A Comparative Analysis of Data Center Network Architectures

Transcription

1 A Compaative Analysis of Data Cente Netwok Achitectues Fan Yao, Jingxin Wu, Guu Venkataamani, Suesh Subamaniam Depatment of Electical and Compute Engineeing, The Geoge Washington Univesity, Washington, DC, USA {albetyao, jingxinwu, guuv, Abstact Advances in data intensive computing and high pefomance computing facilitate apid scaling of data cente netwoks, esulting in a gowing body of eseach exploing new netwok achitectues that enhance scalability, cost effectiveness and pefomance. Undestanding the tadeoffs between these diffeent netwok achitectues could not only help data cente opeatos impove deployments, but also assist system designes to optimize applications unning on top of them. In this pape, we pesent a compaative analysis of seveal well known data cente netwok achitectues using impotant metics, and pesent ou esults on diffeent netwok topologies. We show the tadeoffs between these topologies and pesent implications on pactical data cente implementations. I. INTRODUCTION The compute industy has been actively building lage scale data centes that delive enomous computation powe and stoage capacity needed by data-intensive applications [1], []. Clustes consisting of tens of thousands of nodes have become common in ecent yeas. As netwok sizes incease, loweing the cost of the oveall system infastuctue and achieving highe level of pefomance have become fist ode concens fo data cente opeatos. Data cente achitectues often have diffeent end goals that equie optimization of diffeent chaacteistics [3], []. If the wokload is compute-intensive, data centes need to be equipped with poweful nodes. Fo communication-intensive wokloads, data cente netwoks play a citical ole in deliveing pefomance while making sue that costs ae affodable. Data cente netwok pefomance can typically be chaacteized using well known metics such as bandwidth, eliability, thoughput, powe consumption, latency and cost [1], [], []. Some of these metics ae inteelated. Fo example, cost is dependent on a vaiety of factos, e.g., powe consumption, data cente seves, netwok switches, cables [6], and so on. Data cente netwok infastuctue plays an inceasingly impotant ole in influencing cost and pefomance of the oveall system, something that has been conventionally undeestimated as it does not diectly contibute to data cente pofits. Recently, eseaches have come up with diffeent poposals fo building cost-effective netwok achitectues [1], [7]. Most pio wok have poposed netwok achitectues and topologies with specific goals o have analyzed netwok achitectues based on some single metic [1], [], []. To the best of ou knowledge, a holistic compaative analysis of vaious netwok achitectues is absent in the liteatue. While cost compaison analysis is useful to analyze diffeent data cente achitectues [], we note that quantifying and compaing othe dimensions such as scalability, pefomance and powe can yield futhe insights. In this wok, we conduct a compaative analysis of seveal epesentative data cente netwok achitectues. We pesent a list of key metics to depict pefomance and cost, and analyze ou epesentative achitectues in tems of these metics. The specific contibutions of ou wok ae: 1) We compehensively compae contempoay popula and epesentative data cente topologies by analyzing significant metics in data centes including scalability, latency and hop counts, path divesity, cost and powe. ) We evaluate netwok thoughputs of diffeent topologies unde typical data cente taffic pattens using mininet netwok simulato [8]. To the best of ou knowledge, this is the fist wok that compaes influences of vaious data cente topologies on oveall system thoughput. 3) We summaize cost and pefomance chaacteized by vaious metics and give ecommendations fo pactical data cente topology implementation based on diffeent netwok sizes. II. DATA CENTER ARCHITECTURE CLASSIFICATION We adopt the classification fom [] to categoize data cente netwoks into thee classes: switch-based, seve-based and hybid achitectues. In switch-only achitectues, packet fowading is implemented using switches, wheeas seveonly achitectues use seves fo packet fowading. Seves in seve-only achitectues have dual functions: (i) to un applications, and (ii) to fowad packets between seves. The thid class, hybid achitectues, utilize both switches and seves fo packet fowading. In this wok, we study thee switch-only data cente achitectues (multi-tieed netwok, fat tee, and flattened buttefly); one seve-only achitectue (Camcube), and one hybid achitectue (BCube). We biefly eview these achitectues in this section. A. Switch-only topologies a) -tieed Netwok: -tieed design is a taditional data cente achitectue that is commonly used in many medium-to-lage entepises. A thee-tieed topology (see Figue 1) contains coe switches at the oot level, aggegation switches at the middle level, and access level switches

