Modeling a Photonic Network for Exascale Computing

Photonics technology has become a promising and viable alternative for both on-chip and off-chip computer networks of future Exascale systems. Nevertheless, this technology is not mature enough yet in this context, so research efforts focusing on photonic networks are still required to achieve realistic suitable network implementations. In this context, system-level photonic network simulators can help to guide designers to assess the multiple design choices. Most current research is done on electrical network simulators, whose components work widely different from photonics components. Moreover, photonics technology adds new components that are not present in electrical networks. This paper discusses how a photonics simulation tool can be built by extending an electrical simulation framework. We summarize and compare the working behavior of both technologies -electrical and photonics, and discuss the rationale behind the proposed extensions. Among others, the devised extensions model optical routers, wavelength-division multiplexing, circuit switching, and specific routing algorithms. This work is aimed to provide support to investigate off- chip optical networks in the context of the European Exascale System Interconnect and Storage project (ExaNeSt) project. The experiments presented in this paper study multiple realistic photonic networks configurations and have been performed with excerpts of real traces. Experimental results show that, compared to electrical networks, optical networks can reduce the execution time of the workload by several orders of magnitude. Our study reveals that future optical technologies presenting a 3.2 Tbps aggregate link bandwidth will not provide additional performance benefits over state-of-the-art 1.6 Tbps optical links across the studied workloads, but 1.6 Tbps network links are enough to achieve the highest optical performance on computer networks. Regarding the link configuration, the bandwidth per optical channel is the parameter with highest impact on the network delay and so on the execution time, while for a given optical bandwidth per channel the better strategy is to reduce the phit size.This work was supported by the ExaNest project, funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 671553, and by the Spanish Ministerio de Economía y Competitividad (MINECO) and Plan E funds under Grant TIN2015-66972-C5-1-R.Duro Gómez, J.; Petit Martí, SV.; Sahuquillo Borrás, J.; Gómez Requena, ME. (2017). Modeling a Photonic Network for Exascale Computing. IEEE Computer Society. https://doi.org/10.1109/HPCS.2017.8

Modeling and Analysis of the Performance of Exascale Photonic Networks

"This is the peer reviewed version of the following article: Duro, José, Jose A. Pascual, Salvador Petit, Julio Sahuquillo, and María E. Gómez. 2018. Modeling and Analysis of the Performance of Exascale Photonic Networks. Concurrency and Computation: Practice and Experience 31 (21). Wiley. doi:10.1002/cpe.4773, which has been published in final form at https://doi.org/10.1002/cpe.4773. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving."[EN] Photonics technology has become a promising and viable alternative for both on-chip and off-chip interconnection networks of future Exascale systems. Nevertheless, this technology is not mature enough yet in this context, so research efforts focusing on photonic networks are still required to achieve realistic suitable network implementations. In this regard, system-level photonic network simulators can help guide designers to assess the multiple design choices. Most current research is done on electrical network simulators, whose components work widely different from photonics components. In this work, we summarize and compare the working behavior of both technologies which includes the use of optical routers, wavelength-division multiplexing and circuit switching among others. After implementing them into a well-known simulation framework, an extensive simulation study has been carried out using realistic photonic network configurations with synthetic and realistic traffic. Experimental results show that, compared to electrical networks, optical networks can reduce the execution time of the studied real workloads in almost one order of magnitude. Our study also reveals that the photonic configuration highly impacts on the network performance, being the bandwidth per channel and the message length the most important parameters.This work was supported by the ExaNeSt project, funded by the European Union's Horizon 2020 Research and Innovation Program under grant 671553, and by the Spanish Ministerio de Economía y Competitividad (MINECO) and Plan E funds under grant TIN2015-66972-C5-1-R. Pascual was supported by a HiPEAC Collaboration Grant.Duro-Gómez, J.; Pascual Pérez, JA.; Petit Martí, SV.; Sahuquillo Borrás, J.; Gómez Requena, ME. (2019). Modeling and Analysis of the Performance of Exascale Photonic Networks. Concurrency and Computation Practice and Experience. 31(21):1-12. https://doi.org/10.1002/cpe.4773S1123121Top500 website. Accessed January2018.Kodi, A. K., Neel, B., & Brantley, W. C. (2014). Photonic Interconnects for Exascale and Datacenter Architectures. 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Workload Characterization for Exascale Computing Networks

