Design and performance optimization of asynchronous networks-on-chip

As digital systems continue to grow in complexity, the design of conventional synchronous systems is facing unprecedented challenges. The number of transistors on individual chips is already in the multi-billion range, and a greatly increasing number of components are being integrated onto a single chip. As a consequence, modern digital designs are under strong time-to-market pressure, and there is a critical need for composable design approaches for large complex systems. In the past two decades, networks-on-chip (NoC’s) have been a highly active research area. In a NoC-based system, functional blocks are first designed individually and may run at different clock rates. These modules are then connected through a structured network for on-chip global communication. However, due to the rigidity of centrally-clocked NoC’s, there have been bottlenecks of system scalability, energy and performance, which cannot be easily solved with synchronous approaches. As a result, there has been significant recent interest in combing the notion of asynchrony with NoC designs. Since the NoC approach inherently separates the communication infrastructure, and its timing, from computational elements, it is a natural match for an asynchronous paradigm. Asynchronous NoC’s, therefore, enable a modular and extensible system composition for an ‘object-orient’ design style. The thesis aims to significantly advance the state-of-art and viability of asynchronous and globally-asynchronous locally-synchronous (GALS) networks-on-chip, to enable high-performance and low-energy systems. The proposed asynchronous NoC’s are nearly entirely based on standard cells, which eases their integration into industrial design flows. The contributions are instantiated in three different directions. First, practical acceleration techniques are proposed for optimizing the system latency, in order to break through the latency bottleneck in the memory interfaces of many on-chip parallel processors. Novel asynchronous network protocols are proposed, along with concrete NoC designs. A new concept, called ‘monitoring network’, is introduced. Monitoring networks are lightweight shadow networks used for fast-forwarding anticipated traffic information, ahead of the actual packet traffic. The routers are therefore allowed to initiate and perform arbitration and channel allocation in advance. The technique is successfully applied to two topologies which belong to two different categories – a variant mesh-of-trees (MoT) structure and a 2D-mesh topology. Considerable and stable latency improvements are observed across a wide range of traffic patterns, along with moderate throughput gains. Second, for the first time, a high-performance and low-power asynchronous NoC router is compared directly to a leading commercial synchronous counterpart in an advanced industrial technology. The asynchronous router design shows significant performance improvements, as well as area and power savings. The proposed asynchronous router integrates several advanced techniques, including a low-latency circular FIFO for buffer design, and a novel end-to-end credit-based virtual channel (VC) flow control. In addition, a semi-automated design flow is created, which uses portions of a standard synchronous tool flow. Finally, a high-performance multi-resource asynchronous arbiter design is developed. This small but important component can be directly used in existing asynchronous NoC’s for performance optimization. In addition, this standalone design promises use in opening up new NoC directions, as well as for general use in parallel systems. In the proposed arbiter design, the allocation of a resource to a client is divided into several steps. Multiple successive client-resource pairs can be selected rapidly in pipelined sequence, and the completion of the assignments can overlap in parallel. In sum, the thesis provides a set of advanced design solutions for performance optimization of asynchronous and GALS networks-on-chip. These solutions are at different levels, from network protocols, down to router- and component-level optimizations, which can be directly applied to existing basic asynchronous NoC designs to provide a leap in performance improvement

A feasibility study on integrating electric buses with waste gasification for a green public transport system and solid waste management