2 connected to the hosts. In this wok, we assume that all the coe level and aggegation level switches use GigE pots. Each access switch uses seveal GigE pots connecting hosts as well as one 1 GigE uplink to an aggegation switch. A basic paamete fo multi-tieed netwoks is the ovesubsciption atio. To the best of ou knowledge, thee is no standad definition of ovesubsciption atio; eseaches tend to come up with thei own definitions that may be specific to topologies. Fo example, in a tee-like topology, ovesubsciption atio is typically defined as the atio of bandwidth fo downlinks to the bandwidth fo uplinks. We adopt a moe genealized definition: ovesubsciption is the atio of netwok injection bandwidth to netwok capacity. Specifically, netwok injection bandwidth is the lagest flow sizes that end hosts could inject to the netwok, and netwok capacity [9] is the maximum load on the minimum bisecto unde unifom andom taffic. Note that the ovesubsciption fo multi-tieed topology is highly configuable by vaying the numbe of uplinks and downlinks fo each access level and aggegation level switch [1]. (n 1) flattened buttefly and a k-ay- flattened buttefly. Fo instance, an 8-ay 3-flat can be constucted by copying the 8-ay -flat 8 times, then inteconnecting each switch in one goup with the coesponding 7 switches (one in each of the othe 7 goups). Fig.. netwok. Fig. 1. A multi-tieed netwok. b) Fat Tee Netwok: The design of fat tee netwok is motivated by the fact that the pice diffeential between highend switch (switches with highe link bandwidth o highe numbe of pots) and low-end switches is consideably lage. The main idea behind the constuction of fat tee topology is to eplace the high-end switches in multi-tieed topology by inteconnecting seveal low-end switches. The main diffeence in fat tee is that all of the aggegation level and coe level switches ae eplaced with inteconnections of a set of low end switches. Each subset is called a pod in Figue. As the numbe of uplinks and downlinks fo each pod ae equal, fat tee has full bisection bandwidth. Bisection bandwidth is the maximum bandwidth that can be tansfeed acoss the midpoint of the system [9]. Moeove, since all the switches in the fat tee topology ae inexpensive low-end access level switches, this netwok topology is supposed to be highly scalable and economical. c) Flattened Buttefly Netwok: Flattened buttefly topology [11] was oiginally poposed fo on-chip inteconnection netwok. The k-ay n-flat flattened buttefly () is a multi-dimensional symmetic topology which takes advantage of high-adix switches to ceate low-diamete netwoks. Hee, n denotes the dimension of the topology and k is the numbe of switches in each dimension. Figue 3 shows an 8-ay - flat. Each squae in the figue epesents a switch, and each of the 8 switches inteconnects with the othe 7 switches. In addition, each switch links with 8 host nodes (i.e., seves). A k-ay-n-flat flattened buttefly is constucted fom a k-ay- Fig. 3. Flattened buttefly. B. Seve-only topology d) Camcube: In seve-based data cente achitectues, the data cente is ceated using a set of seves, whee each seve typically has a multi-coe pocesso, and a highpefomance netwok inteface cad (NIC) with multiple pots. The seves ae not only end hosts, but also pefom packet fowading and outing. The camcube is a type of tous topology in which each seve pot is connected diectly to anothe pot on anothe seve. The topology of camcube is a 3D Tous. The pototype has 7 seves (3 3 3) [1]. C. Hybid topology Fig.. BCube 1. e) BCube: The BCube achitectue [13] uses both switches and seves fo outing taffic. BCube k is ecusively defined fom BCube, which contains n seves and an n- pot switch, each seve connecting to a switch pot. Thee ae no diect connections between any two seves. A BCube k netwok can be constucted using n BCube k 1 topologies and n k n-pot switches. In BCube k, thee ae k + 1 levels, and N = n k+1 seves and k + 1 pots fo each seve, each pot connecting a switch at each level. Figue shows an example BCube achitectue. III. COMPARISON METRICS In this section, we define and obtain the pefomance metics fo each of the topologies.