[EN] Exascale computing is the next step in high performance computing provided by systems composed of millions of interconnected processing cores. In order to guide the design and implementation of such systems, multiple workload characterization studies and system performance evaluations are required. This paper provides a workload characterization study in the context of the European Project ExaNeSt, which focuses, among others, on developing the network technology required to implement future exascale systems. In this work, we characterize different ExaNeSt applications from the computer network perspective by analyzing the distribution of messages, the dynamic bandwidth consumption, and the spatial communication patterns among cores. The analysis highlights three main observations; i) message sizes are, in general, below 50 kB; ii) communication patterns are usually bursty; and iii) spatial communication among cores concentrate in hot spots for most applications. Taking into account these observations, one can conclude that in order to unclog congested network links, an exascale network must be designed to briefly support higher-than-average bandwidths in the vicinity of key network cores.This work was supported by the ExaNest project, funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 671553, and by the Spanish Ministerio de Econom´ıa y Competitividad (MINECO) and Plan E funds under Grant TIN2015-66972-C5-1-R.Duro-Gómez, J.; Petit Martí, SV.; Sahuquillo Borrás, J.; Gómez Requena, ME. (2018). Workload Characterization for Exascale Computing Networks. IEEE Computer Society. 383-389. https://doi.org/10.1109/HPCS.2018.00069S38338

Symmetric Interconnection Networks from Cubic Crystal Lattices

Torus networks of moderate degree have been widely used in the supercomputer industry. Tori are superb when used for executing applications that require near-neighbor communications. Nevertheless, they are not so good when dealing with global communications. Hence, typical 3D implementations have evolved to 5D networks, among other reasons, to reduce network distances. Most of these big systems are mixed-radix tori which are not the best option for minimizing distances and efficiently using network resources. This paper is focused on improving the topological properties of these networks. By using integral matrices to deal with Cayley graphs over Abelian groups, we have been able to propose and analyze a family of high-dimensional grid-based interconnection networks. As they are built over $n$-dimensional grids that induce a regular tiling of the space, these topologies have been denoted \textsl{lattice graphs}. We will focus on cubic crystal lattices for modeling symmetric 3D networks. Other higher dimensional networks can be composed over these graphs, as illustrated in this research. Easy network partitioning can also take advantage of this network composition operation. Minimal routing algorithms are also provided for these new topologies. Finally, some practical issues such as implementability and preliminary performance evaluations have been addressed

SpiNNaker: Fault tolerance in a power- and area- constrained large-scale neuromimetic architecture

AbstractSpiNNaker is a biologically-inspired massively-parallel computer designed to model up to a billion spiking neurons in real-time. A full-fledged implementation of a SpiNNaker system will comprise more than 105 integrated circuits (half of which are SDRAMs and half multi-core systems-on-chip). Given this scale, it is unavoidable that some components fail and, in consequence, fault-tolerance is a foundation of the system design. Although the target application can tolerate a certain, low level of failures, important efforts have been devoted to incorporate different techniques for fault tolerance. This paper is devoted to discussing how hardware and software mechanisms collaborate to make SpiNNaker operate properly even in the very likely scenario of component failures and how it can tolerate system-degradation levels well above those expected

High-Performance, Low-Complexity Deadlock Avoidance for Arbitrary Topologies/Routings