Waste management and public transport are two major issues requiring decarbonisation in the face of climate change and environmental concerns related to global warming. Green transport systems are classified as zero or low carbon alternatives to the fossil fuel-based approach and vehicles. These systems rely on zero emission fuels such as hydrogen. Thermochemical processes (e.g., gasification) and biochemical technologies (e.g., fermentation) can convert carbon-based feedstock such as waste to produce desirable products like hydrogen. Waste-to Hydrogen is proposed as a potential solution to provide both sustainable waste management and hydrogen production. Waste-to-Hydrogen (WtH) is a hybrid solution that simultaneously combines sustainable waste management and non-fossil-fuel based hydrogen production. The concept of distributed WtH systems, based on gasification and fermentation, is to support hydrogen fuel cell buses in Glasgow is considered as a potential solution zero emission transport development. Hydrogen has potential to replace petrol and diesel fuels and consequently become part the zero-carbon measures to aid the transition to cleaner energy sources. When hydrogen is produced from renewable or sustainable energy sources it can help decarbonise the energy and transport sector. To be attractive to policymakers and investors it is necessary for the hydrogen from a WtH system to demonstrate its carbon footprint is lower than conventional methods. By supporting the effort to reach carbon emission reduction targets, hydrogen is part of the solution to limit climate change, a global emergency. Providing research to support the roadmap of hydrogen-powered public transport to shape the direction of future technological improvement and policy formulation. As well as the potential to provide a clean versatile fuel through hydrogen, WtH can offer an alternative waste management practice that diverts waste away from landfill and incineration. By utilising and transforming waste into a useful energy resource, a value is applied which can encourage the development of sustainable disposal methods such as WtH conversion processes. Glasgow was chosen as the location for the study due to the large population which would supply regular amounts of waste to be used as feedstock. The city council is also actively trying to decarbonise local industries including transport, this is seen by the strategies and targets in place such as Net Zero by 2045. An aim of this study is to demonstrate how low carbon hydrogen production technologies could fit into the city’s transport and energy plan and support the hydrogen strategy, thereby benefitting the people of Glasgow. Whilst Glasgow does not currently use fuel cell electric buses (FCEB) for public transport, an intention to run a fleet has been presented through the publication of the Scottish Governments Hydrogen Policy Statement (2020) and Hydrogen Action Plan (2022). FCEB fleets in other parts of the UK notably London and Birmingham, have shown the environmental benefit through the annual carbon savings made. FCEBs are classified as zero emissions buses (ZEB) which the UK Department of Transport has stated can reduce carbon emissions by 46 tonnes per year and nitrogen oxide (NOx) by 23kg when compared to a diesel bus (UK Government Department for Transport, 2021). This study contributes to the growing evidence of the benefits of using hydrogen as a transport fuel in terms of the carbon savings as an alternative to conventional fossil fuels. Whilst the main concerns of the underdeveloped industrial status, relatively immature technology and high costs are explored. In practice WtH is currently limited to laboratory and pilot scale systems and requires further investment and policy support for advancements to be made. These bottlenecks and limitations are considered in the discussion section of this study. The research question centres around the economic and environmental feasibility of WtH within Glasgow. A feasible project would show the carbon savings compared to conventional methods in both aspects of waste management and hydrogen production. The feasibility is also a measurement of positive returns on economic investment where total project costs do not outweigh the environmental benefits associated with low carbon technologies. This study critically assesses the current situation for WtH development in terms of the environmental impact and potential carbon savings, economic implications, and cost benefits, plus