycasting the wavelength partitioning problem into a graph edgecoloring problem, it can be shown that for even K’s althougheach WSi, 2 ≤ i ≤ 2K − 1, is utilized multiple times, no twofibers with the same color are connected to the same coupler(for odd K’s the number of subsets should be 2K + 1).III. MULTIDOMAIN SCHEDULING ALGORITHMTo schedule the optical interconnect based on K starcouplers, we consider each server (input node) to have Kqueues, each buffering the traffic destined to a distinct cou-pler. A multicast packet whose destination set is spread over1 ≤ K ′ ≤ K couplers will be copied K ′ times and will beplaced in each of those K ′ queues. Considering this bufferingstrategy, we develop a modified version of the round-robin(RR) multicast scheduling algorithm in [4, Sec. III-B]. Thescheduler starts from the queue of a node that is indicatedby an RR pointer. Then, it sequentially examines every queueof every node and schedules the head-of-line (HOL) packetsthat can be transmitted to all of their destinations withoutcontending with the already scheduled packets. Next, thescheduler examines all HOL packets and those that can betransmitted to a part of their destination set without contentionget scheduled. Depending on the source and destination(s) of ascheduled packet, the corresponding transmitter and receiver(s)are tuned to a wavelength belonging to the desired wavelengthsubset (using first-fit wavelength assignment).IV. DELAY PERFORMANCE EVALUATIONWe study the delay performance of the optical switch inFig. 1(a) based on the interconnection of two 64 × 64 starcouplers (i.e., a 126× 126 switch). We conduct Monte Carlosimulations for different numbers of available wavelengths(W ), multicast degrees, and traffic patterns. We consider twotypes of traffic, Bernoulli and geometric. In the former, duringeach time slot, a packet is generated in each node withprobability 0 ≤ p ≤ 1 and a set of destinations are pickeduniformly at random. In the geometric traffic pattern, burstsof optical packets with the same destination set are generated.The length of each burst follows a geometric distribution,i.e., a discrete exponential distribution (with mean 16). Thedestination set of a packet is chosen uniformly from the setof all nodes excluding the transmitter itself. We assume thatthe fan-out, i.e., the cardinality of the destination set of apacket, follows a truncated geometric distribution with meanF¯ . A total number of 100,000 time slots were simulated, thesecond half of which contribute to the presented results. Inorder to speed up the simulations, we set the maximum queuelength to 500 packets, which is large enough as long as queuesare stable. We distribute the available wavelengths betweenWS1, WS2, and WS3 as uniformly as possible. Specifically,with W = 16, 32, and 64 available wavelengths, 6, 10, and22 wavelengths are respectively assigned to WS1, and theremaining wavelengths are equally distributed between WS2and WS3.Fig. 2 depicts the average delay (i.e., the average number oftime slots a packet spends in the buffer prior to transmission)versus the (normalized) throughput (i.e, the average number ofpackets transmitted in one time slot per node). The maximumachievable throughput increases with an increase in the numberof wavelengths or the average fan-out. As the average fan-outincreases, multicast scheduling allows for a larger number of0.1 0.2 0.3024Bernoulli W = 16ThroughputAveragedelayF¯=1F¯=2F¯=3F¯=4(a)0.2 0.4 0.60246810Bernoulli W = 32ThroughputAveragedelayF¯=1F¯=2F¯=3F¯=4(b)0.4 0.6 0.805101520Bernoulli W = 64ThroughputAveragedelayF¯=1F¯=2F¯=3F¯=4(c)0.1 0.2 0.3020406080100Geometric W = 16ThroughputAveragedelayF¯=1F¯=2F¯=3F¯=4(d)0.2 0.4 0.60100200300Geometric W = 32ThroughputAveragedelayF¯=1F¯=2F¯=3F¯=4(e)0.5 0.60200400Geometric W = 64ThroughputAveragedelayF¯=1F¯=2F¯=4(f)Fig. 2: The delay performance of a 126× 126 switch constructed by interconnecting two 64× 64 couplers as in Fig. 1(a).The dashed lines indicate the upperbound on the achievable throughput under unicast traffic (F¯ = 1) within a 126-port unpartitioned switch.packet copies to be disseminated with a single wavelength,translating to an increased throughput. With W = 16, themaximum throughput under Bernoulli traffic increases from0.15 for F¯ = 1 to 0.38 for F¯ = 4, corresponding to a153% increase. The same trend is observed in Fig. 2(b) andFig. 2(c) with larger numbers of wavelengths. With F¯ = 4 and64 wavelengths, a maximum throughput of 0.83 is achieved,which is 2.8 times the maximum throughput with F¯ = 4 andW = 16. The switch exhibits a similar behavior under thegeometric traffic. However, the delays are increased as queuesbuild up fast under bursty traffic.Compared with an equally sized single-coupler switch(based on a 128× 128 star coupler with 126 used ports), onecan notice the advantage of broadcast domain partitioning inour design. With a single broadcast domain of N ports, Wwavelengths, and an average fan-out of F¯ , the upper boundof the throughput is calculated as WF¯/N , which can only berealized in the absence of output port contentions. With F¯ = 1and W = 16, this upper bound is approximately 0.13, whichis less than the maximum achievable throughput in our design(i.e., 0.15). The same observation can be made with W = 32(0.31 > 0.25) and W = 64 (0.62 > 0.51). In other words,the partitioning technique enables a throughput improvementof 15% for W = 16, 24% for W = 32, and 21% for W = 64.For F¯ = 2, the maximum achievable throughput of our designis close to the upper bound. For example, with W = 32 athroughput of 0.47 is achieved, whereas the theoretical upperbound is 0.51. For higher values of F¯ , the upper boundbecomes loose as it disregards contentions.V. CONCLUSIONWe proposed a novel multicasting mechanism based onthe transparent interconnection of star couplers, leading tobroadcast domain partitioning. Due to partially reusing thespectral resources, the performance of our design can exceedthat of an equally sized single-coupler switch. Besides, anincrease in the number of wavelengths and the fan-out canfurther improve the throughput performance. To reduce thedelays under bursty traffic, a load balancing stage can beemployed at the switch input.REFERENCES[1] P. Samadi, V. Gupta, J. Xu, H. Wang, G. Zussman, and K. Bergman,“Optical multicast system for data center networks,” Opt. Express, vol. 23,no. 17, pp. 22 162–22 180, Aug. 2015.[2] H. Rastegarfar, K. Keykhosravi, E. Agrell, and N. Peyghambarian, “Wave-length reuse for scalable multicasting: a cross-layer perspective,” in Proc.Optical Fiber Communication Conf. (OFC), San Diego, CA, USA, Mar.2018, paper W2A.20.[3] J. Chen, Y. Gong, M. Fiorani, and S. Aleksic, “Optical interconnects atthe top of the rack for energy-efficient data centers,” IEEE Commun.Mag., vol. 53, no. 8, pp. 140–148, Aug. 2015.[4] K. Keykhosravi, H. Rastegarfar, and E. Agrell, “Multicast schedulingof wavelength-tunable, multiqueue optical data center switches,” J. Opt.Commun. Netw., vol. 10, no. 4, pp. 353–364, Apr. 2018.[5] Q. Li et al., “Scaling star-coupler-based optical networks for avionicsapplications,” J. Opt. Commun. Netw., vol. 5, no. 9, pp. 945–956, Sep.2013.

Scalable Interconnection Scheme for Data Center Multicast Applications

https://research.chalmers.se/publication/505179/file/505179_Fulltext.pdf

Scalable Interconnection Scheme for Data Center Multicast Applications

Abstract

Similar works

Full text

Available Versions

Chalmers Research