3 A. Scalability Scalability is the ability of a system, to handle a gowing amount of wok in a capable manne o its ability to be enlaged to accommodate that gowth []. To compae the scalability of diffeent topologies, we need to set ovesubsciption of the topologies to be the same. As we will discuss late, ovesubsciption atio is the majo facto that influences netwok scalability; it is basically a metic to quantify how netwok bandwidth is shaed among all hosts. In this pape, we set the ovesubsciption atio of each topology to 1:1 fo compaison puposes. An 1:1 ovesubsciption indicates that all hosts have the capability to communicate with othe hosts with full link bandwidth. The main impediment to scalability is switch pot count. Accodingly, we evaluate scalability using two metics: (a) how does the size of the data cente netwok (i.e., the numbe of hosts the netwok can suppot) change with switch pot counts?; and (b) how many switch pots ae needed on aveage pe host (i.e., nomalized to the size of the data cente)? In the following, we analyze the consideed topologies and obtain these metics fo them. (a) Fo multi-tieed netwok, assume thee ae e coe switches and f aggegation switches. Fom the achitectue of this netwok, we can see that each coe switch has f pots, and each aggegation switch has e uplink pots. To achieve 1:1 oveall ovesubsciption, both the aggegation level and access ovesubsciptions need to be set as 1:1 [1]. Thus, the numbe of downlink pots fo each aggegation switch is also e; the total numbe of pots fo each aggegation switch is then e. Since we assume that coe and aggegation switches ae of the same type with the same pot count and same link capacity (GigE), we have f = e. Then, the numbe of access switches is ef = f. Also, to get 1:1 ovesubsciption at access level, we set downlinks (1GigE) to hosts and one uplink (GigE) to aggegation switch. (The numbe of links can also be set to othe values, such as uplinks and 8 downlinks, without affecting the esults; fo simplicity, ou choice is one uplink and downlinks.) Then the numbe of hosts can be witten as N = f = f. On the othe hand, assuming that one GigE pot can be viewed as fou 1GigE pots, then the numbe of switch pots pe host can be witten as f f+f f+ f f 8 f =. (b) Fo fat tee topology, the netwok s ovesubsciption atio is a fixed 1:1. Assume f is the pot count pe switch, with link capacity 1GigE. The elationship between numbe of hosts and switch pot count is N = 1 f 3. Theefoe, the numbe of switch pots pe host is f f =. f 3 (c) Fo k-ay n-cube which has c endpoints pe switch (each switch can connect to c hosts), we have the popety that when k = c, the ovesubsciption atio is 1 : 1. Thus the numbe of hosts is N = ck n 1 = k n, and the pot count pe switch is f = (k 1)(n 1) + c = kn n + 1; and k = f+n 1 n. Since when n =, the netwok can achieve good scalability, we choose this value fo ou compaison. Thus, the elationship between the numbe of hosts and switch pot ( ) n ( ) count can be witten as N = k n = f+n 1 n = f+. On the othe hand, the numbe of switch pots pe host is fk n 1 ck = f n 1 k = f f+. Since f = h, we can get that the numbe of switch pots pe host as f f+ = N. (d) Fo BCube k topology, we know that the total numbe of switches is (k + 1)n k, whee n is the pot count of a switch. The total numbe of switch pots is then (k + 1)n k n. The elationship between numbe of hosts and switch pot count is N = n k+1. Fom this, the numbe of switch pots pe host is ((k+1)n k n) n k+1 = k + 1. Fo BCube, N = n 3, and the numbe of switch pots pe seve is 3. Fo BCube 3, N = n, and the numbe of switch pots pe seve is. (e) Since scaling a 3D-tous Camcube would esult in undesiable netwok pefomance due to long outing paths as opposed to othe topologies [], [1], we exclude this topology fo scalability studies. The esults of the scalability compaison among diffeent topologies ae shown in Figue and Figue 6 fo the two metics. As we can see fom Figue, outpefoms the othe two switch-based topologies by suppoting moe hosts fo a given switch pot count. Fo example, when the pot count pe switch is set to 6, the numbe of hosts fo is nealy 8 the numbe of hosts that can be suppoted by the fat tee, and 6 the numbe of hosts in multi-tieed. The hybid achitectue, BCube, also has bette scalability compaed to fat tee and multi-tieed topologies. Figue 6 shows the efficiency of pot utilization fo each topology. We can conclude that BCube equies the least numbe of switch pots pe host (o seve) in the data cente switch pot count Fig.. Numbe of hosts vs. switch pot count. We also analyze the impact of ovesubsciption atio on the scalability of a netwok. Using multi-tieed data cente netwok as an example, we set the ovesubsciption to :1 and 9:1. Fo ovesubsciption :1 (aggegation level :1 and access level :1), the numbe of hosts as a function of pots pe coe switch is N = 16 3 f. Fo ovesubsciption 9:1 (aggegation level 3:1 and access level 3:1), the numbe of hosts as a function of pots pe coe switch is N = 8f. The scalability compaison of diffeent ovesubsciption values fo multi-tieed netwok is shown in Figue 7. We notice that at highe ovesubsciption atio such as 9:1, multi-tieed netwok can scale to lage numbe of hosts making it ideal fo lage scale data centes.