Recently, the use of graph-based network topologies has been proposed as an alternative to traditional networks such as tori or fat-trees due to their very good topological characteristics. However they pose practical implementation challenges such as the lack of deadlock avoidance strategies. Previous proposals are either exceedingly complex, underutilise network resources or lack flexibility. We propose- and prove formally- three generic, low-complexity dead-lock avoidance mechanisms that only require local information. The main strengths of our method are its topology- and routing- independence and that the virtual channel count is bounded by the length of the longest path. We evaluate our proposed mechanisms against previous proposals through an extensive simulation study to measure the impact on the performance using both synthetic and realistic traffic. First we compare against a well-known HPC mechanism for dragonfly and achieved similar performance level. Then we moved to Graph-based networks and show that our mechanisms can greatly outperform traditional, spanning-tree based mechanisms, even if these use a much larger number of virtual channels. Overall, we find that our proposal provides a simple, flexible and high performance deadlock-avoidance solution

Photonic Interconnection Networks for Exascale Computers

[ES] En los últimos años, distintos proyectos alrededor del mundo se han centrado en el diseño de supercomputadores capaces de alcanzar la meta de la computación a exascala, con el objetivo de soportar la ejecución de aplicaciones de gran importancia para la sociedad en diversos campos como el de la salud, la inteligencia artificial, etc. Teniendo en cuenta la creciente tendencia de la potencia computacional en cada generación de supercomputadores, este objetivo se prevee accesible en los próximos años. Alcanzar esta meta requiere abordar diversos retos en el diseño y desarrollo del sistema. Uno de los principales es conseguir unas comunicaciones rápidas y eficientes entre el inmenso número de nodos de computo y los sitemas de memoria. La tecnología fotónica proporciona ciertas ventajas frente a las redes eléctricas, como un mayor ancho de banda en los enlaces, un mayor paralelismo a nivel de comunicaciones gracias al DWDM o una mejor gestión del cableado gracias a su reducido tamaño. En la tesis se ha desarrollado un estudio de viabilidad y desarrollo de redes de interconexión haciendo uso de la tecnología fotónica para los futuros sistemas a exaescala dentro del proyecto europeo ExaNeSt. En primer lugar, se ha realizado un análisis y caracterización de aplicaciones exaescala. Este análisis se ha utilizado para conocer el comportamiento y requisitos de red que presentan las aplicaciones, y con ello guiarnos en el diseño de la red del sistema. El análisis considera tres parámetros: la distribución de mensajes en base a su tamaño y su tipo, el consumo de ancho de banda requerido a lo largo de la ejecución y la matriz de comunicación espacial entre los nodos. El estudio revela la necesidad de una red eficiente y rápida, debido a que la mayoría de las comunaciones se realizan en burst y con mensajes de un tamaño medio inferior a 50KB. A continuación, la tesis se centra en identificar los principales elementos que diferencian las redes fotónicas de las eléctricas. Identificamos una secuencia de pasos en el diseño de un simulador, ya sea haciéndolo desde cero con tecnología fotónica o adaptando un simulador de redes eléctricas existente para modelar la fotónica. Después se han realizado dos estudios de rendimiento y comparativas entre las actuales redes eléctricas y distintas configuraciones de redes fotónicas utilizando topologías clásicas. En el primer estudio, realizado tanto con tráfico sintético como con trazas de ExaNeSt en un toro, fat tree y dragonfly, se observa como la tecnología fotónica supone una clara mejora respecto a la eléctrica. Además, el estudio muestra que el parámetro que más afecta al rendimiento es el ancho de banda del canal fotónico. El segundo estudio muestra el comportamiento y rendimiento de aplicaciones reales en simulaciones a gran escala en una topología jellyfish. En este estudio se confirman las conclusiones obtenidas en el anterior, revelando además que la tecnología fotónica permite reducir la complejidad de algunas topologías, y por ende, el coste de la red. En los estudios realizados se ha observado una baja utilización de la red debido a que las topologías utilizadas para redes eléctricas no aprovechan las características que proporciona la tecnología fotónica. Por ello, se ha propuesto Segment Switching, una estrategia de conmutación orientada a reducir la longitud de las rutas mediante el uso de buffers intermedios. Los resultados experimentales muestran que cada topología tiene sus propios requerimientos. En el caso del toro, el mayor rendimiento se obtiene con un mayor número de buffers en la red. En el fat tree el parámetro más