OFAR-CM: Efficient Dragonfly networks with simple congestion management

Dragonfly networks are appealing topologies for large-scale Data center and HPC networks, that provide high throughput with low diameter and moderate cost. However, they are prone to congestion under certain frequent traffic patterns that saturate specific network links. Adaptive non-minimal routing can be used to avoid such congestion. That kind of routing employs longer paths to circumvent local or global congested links. However, if a distance-based deadlock avoidance mechanism is employed, more Virtual Channels (VCs) are required, what increases design complexity and cost. OFAR (On-the-Fly Adaptive Routing) is a previously proposed routing that decouples VCs from deadlock avoidance, making local and global misrouting affordable. However, the severity of congestion with OFAR is higher, as it relies on an escape sub network with low bisection bandwidth. Additionally, OFAR allows for unlimited misroutings on the escape sub network, leading to unbounded paths in the network and long latencies. In this paper we propose and evaluate OFAR-CM, a variant of OFAR combined with a simple congestion management (CM) mechanism which only relies on local information, specifically the credit count of the output ports in the local router. With simple escape sub networks such as a Hamiltonian ring or a tree, OFAR outperforms former proposals with distance-based deadlock avoidance. Additionally, although long paths are allowed in theory, in practice packets arrive at their destination in a small number of hops. Altogether, OFAR-CM constitutes the first practicable mechanism to the date that supports both local and global misrouting in Dragonfly networks.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. ERC-2012-Adg-321253- RoMoL, the Spanish Ministry of Science under contracts TIN2010-21291-C02-02, TIN2012-34557, and by the European HiPEAC Network of Excellence. M. García participated in this work while affiliated with the University of Cantabria.Peer ReviewedPostprint (author's final draft

Network unfairness in dragonfly topologies

Dragonfly networks arrange network routers in a two-level hierarchy, providing a competitive cost-performance solution for large systems. Non-minimal adaptive routing (adaptive misrouting) is employed to fully exploit the path diversity and increase the performance under adversarial traffic patterns. Network fairness issues arise in the dragonfly for several combinations of traffic pattern, global misrouting and traffic prioritization policy. Such 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. This paper reviews the main causes behind network unfairness in dragonflies, including a new adversarial traffic pattern which can easily occur in actual systems and congests all the global output links of a single router. A solution for the observed unfairness is evaluated using age-based arbitration. Results show that age-based arbitration mitigates fairness issues, especially when using in-transit adaptive routing. However, when using source adaptive routing, the saturation of the new traffic pattern interferes with the mechanisms employed to detect remote congestion, and the problem grows with the network size. This makes source adaptive routing in dragonflies based on remote notifications prone to reduced performance, even when using age-based arbitration.Peer ReviewedPostprint (author's final draft

Foc, pluges i resposta hidrològica del sòl a les muntanyes d'Alacant

El foc afecta el sòl mitjançant l'increment de la temperatura i la deposició de cendres, però també per la pèrdua de vegetació i horitzons orgànics, modificant la incidència de la pluja sobre la superfície. El risc de degradació postincendi està controlat, entre altres factors, pel clima, la vegetació prèvia i el tipus de sòl. Les localitats d'estudi es situaren en zones de transició entre climes semiàrid i subhumit. Se seguí la dinàmica temporal de l'escolament superficial i de la producció de sediments mitjançant simulació de pluja. En ambient semiàrid, la capacitat d'infiltració no es modificà tot just després de l'incendi. Tant en ambient semiàrid com subhumit, s'observà una pèrdua de la capacitat d'infiltració comparant els períodes previ i posterior a l'acció de la pluja sobre el sòl cremat. La resposta de la vegetació va ser un factor fonamental en el control de la degradació del sòl posterior al foc.Fuego, lluvias y respuesta hidrológica del suelo en las montañas alicantinas. El fuego afecta al suelo a través del incremento de la temperatura y la deposición de cenizas, pero también por la pérdida de vegetación y horizontes orgánicos, modificándose la incidencia de la lluvia sobre la superficie. El riesgo de degradación post-incendio está controlado, entre otros factores, por el clima, la vegetación previa y el tipo de suelo. Las localidades de estudio se situaron en zonas de transición entre climas semiárido y subhúmedo. Se estudió la dinámica temporal de las producciones de escorrentía y sedimentos mediante simulaciones de lluvia. En ambiente semiárido, la capacidad de infiltración no se vio modificada directamente por el fuego. Se observó una pérdida general de capacidad de infiltración comparando los periodos previo y posterior a la acción de las lluvias sobre el suelo quemado. La respuesta de la vegetación fue un factor fundamental en el control de la degradación del suelo posterior al fuego.Forest fires, rainstorms and soil hydrological response in the Alicante rangelands (SE Iberian Peninsula). Forest fires directly affect the soil surface through the increment of temperature and the deposition of ashes. Fires also result in the reduction or elimination of aboveground biomass and organic layers, thereby modifying the impact of raindrops on the soil surface. The post-fire degradation risk is controlled by diverse factors, e.g. climate, pre-fire vegetation and soil characteristics. Study sites were located in transitional areas between semiarid and subhumid climates. We assessed the temporal dynamics of runoff and sediment yield using a rainfall simulator. Under semiarid climate, the infiltration capacity was not directly modified by the fire. Both under semiarid and under subhumid climates, an increase in runoff and sediment production was observed by comparing the post-fire results obtained prior to and after than the rainy season. Plant response was one of the main factors controlling the post-fire soil degradation risk