4 No. of switch pots (nomalized by no. of seves) Fig. 6. Numbe of switch pots pe host vs. numbe of hosts :1 :1 1: switch pot count Fig. 7. Scalability of multi-tieed netwok fo diffeent ovesubsciption atios. B. Path Divesity Path divesity is an impotant metic fo two easons: a multiplicity of paths can impove load balance and enhance thoughput by distibuting the taffic load, and the netwok is moe immune to link and switch failues. In this pape, we define path divesity as the numbe of diffeent shotest paths between a pai of hosts [9]. We conside both the maximum (ove all host pais) numbe of shotest paths between a pai of hosts, and the aveage numbe (ove all host pais) of shotest paths. These paths ae not necessaily disjoint. As a second measue, we conside the numbe of disjoint paths (not necessaily shotest), both maximum and aveage ove all host pais. In ode to maintain fainess of the compaison in tems of path divesity, we need to set ovesubsciption of the netwoks the same. Hee we set it to be 1:1 fo all achitectues to equalize the pefomance. Fo multi-tieed topology, we know that if the numbe of coe switches is e, the numbe of hosts is N = f = 8e with 1:1 ovesubsciption atio. By analyzing the topology, we deduce some paametes: p = 1e is the numbe of pais of hosts unde the same access switch, q = 16e (e 1) is the numbe of pais of hosts unde the same aggegation switch but diffeent access switches, and = 8e (8e 1) is the total numbe of host pais. Then, the aveage numbe of diffeent shotest paths is p 1+q 1+( p q) e, and the maximum path divesity is e. Fo k-ay fat tee topology, we know that the numbe of hosts is N = 1 k3. We also deduce paametes p, q, fo this topology. Let p = k3 8 ( k 1), q = k 16 ( k 1), and = k3 8 ( k3 p 1+q k +( p q) k 1). Then the aveage numbe of shotest paths is, and the maximum path divesity is k. Fo FLBLY, we have chosen n =. Hence, the maximum numbe of shotest paths is (n 1)! =. The aveage numbe is calculated using the numbe of shotest paths between pais of hosts that ae in the same o adjacent dimension. Fo BCube, we have chosen k =, since a BCube netwok is aleady capable of connecting desied numbe of hosts fo ou compaison. The maximum numbe of diffeent shotest paths is k + 1 = 3. The aveage numbe of shotest paths is computed by exhaustively calculating the numbe of shotest paths fo each host pai in the BCube. A compaative analysis of diffeent topologies is shown in Figues 8 and 9. Avg. no. of paths Fig. 8. Aveage numbe of shotest paths between a pai of hosts. Max. no. of paths Fig. 9. Maximum numbe of shotest paths between a pai of hosts. A second measue we choose to quantify path divesity is the numbe of disjoint paths between a pai of hosts. We exclude the link connecting hosts and access switches when computing numbe of disjoint paths. Fo multi-tieed topology, the maximum and aveage numbe of disjoint paths ae both 1. Fo fat tee topology, the maximum numbe of disjoint paths is k k p 1+q, and the aveage is +( p q) k. Fo ecusive o multiple-level topologies like flattened buttefly and BCube, we can detemine the numbe of disjoint shotest paths fo any pai of nodes by utilizing the Hamming distance between the souce node and destination node, when the nodes ae labeled by its base and dimension (o level) in the topology. Note that the minimum numbe of disjoint shotest paths fo all these topologies is 1. The esults ae shown in Figue 1. Fo both cases, the aveage numbe of shotest paths fo fat