importante es el tamaño del buffer, obteniendo unas prestaciones similares una configuración con buffers en todos los switches que la que los ubica solo en el nivel superior. En resumen, esta tesis estudia el uso de la tecnología fotónica para las redes de sistemas a exascala y propone aprovechar[CA] Els darrers anys, múltiples projectes de recerca a tot el món s'han centrat en el disseny de superordinadors capaços d'assolir la barrera de computació exascala, amb l'objectiu de donar suport a l'execució d'aplicacions importants per a la nostra societat, com ara salut, intel·ligència artificial, meteorologia, etc. Segons la tendència creixent en la potència de càlcul en cada generació de superordinadors, es preveu assolir aquest objectiu en els propers anys. No obstant això, assolir aquest objectiu requereix abordar diferents reptes importants en el disseny i desenvolupament del sistema. Un dels principals és aconseguir comunicacions ràpides i eficients entre l'enorme nombre de nodes computacionals i els sistemes de memòria. La tecnologia fotònica proporciona diversos avantatges respecte a les xarxes elèctriques actuals, com ara un major ample de banda als enllaços, un major paral·lelisme de la xarxa gràcies a DWDM o una millor gestió del cable a causa de la seva mida molt més xicoteta. En la tesi, s'ha desenvolupat un estudi de viabilitat i desenvolupament de xarxes d'interconnexió mitjançant tecnologia fotònica per a futurs sistemes exascala dins del projecte europeu ExaNeSt. En primer lloc, s'ha dut a terme un estudi de caracterització d'aplicacions exascala dels requisits de xarxa. Els resultats de l'anàlisi ajuden a entendre els requisits de xarxa de les aplicacions exascale i, per tant, ens guien en el disseny de la xarxa del sistema. Aquesta anàlisi considera tres paràmetres principals: la distribució dels missatges en funció de la seva mida i tipus, el consum d'ample de banda requerit durant tota l'execució i els patrons de comunicació espacial entre els nodes. L'estudi revela la necessitat d'una xarxa d'interconnexió ràpida i eficient, ja que la majoria de comunicacions consisteixen en ràfegues de transmissions, cadascuna amb una mida mitjana de missatge de 50 KB. A continuació, la tesi se centra a identificar els principals elements que diferencien les xarxes fotòniques de les elèctriques. Identifiquem una seqüència de passos en el disseny i implementació d'un simulador: tractar la tecnologia fotònica des de zero o per ampliar un simulador de xarxa elèctrica existent per modelar la fotònica. Després, es presenten dos estudis principals de comparació de rendiment entre xarxes elèctriques i diferents configuracions de xarxes fotòniques mitjançant topologies clàssiques. En el primer estudi, realitzat tant amb trànsit sintètic com amb traces d'ExaNeSt en un toro, fat tree i dragonfly, vam trobar que la tecnologia fotònica representa una millora notable respecte a la tecnologia elèctrica. A més, l'estudi mostra que el paràmetre que més afecta el rendiment és l'amplada de banda del canal fotònic. Aquest darrer estudi analitza el rendiment d'aplicacions reals en simulacions a gran escala en una topologia jellyfish. Els resultats d'aquest estudi corroboren les conclusions obtingudes en l'anterior, revelant també que la tecnologia fotònica permet reduir la complexitat d'algunes topologies i, per tant, el cost de la xarxa. En els estudis anteriors ens adonem que la xarxa estava infrautilitzada principalment perquè les topologies estudiades per a xarxes elèctriques no aprofiten les característiques proporcionades per la tecnologia fotònica. Per aquest motiu, proposem Segment Switching, una estratègia de commutació destinada a reduir la longitud de les rutes mitjançant la implementació de memòries intermèdies en nodes intermedis al llarg de la ruta. Els resultats experimentals mostren que cadascuna de les topologies estudiades presenta diferents requisits de memòria intermèdia. Per al toro, com més gran siga el nombre de memòries intermèdies a la xarxa, major serà el rendiment. Per al fat tree, el paràmetre clau és la mida de la memòria intermèdia, aconseguint un rendiment similar tant amb una configuració amb memòria intermèdia en tots els co[EN] In the last recent years, multiple research projects around the world have focused on the design of supercomputers able to reach the exascale computing barrier, with the aim of supporting the execution of important applications for our society, such as health, artificial intelligence, meteorology, etc. According to the growing trend in the computational power in each supercomputer generation, this objective is expected to be reached in the coming years. However, achieving this goal requires addressing distinct major challenges in the design and development of the system. One of the main ones is to achieve fast and efficient communications between the huge number of computational nodes and the memory systems. Photonics technology provides several