A RECONFIGURABLE AND EXTENSIBLE EXPLORATION PLATFORM FOR FUTURE HETEROGENEOUS SYSTEMS

Accelerator-based -or heterogeneous- computing has become increasingly important in a variety of scenarios, ranging from High-Performance Computing (HPC) to embedded systems. While most solutions use sometimes custom-made components, most of today’s systems rely on commodity highend CPUs and/or GPU devices, which deliver adequate performance while ensuring programmability, productivity, and application portability. Unfortunately, pure general-purpose hardware is affected by inherently limited power-efficiency, that is, low GFLOPS-per-Watt, now considered as a primary metric. The many-core model and architectural customization can play here a key role, as they enable unprecedented levels of power-efficiency compared to CPUs/GPUs. However, such paradigms are still immature and deeper exploration is indispensable. This dissertation investigates customizability and proposes novel solutions for heterogeneous architectures, focusing on mechanisms related to coherence and network-on-chip (NoC). First, the work presents a non-coherent scratchpad memory with a configurable bank remapping system to reduce bank conflicts. The experimental results show the benefits of both using a customizable hardware bank remapping function and non-coherent memories for some types of algorithms. Next, we demonstrate how a distributed synchronization master better suits many-cores than standard centralized solutions. This solution, inspired by the directory-based coherence mechanism, supports concurrent synchronizations without relying on memory transactions. The results collected for different NoC sizes provided indications about the area overheads incurred by our solution and demonstrated the benefits of using a dedicated hardware synchronization support. Finally, this dissertation proposes an advanced coherence subsystem, based on the sparse directory approach, with a selective coherence maintenance system which allows coherence to be deactivated for blocks that do not require it. Experimental results show that the use of a hybrid coherent and non-coherent architectural mechanism along with an extended coherence protocol can enhance performance. The above results were all collected by means of a modular and customizable heterogeneous many-core system developed to support the exploration of power-efficient high-performance computing architectures. The system is based on a NoC and a customizable GPU-like accelerator core, as well as a reconfigurable coherence subsystem, ensuring application-specific configuration capabilities. All the explored solutions were evaluated on this real heterogeneous system, which comes along with the above methodological results as part of the contribution in this dissertation. In fact, as a key benefit, the experimental platform enables users to integrate novel hardware/software solutions on a full-system scale, whereas existing platforms do not always support a comprehensive heterogeneous architecture exploration

Proximity coherence for chip-multiprocessors

Many-core architectures provide an efficient way of harnessing the growing numbers of transistors available in modern fabrication processes; however, the parallel programs run on these platforms are increasingly limited by the energy and latency costs of communication. Existing designs provide a functional communication layer but do not necessarily implement the most efficient solution for chip-multiprocessors, placing limits on the performance of these complex systems. In an era of increasingly power limited silicon design, efficiency is now a primary concern that motivates designers to look again at the challenge of cache coherence. The first step in the design process is to analyse the communication behaviour of parallel benchmark suites such as Parsec and SPLASH-2. This thesis presents work detailing the sharing patterns observed when running the full benchmarks on a simulated 32-core x86 machine. The results reveal considerable locality of shared data accesses between threads with consecutive operating system assigned thread IDs. This pattern, although of little consequence in a multi-node system, corresponds to strong physical locality of shared data between adjacent cores on a chip-multiprocessor platform. Traditional cache coherence protocols, although often used in chip-multiprocessor designs, have been developed in the context of older multi-node systems. By redesigning coherence protocols to exploit new patterns such as the physical locality of shared data, improving the efficiency of communication, specifically in chip-multiprocessors, is possible. This thesis explores such a design – Proximity Coherence – a novel scheme in which L1 load misses are optimistically forwarded to nearby caches via new dedicated links rather than always being indirected via a directory structure.EPSRC DTA research scholarshi

A consistency checker for memory subsystem traces

Verifying the memory subsystem in a modern shared-memory multiprocessor is a big challenge. Optimized implementations are highly sophisticated, yet must provide subtle consistency and liveness guarantees for the correct execution of concurrent programs. We present a tool that supports efficient specification-based testing of the memory subsystem against a range of formally specified consistency models. Our tool operates directly on the memory subsystem interface, promoting a compositional approach to system-on-chip verification, and can be used to search for simple failure cases – assisting rapid debug. It has recently been incorporated into the development flows of two open-source implementations – Berkeley’s Rocket Chip (RISCV) and Cambridge’s BERI (MIPS) – where it has uncovered a number of serious bugs.This work was supported by DARPA/AFRL contracts FA8750-10-C-0237 (CTSRD) and FA8750-11-C-0249 (MRC2), and EPSRC grant EP/K008528/1 (REMS).This is the author accepted manuscript