transport and climate policy. The novelty of the study establishes a procedure for defining how WtH could support the growing hydrogen industry as a low carbon hydrogen production technique. The results from the environmental impact analysis and economic assessment add data sets to existing research in academia and energy industry. Life cycle assessment (LCA), cost benefit analysis (CBA) and multi-bjective optimization (MOO) have been conducted to determine the feasibility of WtH projects to support green transport systems and sustainable waste management schemes. A variety of WtH scenarios were designed based on biomass waste feedstock, hydrogen production reactors, and upstream and downstream system components. The WtH systems selected use thermochemical and biochemical technologies to convert the different waste feedstocks available in Glasgow with suitable operational conditions according to the waste characteristics. The waste considered in this study is biodegradable, carbon based and organic including household, plastics, waste wood products, as well as the wet fraction of waste such as food and sewage sludge. Five scenarios, four WtH technologies and one conventional hydrogen production technology of steam methane reforming (SMR), were designed to allow for comparison of environmental and economic results. The scenarios differ in waste feedstock type and technology leading to differences in hydrogen production rates, hydrogen yields, and process carbon emissions. Waste that is less suitable for thermochemical conversion processes can be utilised by biochemical technology to ensure the most efficient and least energy intensive method is applied. The environmental approach for this work focuses on the LCA method to evaluate environmental performance through the carbon saving potential using global warming potential (GWP) as the impact indicator for the WtH technologies. It was shown that WtH technologies could reduce <55% of CO2-eq emissions per kg H2 compared to SMR. Gasification treating municipal solid waste and waste wood had global warming potentials of 4.99 and 4.11 kg CO2-eq/kg H2 respectively, which were lower than dark fermentation treating wet waste at 6.6 kg CO2-eq/kg H2 and combined dark and photo fermentation at 6.4 kg CO2-eq/kg H2. The distance emissions of WtH-based electric fuel cell bus scenarios were 0.33-0.44 kg CO2-eq/km as compared to 0.89 kg CO2-eq/km for the SMR-based scenario. The economic assessment in this study uses cost benefit analysis to determine whether the carbon savings outweigh the expected cost of a WtH system. The CBA was conducted to compare the economic feasibility of the different WtH systems with the conventional SMR. A database was that includes, direct cost data on construction, maintenance, operations, infrastructure, and storage, along with indirect cost data comprising environmental impacts and externalities, cost of pollution, carbon taxes and subsidies was collated. The results are in the form of economic indicators Net present value (NPV), Internal rate of return (IRR), Benefit cost ratio (BCR) and Levelized cost of hydrogen (LCoH). The LCoH was calculated as 0.49 GBP/kg for the gasification systems using MSW feedstock and 0.52 GBP/kg for waste wood gasification. The LCoH for dark fermentation was calculated to be 0.52 GBP/kg and 0.59 GBP/kg for combined dark and photo fermentation systems. Sensitivity analysis was conducted to identify the most significant influential factors of distributed WtH systems. The results indicate that the conversion efficiency and the energy density of the waste had the largest impact for biochemical technology and thermochemical technologies, respectively. It is concluded that WtH could be economically feasible for hydrogen production in Glasgow. However, limitations including high capital expenditure will require cost reduction through technical advancements and carbon tax on conventional hydrogen production methods to improve the outlook for WtH. The multi-objective optimisation results suggest that optimisation is possible with the best solution calculated to minimise both total cost and GWP for the four Scenarios assessed in this work. The results from the three analysis types in this work, indicate the feasibility of WtH in Glasgow. The results suggest there is potential to utilise the waste generated within Glasgow to produce hydrogen, reduce the environmental impact of waste management practices, and provide economic benefit to both the energy and transport industry