advantages over current electrical networks, such as higher bandwidth in the links, greater network parallelism thanks to DWDM, or better cable management due to its much smaller size. In this thesis, a feasibility study and development of interconnection networks have been developed using photonics technology for future exascale systems within the European project ExaNeSt. First, a characterization study of exascale applications from the network requirements has been carried out. The results of the analysis help understand the network requirements of exascale applications, and thereby guide us in the design of the system network. This analysis considers three main parameters: the distribution of the messages based on their size and type, the required bandwidth consumption throughout the execution, and the spatial communication patterns between the nodes. The study reveals the need for a fast and efficient interconnection network, since most communications consist of bursts of transmissions, each with an average message size of 50 KB. Next, this dissertation concentrates on identifying the main elements that differentiate photonic networks from electrical ones. We identify a sequence of steps in the design and implementation of a simulator either i) dealing with photonic technology from scratch or ii) to extend an existing electrical network simulator in order to model photonics. After that, two main performance comparison studies between electrical networks and different configurations of photonic networks are presented using classical topologies. In the former study, carried out with both synthetic traffic and traces of ExaNeSt in a torus, fat tree and dragonfly, we found that photonic technology represents a noticeable improvement over electrical technology. Furthermore, the study shows that the parameter that most affects the performance is the bandwidth of the photonic channel. The latter study analyzes performance of real applications in large-scale simulations in a jellyfish topology. The results of this study corroborates the conclusions obtained in the previous, also revealing that photonic technology allows reducing the complexity of some topologies, and therefore, the cost of the network. In the previous studies we realize that the network was underutilized mainly because the studied topologies for electrical networks do not take advantage of the features provided by photonic technology. For this reason, we propose Segment Switching, a switching strategy aimed at reducing the length of the routes by implementing buffers at intermediate nodes along the path. Experimental results show that each of the studied topologies presents different buffering requirements. For the torus, the higher the number of buffers in the network, the higher the performance. For the fat tree, the key parameter is the buffer size, achieving similar performance a configuration with buffers on all switches that locating buffers only at the top level. In summary, this thesis studies the use of photonic technology for networks of exascale systems, and proposes to take advantage of the characteristics of this technology in current electrical network topologies.This thesis has been conceived from the work carried out by Polytechnic University of Valencia in the ExaNeSt European projectDuro Gómez, J. (2021). Photonic Interconnection Networks for Exascale Computers [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/166796TESI

Task mapping in rectangular twisted tori

Twisted torus topologies have been proposed as an alternative to toroidal rectangular networks, improving distance parameters and providing network symmetry. However, twisting is apparently less amenable to task mapping algorithms of real life applications. In this paper we make an analytical study of different mapping and concentration techniques on 2D twisted tori that try to compensate for the twisted peripheral links. We introduce a performance model based on the network average distance and the detection of the set of links which receive the highest load. The model also considers the amount of local and global communications in the network. Our model shows that the twisted torus can improve latency and maximum throughput over rectangular torus, especially when global communications dominate over local ones and when some concentration is employed. Simulation results corroborate our synthetic model. For real applications from the NPB benchmark suite, the use of the twisted topologies with an appropriate mapping provides overall average application speedups of 2.9%, which increase to 4.9% when concentrated topologies (c = 2) are considered.This work has been supported by the Spanish Ministry of Science under contracts TIN2010-21291-C02-02, TIN-2007- 60625, AP2010-4900 and CONSOLIDER Project CSD2007-00050, and by the European HiPEAC Network of Excellence. M. Moreto is supported by a MEC/Fulbright Fellowship.Postprint (author’s final draft