Technical Session 1

Chair: Jesus Escudero-Sahuquillo, Technical University of Valencia, Spain

All the speakers will have 20 minutes for their presentations, plus 4 minutes for questions and answers

• Throughput Unfairness in Dragonfly Networks under Realistic Traffic Patterns
  Pablo Fuentes, Enrique Vallejo, Cristobal Camarero, Ramon Beivide (University of Cantabria) and Mateo Valero (UPC/BSC)Abstract: Dragonfly networks have a two-level hierarchical arrangement of the network routers, and allow for a competitive cost-performance solution in large systems. Non-minimal adaptive routing is employed to fully exploit the path diversity and increase the performance under adversarial traffic patterns. Previous works have demonstrated the presence of throughput unfairness under certain adversarial traffic patterns, and proposed different alternatives to effectively combat such effect. Throughput unfairness prevents a balanced use of the resources across the network nodes and degrades severely the performance of any application running on an affected node. In this paper we introduce a new traffic pattern denoted adversarial consecutive (ADVc), which portrays a real use case, and evaluate its impact on network performance and throughput fairness. Furthermore, we assess the limitations of global misrouting policies to alleviate this effect, and the impact of transit-over injection priority on throughput unfairness under ADVc traffic.
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• VEF Traces: A Framework for Modelling MPI Traffic in Interconnection Network Simulators
  Francisco J. Andujar, Juan A. Villar, Jose L. Sanchez, Francisco J. Alfaro (University of Castilla-La Mancha) and Jesus Escudero-Sahuquillo (Technical University of Valencia)Abstract: Simulation is often used to evaluate the behaviour and measure the performance of computing systems. Specifically, in high-performance interconnection networks, the simulation has been extensively considered to verify the behaviour of the network itself and to evaluate its performance. In this context, network simulation must be fed with network traffic, also referred to as network workload, whose nature has been traditionally synthetic. These workloads can be used for the purpose of driving studies on network performance, but often such workloads are not accurate enough if a realistic evaluation is pursued. For this reason, other non-synthetic workloads have gained popularity over last decades since they are best to capture the realistic behaviour of existing applications. In this paper, we present the VEF traces framework, a self-related trace model, and all their associated tools. The main novelty of this framework is that, unlike existing ones, it does not provide a network simulation framework, but only offers an MPI task simulation framework, which allows to use the MPI-based network traffic by any third-party network simulator, since this framework does not depend on any specific simulation platform.
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• SlimUpdate: Minimal Routing Update for Performance-based Reconfigurations in Fat-Trees
  Feroz Zahid (Simula Labs), Ernst Gunnar Gran (Simula Labs), Bartosz Bogdanski (Oracle), Bjørn Dag Johnsen (Oracle) and Tor Skeie (University of Oslo)Abstract: As the size of high-performance computing systems grows, the number of events requiring a network reconfiguration, as well as the complexity of each reconfiguration, is likely to increase. In large systems, the probability of component failure is high. At the same time, with more network components, ensuring high utilization of network resources becomes challenging. Reconfiguration in interconnection networks, like InfiniBand (IB), typically involves computation and distribution of a new set of routes in order to maintain connectivity and performance. In general, current routing algorithms do not consider the existing routes in a network when calculating new ones. Such configuration-oblivious routing might result in substantial modifications to the existing paths, and the reconfiguration becomes costly as it potentially involves a large number of source-destination pairs. In this paper, we propose a novel routing algorithm for IB based fat-tree topologies, SlimUpdate. SlimUpdate employs techniques to preserve existing forwarding entries in switches to ensure a minimal routing update, without any performance penalty, and with minimal computational overhead. We present an implementation of SlimUpdate in OpenSM, and compare it with the current de facto fat-tree routing algorithm. Our experiments and simulations show a decrease of up to 80% in the number of total path modifications when using SlimUpdate routing, while achieving similar or even better performance than the fat-tree routing in most reconfiguration scenarios.
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• Fault-Tolerant Routing for Exascale Supercomputer: The BXI Routing Architecture
  Pierre Vignéras and Jean-Noël Quintin (Atos/Bull)Abstract: BXI, Bull eXascale Interconnect, is the new interconnection network developed by Atos for High Performance Computing. It has been designed to meet the requirements of exascale supercomputers. At such scale, faults have to be expected and dealt with transparently so that applications remain unaffected by them. BXI features various mechanisms for this purpose, one of which is the BXI routing component presented in this paper. The BXI routing module computes the full routing tables for a 64k nodes fat-tree in a few minutes. But with partial re-computation it can withstand numerous inter-router link failures without any noticeable impact on running applications.
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• Multipath Load Balancing for M × N Communication Patterns on the Blue Gene/Q Supercomputer Interconnection Network
  Huy Bui (University of Illinois), Robert Jacob (ANL), Preeti Malakar (ANL), Venkatram Vishwanath (ANL), Andrew Johnson (University of Illinois), Michael Papka (ANL) and Jason Leigh (University of Hawai’i at Mānoa)Abstract: Achievable networking performance of applications in a supercomputer depends on the exact combination of the communication patterns of the applications and the routing algorithms used by the supercomputer. In order to achieve the highest networking performance for the applications the routing algorithms need to be designed optimally for those communication patterns. However, while communication patterns usually have a wide variation from application to application and even from phase to phase in an application, routing algorithms have a limited variation and usually are optimized for typical communication patterns. This results in high networking perfor- mance for favored communication patterns but low networking performance for others. In this paper we present approaches for improving networking performance by rebalancing load on physical links on the Blue Gene Q supercomputer. We realize our approaches in a framework called OPTIQ and demonstrate the efficacy of our framework via a set of benchmarks. Our results show that we can achieve 30% higher throughput on experiment with data and patterns from a real application. The improvemnt can be up to serveral times higher throughput than default MPI Alltoallv used in the Blue Gene Q supercomputer for certain communication patterns.
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Technical Session 2

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Panel

How can we dramatically increase network scalability?

Moderator: 

• Pedro Javier Garcia, University of Castilla-La Mancha, Spain

Panelists:

• Dhabaleswar K Panda, Ohio State University, USA
• ​Eitan Zahavi, Mellanox, Israel
• Satoshi Matsuoka, Tokyo Institute of Technology, Japan

The panelists will have 20-25 minutes for their talks, followed by 20 minutes for questions and answers 

Topic:

In their talks, the panelists will address the next questions:

• In order to reach Exascale performance, what are the necessary changes we need to introduce in the interconnection network?
• Using photonics to reduce signal attenuation seems to be mandatory, but at what levels? Just for node to node interconnects? Within the motherboard? Within the processor chip?
• Most of the latency is not in the interconnect hardware. How should communication protocols be modified? How will those changes affect the programming model for massively parallel applications?
• The interconnect is consuming an increasing fraction of the computer power. In addition to using photonics to reduce losses, what other techniques should be implemented to reduce power consumption? How will they affect network congestion and message latency (average latency and jitter)?

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