On Multicast in Asynchronous Networks-on-Chip: Techniques, Architectures, and FPGA Implementation

In this era of exascale computing, conventional synchronous design techniques are facing unprecedented challenges. The consumer electronics market is replete with many-core systems in the range of 16 cores to thousands of cores on chip, integrating multi-billion transistors. However, with this ever increasing complexity, the traditional design approaches are facing key issues such as increasing chip power, process variability, aging, thermal problems, and scalability. An alternative paradigm that has gained significant interest in the last decade is asynchronous design. Asynchronous designs have several potential advantages: they are naturally energy proportional, burning power only when active, do not require complex clock distribution, are robust to different forms of variability, and provide ease of composability for heterogeneous platforms. Networks-on-chip (NoCs) is an interconnect paradigm that has been introduced to deal with the ever-increasing system complexity. NoCs provide a distributed, scalable, and efficient interconnect solution for today’s many-core systems. Moreover, NoCs are a natural match with asynchronous design techniques, as they separate communication infrastructure and timing from the computational elements. To this end, globally-asynchronous locally-synchronous (GALS) systems that interconnect multiple processing cores, operating at different clock speeds, using an asynchronous NoC, have gained significant interest. While asynchronous NoCs have several advantages, they also face a key challenge of supporting new types of traffic patterns. Once such pattern is multicast communication, where a source sends packets to arbitrary number of destinations. Multicast is not only common in parallel computing, such as for cache coherency, but also for emerging areas such as neuromorphic computing. This important capability has been largely missing from asynchronous NoCs. This thesis introduces several efficient multicast solutions for these interconnects. In particular, techniques, and network architectures are introduced to support high-performance and low-power multicast. Two leading network topologies are the focus: a variant mesh-of-trees (MoT) and a 2D mesh. In addition, for a more realistic implementation and analysis, as well as significantly advancing the field of asynchronous NoCs, this thesis also targets synthesis of these NoCs on commercial FPGAs. While there has been significant advances in FPGA technologies, there has been only limited research on implementing asynchronous NoCs on FPGAs. To this end, a systematic computeraided design (CAD) methodology has been introduced to efficiently and safely map asynchronous NoCs on FPGAs. Overall, this thesis makes the following three contributions. The first contribution is a multicast solution for a variant MoT network topology. This topology consists of simple low-radix switches, and has been used in high-performance computing platforms. A novel local speculation technique is introduced, where a subset of the network’s switches are speculative that always broadcast every packet. These switches are very simple and have high performance. Speculative switches are surrounded by non-speculative ones that route packets based on their destinations and also throttle any redundant copies created by the former. This hybrid network architecture achieved significant performance and power benefits over other multicast approaches. The second contribution is a multicast solution for a 2D-mesh topology, which is more complex with higher-radix switches and also is more commonly used. A novel continuous-time replication strategy is introduced to optimize the critical multi-way forking operation of a multicast transmission. In this technique, a multicast packet is first stored in an input port of a switch, from where it is sent through distinct output ports towards different destinations concurrently, at each output’s own rate and in continuous time. This strategy is shown to have significant latency and energy benefits over an approach that performs multicast using multiple distinct serial unicasts to each destination. Finally, a systematic CAD methodology is introduced to synthesize asynchronous NoCs on commercial FPGAs. A two-fold goal is targeted: correctness and high performance. For ease of implementation, only existing FPGA synthesis tools are used. Moreover, since asynchronous NoCs involve special asynchronous components, a comprehensive guide is introduced to map these elements correctly and efficiently. Two asynchronous NoC switches are synthesized using the proposed approach on a leading Xilinx FPGA in 28 nm: one that only handles unicast, and the other that also supports multicast. Both showed significant energy benefits with some performance gains over a state-of-the-art synchronous switch

Strategies and tools for the exploitation of massively parallel computer systems

The aim of this thesis is to develop software and strategies for the exploitation of parallel computer hardware, in particular distributed memory systems, and embedding these strategies within a parallelisation tool to allow the automatic generation of these strategies. The parallelisation of four structured mesh codes using the Computer Aided Parallelisation Tools provided a good initial parallelisation of the codes. However, investigation revealed that simple optimisation of the communications within these codes provided an even better improvement in performance. The dominant factor within the communications was the data transfer time with communication start-up latencies also significant. This was significant throughout the codes but especially in sections of pipelined code where there were large amounts of communication present. This thesis describes the development and testing of the methods used to increase the performance of these communications by overlapping them with unrelated calculation. This method of overlapping the communications was applied to the exchange of data communications as well as the pipelined communications. The successful application by hand provided the motivation for these methods to be incorporated and automatically generated within the Computer Aided Parallelisation Tools. These methods were integrated within these tools as an additional stage of the parallelisation. This required a generic algorithm that made use of many of the symbolic algebra tests and symbolic variable manipulation routines within the tools. The automatic generation of overlapped communications was applied to the four codes previously parallelised as well as a further three codes, one of which was a real world Computational Fluid Dynamics code. The methods to apply automatic generation of overlapped communications to unstructured mesh codes were also discussed. These methods are similar to those applied to the structured mesh codes and their automation is viewed to be of a similar fashion