THE SUBANTARCTIC TIERRA DEL FUEGO: NOTHOFAGUS FORESTS OF DISTRIBUTION, STRUCTURE AND PRODUCTION

Evergreen Nothofagus betuloides and deciduous N. pumilio form the main forest types in Tierra del Fuego. These forests were sampled along two altitudinal gradients to study their structure and dynamics and assess the causes of their distribution. The distribution pattems of the two species of Nothofagus seem to respond to different climates and soils. The dornÍnant soil processes are hydromorphy in the evergreen forest and podzolization in the deciduous one. N. betuloides is an evergreen resilient to short-term environmental fluctuations, due to its ability to retain nutrients. Leaves on the tree may last up to 7 years, with an average density of 17 mg cm,2 . In contrast, the leaves of N. pumilio are shed in autumn and reach only 8 mg cm,2 . In both types of forests the following features can be outlined. Old-growth forest stands develop in the middle and lower slopes. The distribution of diameter sizes of the trees usually shows a pronounced bimodality. Recruitment is discontinuous as shown by the spatial pattem of tree sizes, and regeneration is vegetative in the upper slopes. Leaf area indices range between 2.3 and 2.8 m 2 m,2 for the deciduous forest and from 2.5 to 4.5 m 2 m,2 for the evergreen forest. It appears that tree standing biomass ranges between 6.0 and 1 3.6 kg e m'2 in the deciduous forest, and between 10.5 and 1 5.6 Kg e m,2 in the evergreen forest. Production varies from 148.0 to 372.1 g e m ,2 yr , 1 , and from 204.6 to 346.4 g e m ,2 yr,l , respecti vely. In relation to boreal forests, biomass can be considered high, but production is medium to low. The relation of wood production to leaf biomass shows that N. betuloides be ha ves similar to coniferous forests and N. pumilio as hardwood forests. There is much accumulation of organic matter on the forest floor, especially coarse wood debris, reflecting the stage of development of these forests and the unfavourable conditions for litter decomposition

Analysis of soil carbon in Spanish agricultural soils: relevant aspects in relation to the organic farming reconversion in the Mediterranean area

Los niveles de carbono orgánico en suelos agrícolas son especialmente relevantes en los modelos de gestión que se basan en la reserva orgánica de nutrientes. La disminución de materia orgánica que ocurre en los suelos roturados puede en algunos casos determinar la degradación del suelo. Este hecho puede ser especialmente relevante en condiciones climáticas límite (secas y semiáridas). A partir del análisis de una base de datos de suelos españoles se discute la distribución del C en el horizonte superficial de suelo según el clima y según el uso. Para cada área climática, mediante comparación entre suelos forestales y agrícolas se observa que los suelos de clima semiárido presentan una mayor resistencia a la pérdida de C y una menor resiliencia que los suelos de climas más húmedos. Este hecho nos hace pensar que los suelos semiáridos pueden estar cerca de los umbrales de degradación y que por lo tanto su gestión debería evitar les pérdidas de materia orgánica.Organic carbon levels in agricultural soils are specially relevant in the management models based on the organic reserve of the soil. Losses of soil organic matter that occur associated to tillage practices can enhance, in some cases, soil degradation. This fact may be specially relevant in dry and semiarid climates. By analysing a soil data base of the Spanish forest and agricultural soils we discuss the distribution of C in the soil surface horizon across climates and land uses. In each climatic area, by comparing forest with agricultural soils we found that soils in semiarid climate show greater resistance to soil C losses and lower resilience than soils of wetter climates. This fact suggest that semiarid soils can be at or near soil degradation thresholds and thus its management should avoid any losses of soil organic matter.Este trabajo ha sido posible gracias a un convenio establecido entre la Dirección General de Evaluación Ambiental del Ministerio de Medio Ambiente y la Universidad de Barcelona