CROSS-LAYER CUSTOMIZATION FOR LOW POWER AND HIGH PERFORMANCE EMBEDDED MULTI-CORE PROCESSORS

Due to physical limitations and design difficulties, computer processor architecture has shifted to multi-core and even many-core based approaches in recent years. Such architectures provide potentials for sustainable performance scaling into future peta-scale/exa-scale computing platforms, at affordable power budget, design complexity, and verification efforts. To date, multi-core processor products have been replacing uni-core processors in almost every market segment, including embedded systems, general-purpose desktops and laptops, and super computers. However, many issues still remain with multi-core processor architectures that need to be addressed before their potentials could be fully realized. People in both academia and industry research community are still seeking proper ways to make efficient and effective use of these processors. The issues involve hardware architecture trade-offs, the system software service, the run-time management, and user application design, which demand more research effort into this field. Due to the architectural specialties with multi-core based computers, a Cross-Layer Customization framework is proposed in this work, which combines application specific information and system platform features, along with necessary operating system service support, to achieve exceptional power and performance efficiency for targeted multi-core platforms. Several topics are covered with specific optimization goals, including snoop cache coherence protocol, inter-core communication for producer-consumer applications, synchronization mechanisms, and off-chip memory bandwidth limitations. Analysis of benchmark program execution with conventional mechanisms is made to reveal the overheads in terms of power and performance. Specific customizations are proposed to eliminate such overheads with support from hardware, system software, compiler, and user applications. Experiments show significant improvement on system performance and power efficiency

From Parallel Programs to Customized Parallel Processors

The need for fast time to market of new embedded processor-based designs calls for a rapid design methodology of the included processors. The call for such a methodology is even more emphasized in the context of so called soft cores targeted to reconfigurable fabrics where per-design processor customization is commonplace. The C language has been commonly used as an input to hardware/software co-design flows. However, as C is a sequential language, its potential to generate parallel operations to utilize naturally parallel hardware constructs is far from optimal, leading to a customized processor design space with limited parallel resource scalability. In contrast, when utilizing a parallel programming language as an input, a wider processor design space can be explored to produce customized processors with varying degrees of utilized parallelism. This Thesis proposes a novel Multicore Application-Specific Instruction Set Processor (MCASIP) co-design methodology that exploits parallel programming languages as the application input format. In the methodology, the designer can explicitly capture the parallelism of the algorithm and exploit specialized instructions using a parallel programming language in contrast to being on the mercy of the compiler or the hardware to extract the parallelism from a sequential input. The Thesis proposes a multicore processor template based on the Transport Triggered Architecture, compiler techniques involved in static parallelization of computation kernels with barriers and a datapath integrated hardware accelerator for low overhead software synchronization implementation. These contributions enable scaling the customized processors both at the instruction and task levels to efficiently exploit the parallelism in the input program up to the implementation constraints such as the memory bandwidth or the chip area. The different contributions are validated with case studies, comparisons and design examples

Extending the HybridThread SMP Model for Distributed Memory Systems

Memory Hierarchy is of growing importance in system design today. As Moore\u27s Law allows system designers to include more processors within their designs, data locality becomes a priority. Traditional multiprocessor systems on chip (MPSoC) experience difficulty scaling as the quantity of processors increases. This challenge is common behavior of memory accesses in a shared memory environment and causes a decrease in memory bandwidth as processor numbers increase. In order to provide the necessary levels of scalability, the computer architecture community has sought to decentralize memory accesses by distributing memory throughout the system. Distributed memory offers greater bandwidth due to decoupled access paths. Today\u27s million gate Field Programmable Gate Arrays (FPGA) offer an invaluable opportunity to explore this type of memory hierarchy. FPGA vendors such as Xilinx provide dual-ported on-chip memory for decoupled access in addition to configurable sized memories. In this work, a new platform was created around the use of dual-ported SRAMs for distributed memory to explore the possible scalability of this form of memory hierarchy. However, developing distributed memory poses a tremendous challenge: supporting a linear address space that allows wide applicability to be achieved. Many have agreed that a linear address space eases the programmability of a system. Although the abstraction of disjointed memories via underlying architecture and/or new programming presents an advantage in exploring the possibilities of distributed memory, automatic data partitioning and migration remains a considerable challenge. In this research this challenge was dealt with by the inclusion of both a shared memory and distributed memory model. This research is vital because exposing the programmer to the underlying architecture while providing a linear address space results in desired standards of programmability and performance alike. In addition, standard shared memory programming models can be applied allowing the user to enjoy full scalable performance potential