5 tee is highe with inceasing numbe of hosts, making it ideal fo fault toleance puposes. Avg. no. of paths Fig. 1. Aveage numbe of disjoint shotest paths between a pai of hosts. Fo the Camcube, we can easily see that the numbe of diffeent shotest paths is n! whee n = log N is the dimension of the Camcube. Howeve, we omit plotting esults fo Camcube due to the same eason mentioned in III-A. Avg. no. of hops C. Hop Count Fig. 11. Aveage hop count. The aveage hop count is the aveage numbe of hops on the shotest path between a pai of hosts (aveaged ove all host pais). This metic is useful in deducing packet latencies. Simila to the compaison with espect to path divesity, we choose an ovesubsciption atio of 1:1 to nomalize the pefomance, and the paametes ae the same as those fo path divesity compaison. Fo multi-tieed and fat tee topologies, we use the same paametes p, q and that we defined ealie fo the path divesity compaison. The aveage hop count fo multi-tieed topology is p +q +( p q) 6, and the maximum hop count is 6. The aveage hop count fo fat tee topology is p +q +( p q) 6. The maximum hop count is 6. Fo and BCube, the hop counts ae calculated using Hamming distance. The compaison esults ae shown in Figue 11. We notice that fo smalle numbe of hosts, and BCube have lowe hop count than othes making them moe suitable fo smalle scale data centes. D. Thoughput In a netwok, thoughput (o accepted taffic) is the ate (bits/sec) at which taffic is deliveed to the destination nodes [9]. In ou expeiment, we show nomalized thoughput ove maximum achievable injection bandwidth. We pefomed simulations using Mininet [8], a softwae netwok emulato, and netwokx [1] that statically analyzes netwok constucts and featues. We an ou simulations on a Xeon x7 3.GHz quad-coe CPU machine with 8G DRAM. At pesent, we have esults fo switch-only topologies, as the simulato equies consideable econfiguation effot to make it wok fo othe topologies. We compae ou topology esults with a sta topology whee evey pai of hosts ae connected though a single non-blocking switch using a dedicated pai of links. In ou expeiments, each topology has 16 hosts with 1 Mbps link capacity (except fo the aggegation level links with capacity Mbps in multi-tieed topology). We study the topologies using seveal types of wokloads: hotspot, andom and stide [16]. Each host in Mininet uns a shell pogam and communication among pogams is modeled fo the above-mentioned wokload pattens. The aveage thoughput fo the wokloads in each achitectue is measued by aveaging ove thee independent uns. The esults fo the thee types of wokloads (andom, stide and hotspot) ae shown in Figue 1. Note that although fat tee and multitieed netwoks have the same physical bisection bandwidth, the achievable thoughput fo fat tee is much bette. This is due to the lage numbe of disjoint paths between nodes in fat tee, that could potentially balance the load and educe taffic congestion. zed thoughput Nomaliz Non blocking Fat tee Fbfly tieed hotspot andom stide hotspot andom stide fixed path andom path Fig. 1. Thoughput compaison fo diffeent topologies. Fixed path: statically choose one path fom all the available paths; Random path: geneate a andom numbe as an index to the available paths. E. Cost We model the cost of a netwok topology as the capital cost fo netwok infastuctue. Fo switch-based topologies, netwok components include NICs, switches, and cables. Fo seve-based data centes, the cost involves switches (if any), NICs and CPU coes (that ae esponsible fo packet fowading). Table 1 shows the costs of a data cente with appoximately K hosts using diffeent topologies. Assume that P x is the pice pe GigE pot, P y is the pice fo a 1GigE pot, C is the pice fo 1GigE NIC pot, and N is the numbe of seves. Fo multi-tieed netwok, with a coe level switches, b aggegation level switches and c access switches, the switch cost is: (a + b)bp x + c(p y + P x ). The NIC cads cost would be.k C in ou setting.

