Adaptive Routing Strategies for Modern High Performance Networks

Today’s scalable high-performance applications heavily depend on the bandwidth characteristics of their commu-nication patterns. Contemporary multi-stage interconnec-tion networks suffer from network contention which might decrease application performance. Our experiments show that the effective bisection bandwidth of a non-blocking 512-node Clos network is as low as 38 % if the network is routed statically. In this paper, we propose and ana-lyze different adaptive routing schemes for those networks. We chose Myrinet/MX to implement our proposed routing schemes. Our best adaptive routing scheme is able to in-crease the effective bisection bandwidth to 77 % for 512 nodes and 100 % for smaller node counts. Thus, we show that our proposed adaptive routing schemes are able to im-prove network throughput significantly.

FlexVC: Flexible virtual channel management in low-diameter networks

Deadlock avoidance mechanisms for lossless lowdistance networks typically increase the order of virtual channel (VC) index with each hop. This restricts the number of buffer resources depending on the routing mechanism and limits performance due to an inefficient use. Dynamic buffer organizations increase implementation complexity and only provide small gains in this context because a significant amount of buffering needs to be allocated statically to avoid congestion. We introduce FlexVC, a simple buffer management mechanism which permits a more flexible use of VCs. It combines statically partitioned buffers, opportunistic routing and a relaxed distancebased deadlock avoidance policy. FlexVC mitigates Head-of-Line blocking and reduces up to 50% the memory requirements. Simulation results in a Dragonfly network show congestion reduction and up to 37.8% throughput improvement, outperforming more complex dynamic approaches. FlexVC merges different flows of traffic in the same buffers, which in some cases makes more difficult to identify the traffic pattern in order to support nonminimal adaptive routing. An alternative denoted FlexVCminCred improves congestion sensing for adaptive routing by tracking separately packets routed minimally and nonminimally, rising throughput up to 20.4% with 25% savings in buffer area.This work has been supported by the Spanish Government (grant SEV2015-0493 of the Severo Ochoa Program), the Spanish Ministry of Economy, Industry and Competitiveness (contracts TIN2015-65316), the Spanish Research Agency (AEI/FEDER, UE - TIN2016-76635-C2-2-R), the Spanish Ministry of Education (FPU grant FPU13/00337), the Generalitat de Catalunya (contracts 2014-SGR-1051 and 2014- SGR-1272), the European Union FP7 programme (RoMoL ERC Advanced Grant GA 321253), the European HiPEAC Network of Excellence and the European Union’s Horizon 2020 research and innovation programme (Mont-Blanc project under grant agreement No 671697).Peer ReviewedPostprint (author's final draft

Enhancing HPC on Virtual Systems in Clouds through Optimizing Virtual Overlay Networks

Virtual Ethernet overlay provides a powerful model for realizing virtual distributed and parallel computing systems with strong isolation, portability, and recoverability properties. However, in extremely high throughput and low latency networks, such overlays can suffer from bandwidth and latency limitations, which is of particular concern in HPC environments. Through a careful and quantitative analysis, I iden- tify three core issues limiting performance: delayed and excessive virtual interrupt delivery into guests, copies between host and guest data buffers during encapsulation, and the semantic gap between virtual Ethernet features and underlying physical network features. I propose three novel optimizations in response: optimistic timer- free virtual interrupt injection, zero-copy cut-through data forwarding, and virtual TCP offload. These optimizations improve the latency and bandwidth of the overlay network on 10 Gbps Ethernet and InfiniBand interconnects, resulting in near-native performance for a wide range of microbenchmarks and MPI application benchmarks