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

Architectural support for task dependence management with flexible software scheduling

The growing complexity of multi-core architectures has motivated a wide range of software mechanisms to improve the orchestration of parallel executions. Task parallelism has become a very attractive approach thanks to its programmability, portability and potential for optimizations. However, with the expected increase in core counts, finer-grained tasking will be required to exploit the available parallelism, which will increase the overheads introduced by the runtime system. This work presents Task Dependence Manager (TDM), a hardware/software co-designed mechanism to mitigate runtime system overheads. TDM introduces a hardware unit, denoted Dependence Management Unit (DMU), and minimal ISA extensions that allow the runtime system to offload costly dependence tracking operations to the DMU and to still perform task scheduling in software. With lower hardware cost, TDM outperforms hardware-based solutions and enhances the flexibility, adaptability and composability of the system. Results show that TDM improves performance by 12.3% and reduces EDP by 20.4% on average with respect to a software runtime system. Compared to a runtime system fully implemented in hardware, TDM achieves an average speedup of 4.2% with 7.3x less area requirements and significant EDP reductions. In addition, five different software schedulers are evaluated with TDM, illustrating its flexibility and performance gains.This work has been supported by the RoMoL ERC Advanced Grant (GA 321253), by the European HiPEAC Network of Excellence, by the Spanish Ministry of Science and Innovation (contracts TIN2015-65316-P, TIN2016-76635-C2-2-R and TIN2016-81840-REDT), by the Generalitat de Catalunya (contracts 2014-SGR-1051 and 2014-SGR-1272), and by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 671697 and No. 671610. M. Moretó has been partially supported by the Ministry of Economy and Competitiveness under Juan de la Cierva postdoctoral fellowship number JCI-2012-15047.Peer ReviewedPostprint (author's final draft

Solving multiprocessor drawbacks with kilo-instruction processors

Nowadays, a good multiprocessor system design has to deal with many drawbacks in order to achieve a good tradeoff between complexity and performance. For example, while solving problems like coherence and consistency is essential for correctness the way to solve processor stalls due to critical sections and synchronization points is desirable for performance. And none of these drawbacks has a straightforward solution. We show in our paper how the multi-checkpointing mechanism of the Kilo-Instruction Processors can be correctly leveraged in order to achieve a good complexity-effective multiprocessor design. Specifically, we describe a Kilo-Instruction Multiprocessor that transparently, i.e. without any software support, uses transaction-based memory updates. Our model simplifies the coherence and consistency hardware and gives the potential for easily applying different desirable speculative mechanisms to enhance performance when facing some synchronization constructs of current parallel applications.Postprint (published version

Implicit transactional memory in chip multiprocessors

Chip Multiprocessors (CMPs) are an efficient way of designing and use the huge amount of transistors on a chip. Different cores on a chip can compose a shared memory system with a very low-latency interconnect at a very low cost. Unfortunately, consistency models and synchronization styles of popular programming models for multiprocessors impose severe performance losses. Known architectural approaches to combat these losses are too complex, too specialized, or not transparent to the software. In this article, we introduce “implicit transactional memory” as a generalized architectural concept to remove such performance losses. We show how the concept of implicit transactions can be implemented at a low complexity by leveraging the multi-checkpoint mechanism of the Kilo-Instruction Processor. By relying on a general speculation substrate, it supports even the strictest consistency model – sequential consistency – potentially as effectively as weaker models and it allows multiple threads to speculatively execute critical sections, beyond barriers and event synchronizations.Postprint (published version