RA-LPEL: A Resource-Aware Light-Weight Parallel Execution Layer for Reactive Stream Processing Networks on The SCC Many-core Tiled Architecture

In computing the available computing power has continuously fallen short of the demanded computing performance. As a consequence, performance improvement has been the main focus of processor design. However, due to the phenomenon called “Power Wall” it has become infeasible to build faster processors by just increasing the processor’s clock speed. One of the resulting trends in hardware design is to integrate several simple and power-efficient cores on the same chip. This design shift poses challenges of its own. In the past, with increasing clock frequency the programs became automatically faster as well without modifications. This is no longer true with many-core architectures. To achieve maximum performance the programs have to run concurrently on more than one core, which forces the general computing paradigm to become increasingly parallel to leverage maximum processing power. In this thesis, we will focus on the Reactive Stream Program (RSP). In stream processing, the system consists of computing nodes, which are connected via communication streams. These streams simplify the concurrency management on modern many-core architectures due to their implicit synchronisation. RSP is a stream processing system that implements the reactive system. The RSPs work in tandem with their environment and the load imposed by the environment may vary over time. This provides a unique opportunity to increase performance per watt. In this thesis the research contribution focuses on the design of the execution layer to run RSPs on tiled many-core architectures, using the Intel’s Single-chip Cloud Computer (SCC) processor as a concrete experimentation platform. Further, we have developed a Dynamic Voltage and Frequency Scaling (DVFS) technique for RSP deployed on many-core architectures. In contrast to many other approaches, our DVFS technique does not require the capability of controlling the power settings of individual computing elements, thus making it applicable for modern many-core architectures, with which power can be changed only for power islands. The experimental results confirm that the proposed DVFS technique can effectively improve the energy efficiency, i.e. increase the performance per watt, for RSPs

Investigation of subunits of the cytoplasmic dynein complex using novel mouse models

Cytoplasmic dynein is a multisubunit complex responsible for the transport of cellular components from the cell periphery towards the nucleus. The role of the dynein complex in vesicle trafficking, organelle positioning and chromosome segregation during mitosis has been extensively studied but still little is known of specific roles of distinct subunits of the complex. Cytoplasmic dynein is a dimeric complex consisting of heavy chains, intermediate chains, light intermediate chains and three light chains. In order to investigate the roles of the cytoplasmic dynein subunits, two mouse lines with chemically generated single point mutations in the intermediate chain 1 and 2 genes (Dync1i1, Dync1i2) were subjected to a behavioural analysis. The mouse line carrying a mutation in the intermediate chain 2 showed working memory deficits which suggested impairment in hippocampal functions. In order to examine the effects of mutation at the cellular level primary mouse embryonic fibroblasts (MEFs) lines were derived from embryos carrying mutations in the intermediate chains and used as a model system. Cell functions, such as trafficking of epidermal growth factor (EGF) positive endosomes, Golgi assembly were examined. Furthermore, biochemical analyses were performed focused on the expression of dynein subunits and their assembly in the functional complex. Alternative splicing is known to produce multiple isoforms of the intermediate chains. The analysis of various splice variants of these genes in a panel of mouse tissues resulted in detecting new isoforms which were compared with bioinformatics data available for human and rat thus establishing the splicing pattern of the mouse intermediate chains. Legs at odd angles (Dync1h1Loa) is another mutant mouse line carrying a point mutation in the dynein heavy chain which results in neurological defects. Here the effects of the Loa mutation in the trafficking of membranous organelles were investigated by an infection of cultured MEFs with Salmonella enterica serovar Typhimurium. Furthermore, upon the induction of a cellular stress the wildtype and the Loa homozygous cells showed significant differences in stress granule assembly suggesting the impairment in the stress signaling