A multipath routing method for tolerating permanent and non-permanent faults

The intensive and continuous use of high-performance computers for executing computationally intensive applications, coupled with the large number of elements that make them up, dramatically increase the likelihood of failures during their operation. The interconnection network is a critical part of such systems, therefore, network faults have an extremely high impact because most routing algorithms are not designed to tolerate faults. In such algorithms, just a single fault may stall messages in the network, preventing the finalization of applications, or may lead to deadlocked confi gurations. This work focuses on the problem of fault tolerance for high-speed interconnection networks by designing a fault-tolerant routing method to solve an unbounded number of dynamic faults (permanent and non- permanent). To accomplish this task we take advantage of the communication path redundancy, by means of a multipath routing approach. Experiments show that our method allows applications to finalize their execution in the presence of several number of faults, with an average performance value of 97% compared to the fault-free scenarios.Presentado en el IX Workshop Procesamiento Distribuido y Paralelo (WPDP)Red de Universidades con Carreras en Informática (RedUNCI

Routing on the Channel Dependency Graph:: A New Approach to Deadlock-Free, Destination-Based, High-Performance Routing for Lossless Interconnection Networks

In the pursuit for ever-increasing compute power, and with Moore's law slowly coming to an end, high-performance computing started to scale-out to larger systems. Alongside the increasing system size, the interconnection network is growing to accommodate and connect tens of thousands of compute nodes. These networks have a large influence on total cost, application performance, energy consumption, and overall system efficiency of the supercomputer. Unfortunately, state-of-the-art routing algorithms, which define the packet paths through the network, do not utilize this important resource efficiently. Topology-aware routing algorithms become increasingly inapplicable, due to irregular topologies, which either are irregular by design, or most often a result of hardware failures. Exchanging faulty network components potentially requires whole system downtime further increasing the cost of the failure. This management approach becomes more and more impractical due to the scale of today's networks and the accompanying steady decrease of the mean time between failures. Alternative methods of operating and maintaining these high-performance interconnects, both in terms of hardware- and software-management, are necessary to mitigate negative effects experienced by scientific applications executed on the supercomputer. However, existing topology-agnostic routing algorithms either suffer from poor load balancing or are not bounded in the number of virtual channels needed to resolve deadlocks in the routing tables. Using the fail-in-place strategy, a well-established method for storage systems to repair only critical component failures, is a feasible solution for current and future HPC interconnects as well as other large-scale installations such as data center networks. Although, an appropriate combination of topology and routing algorithm is required to minimize the throughput degradation for the entire system. This thesis contributes a network simulation toolchain to facilitate the process of finding a suitable combination, either during system design or while it is in operation. On top of this foundation, a key contribution is a novel scheduling-aware routing, which reduces fault-induced throughput degradation while improving overall network utilization. The scheduling-aware routing performs frequent property preserving routing updates to optimize the path balancing for simultaneously running batch jobs. The increased deployment of lossless interconnection networks, in conjunction with fail-in-place modes of operation and topology-agnostic, scheduling-aware routing algorithms, necessitates new solutions to solve the routing-deadlock problem. Therefore, this thesis further advances the state-of-the-art by introducing a novel concept of routing on the channel dependency graph, which allows the design of an universally applicable destination-based routing capable of optimizing the path balancing without exceeding a given number of virtual channels, which are a common hardware limitation. This disruptive innovation enables implicit deadlock-avoidance during path calculation, instead of solving both problems separately as all previous solutions

An In-Depth Analysis of the Slingshot Interconnect

The interconnect is one of the most critical components in large scale computing systems, and its impact on the performance of applications is going to increase with the system size. In this paper, we will describe Slingshot, an interconnection network for large scale computing systems. Slingshot is based on high-radix switches, which allow building exascale and hyperscale datacenters networks with at most three switch-to-switch hops. Moreover, Slingshot provides efficient adaptive routing and congestion control algorithms, and highly tunable traffic classes. Slingshot uses an optimized Ethernet protocol, which allows it to be interoperable with standard Ethernet devices while providing high performance to HPC applications. We analyze the extent to which Slingshot provides these features, evaluating it on microbenchmarks and on several applications from the datacenter and AI worlds, as well as on HPC applications. We find that applications running on Slingshot are less affected by congestion compared to previous generation networks.Comment: To be published in Proceedings of The International Conference for High Performance Computing Networking, Storage, and Analysis (SC '20) (2020