6 has a constant numbe of pots pe seve, namely. Theefoe, the switch cost is NP y, and the cost fo NIC cads is NC. In this setting, N is equal to.3k. In, we set k = c in ode to achieve full bisection bandwidth. The numbe of pots pe switch is P t = (k 1)(n 1) + c. The cost fo switches is P t K n 1 P y, and the cost fo NIC cads is NC. Fo BCube with base N and K levels, the numbe of switches is N K (K + 1). The numbe of pots pe switches ae also N. We use BCube, so K =. Theefoe, each seve has 3 NIC pots, and the cost fo NIC cads is 3N K+1. Camcube is a 3D tous, so we assume a 16 base 3-cube tous. Each seve has 6 NIC pots. Fo the cost numbes, we efe to Popa et al. [] fo the pice of netwok components and also conside the data evealed by Mudigonda et al. [6]. We assume that switch cost is $ pe 1GigE switch pot and $ pe GigE switch pot, $1 pe Ethenet NIC pot, and $ pe CPU coe. Fo BCube, we assume two exta coes pe seve, fo Camcube, we assume packet fowading coes. TABLE I COST COMPARISON OF DATA CENTER TOPOLOGIES. Cost(k$) Switch NIC CPU coe Total -tieed Camcube BCube F. Powe Consumption The topology paametes fo powe consumption compaison ae the same as fo the cost compaison. Hee, we quote the powe consumption of netwok components fom []: 1W fo 1GigE switch pot and GigE switch pot, 1W fo NIC, and 1W pe CPU coe. Table shows the powe consumption of data centes of appoximately K hosts using diffeent topologies. TABLE II POWER CONSUMPTION COMPARISON OF DATA CENTER TOPOLOGIES. Powe(kW) Switch NIC CPU coe Total -tieed Camcube 16 7 BCube G. Tade-offs In this section, we pesent an oveall compaison of the metics discussed in the pevious sections based on diffeent netwok sizes. To be moe specific, we consideed thee netwok scales: small, S (N 1), medium, M (1 < N 1), and lage, L (1 < N ). Table III shows the oveall compaison. The lettes in the table show the bestpefoming achitectue fo a paticula metic. Some useful insights can be gained fom these esults. Fo example, the multi-tieed achitectue is well-suited fo small and medium data centes, if powe is the majo concen. Camcube is costeffective fo small to medium-sized data centes but does not scale well. Both fat tee and flattened buttefly could be used to build lage scale high pefomance data centes. While fat tee has bette fault toleance and latency, flattened buttefly is less expensive and moe scalable. TABLE III A SUMMARY TABLE OF COMPARISON AMONG TOPOLOGIES. Scalability Latency Path divesity Cost Powe -tieed S M L S M L S L S M L L Camcube M BCube M S IV. CONCLUSION In this wok, we pesented a compaative analysis of seveal popula data cente netwok topologies such as, tieed netwoks, Flattened Buttefly, Camcube and BCube. The metics that we chose fo compaison wee scalability, path divesity, hop count, thoughput and cost. We find that diffeent topologies scale diffeently fo vaious metics, and we conclude that data cente designes have to conside such chaacteistics to maximize thei pefomance while minimizing cost and powe. As futue wok, we will study enegy optimization stategies and application-specific constaints to bette undestand data cente needs and design. REFERENCES [1] D. Abts, M. R. Maty, P. M. Wells, P. Klausle, and H. Liu, Enegy popotional datacente netwoks, in ISCA, 1. [] L. Popa, S. Ratnasamy, G. Iannaccone, A. Kishnamuthy, and I. Stoica, A cost compaison of datacente netwok achitectues, in Co-NEXT, 1. [3] A. Geenbeg, J. R. Hamilton, N. Jain, S. Kandula, C. Kim, P. Lahii, D. A. Maltz, P. Patel, and S. Sengupta, Vl: a scalable and flexible data cente netwok, in SIGCOMM, 9. [] R. Nianjan Mysoe, A. Pambois, N. Faington, N. Huang, P. Mii, S. Radhakishnan, V. Subamanya, and A. Vahdat, Potland: a scalable fault-toleant laye data cente netwok fabic, in SIGCOMM, 9. [] M. Al-Faes, A. Loukissas, and A. 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