Heterogeneity-awareness in multithreaded multicore processors

During the last decades, Computer Architecture has experienced a great series of revolutionary changes. The increasing transistor count on a single chip has led to some of the main milestones in the field, from the release of the first Superscalar (1965) to the state-of-the-art Multithreaded Multicore Architectures, like the Intel Core i7 (2009).Moore's Law has continued for almost half of a century and is not expected to stop for at least another decade, and perhaps much longer. Moore observed a trend in the process technology advances. So, the number of transistors that can be placed inexpensively on an integrated circuit has increased exponentially, doubling approximately every two years. Nevertheless, having more available transistors can not be always directly translated into having more performance.The complexity of state-of-the-art software has reached heights unthinkable in prior ages, both in terms of the amount of computation and the complexity involved. If we deeply analyze this complexity in software we would realize that software is comprised of smaller execution processes that, although maintaining certain spatial/temporal locality, imply an inherently heterogeneous behavior. That is, during execution time the hardware executes very different portions of software, with huge differences in terms of behavior and hardware requirements. This heterogeneity in the behaviour of the software is not specific of the latest videogame, but it is inherent to software programming itself, since the very beginning of Algorithmics.In this PhD dissertation we deeply analyze the inherent heterogeneity present in software behavior. We identify the main issues and sources of this heterogeneity, that hamper most of the state-of-the-art processor designs from obtaining their maximum potential. Hence, the heterogeneity in software turns most of the current processors, commonly called general-purpose processors, into overdesigned. That is, they have much more hardware resources than really needed to execute the software running on them. This fact would not represent a main problem if we were not concerned on the additional power consumption involved in software computation.The final goal of this PhD dissertation consists in assigning each portion of software exactly the amount of hardware resources really needed to fully exploit its maximal potential; without consuming more energy than the strictly needed. That is, obtaining complexity-effective executions using the inherent heterogeneity in software behavior as steering indicator. Thus, we start deeply analyzing the heterogenous behaviour of the software run on top of general-purpose processors and then matching it on top of a heterogeneously distributed hardware, which explicitly exploit heterogeneous hardware requirements. Only by being heterogeneity-aware in software, and appropriately matching this software heterogeneity on top of hardware heterogeneity, may we effectively obtain better processor designs.The PhD dissertation is comprised of four main contributions that cover both multithreaded single-core (hdSMT) and multicore (TCA Algorithm, hTCA Framework and MFLUSH) scenarios, deeply explained in their corresponding chapters in the PhD dissertation memory. Overall, these contributions cover a significant range of the Heterogeneity-Aware Processors' design space. Within this design space, we have focused on the state-of-the-art trend in processor design: Multithreaded Multicore (CMP+SMT) Processors.We make special emphasis on the MPsim simulation tool, specifically designed and developed for this PhD dissertation. This tool has already gone beyond this PhD dissertation, becoming a reference tool by an important group of researchers spread over the Computer Architecture Department (DAC) at the Polytechnic University of Catalonia (UPC), the Barcelona Supercomputing Center (BSC) and the University of Las Palmas de Gran Canaria (ULPGC)

MURAC: A unified machine model for heterogeneous computers

Includes bibliographical referencesHeterogeneous computing enables the performance and energy advantages of multiple distinct processing architectures to be efficiently exploited within a single machine. These systems are capable of delivering large performance increases by matching the applications to architectures that are most suited to them. The Multiple Runtime-reconfigurable Architecture Computer (MURAC) model has been proposed to tackle the problems commonly found in the design and usage of these machines. This model presents a system-level approach that creates a clear separation of concerns between the system implementer and the application developer. The three key concepts that make up the MURAC model are a unified machine model, a unified instruction stream and a unified memory space. A simple programming model built upon these abstractions provides a consistent interface for interacting with the underlying machine to the user application. This programming model simplifies application partitioning between hardware and software and allows the easy integration of different execution models within the single control ow of a mixed-architecture application. The theoretical and practical trade-offs of the proposed model have been explored through the design of several systems. An instruction-accurate system simulator has been developed that supports the simulated execution of mixed-architecture applications. An embedded System-on-Chip implementation has been used to measure the overhead in hardware resources required to support the model, which was found to be minimal. An implementation of the model within an operating system on a tightly-coupled reconfigurable processor platform has been created. This implementation is used to extend the software scheduler to allow for the full support of mixed-architecture applications in a multitasking environment. Different scheduling strategies have been tested using this scheduler for mixed-architecture applications. The design and implementation of these systems has shown that a unified abstraction model for heterogeneous computers provides important usability benefits to system and application designers. These benefits are achieved through a consistent view of the multiple different architectures to the operating system and user applications. This allows them to focus on achieving their performance and efficiency goals by gaining the benefits of different execution models during runtime without the complex implementation details of the system-level synchronisation and coordination

Dynamic Processor Reconfiguration for Power, Performance and Reliability Management

Technology advancements allowed more transistors to be packed in a smaller area, while the improved performance helped in achieving higher clock frequencies. This, unfortunately led to a power density problem, forcing processor industry to lower the clock frequency and integrate multiple cores on the same die. Depending on core characteristics, the multiple cores in the die could be symmetric or asymmetric. Asymmetric multi-core processors (AMPs) have been proposed as an alternative to symmetric multi-cores to improve power efficiency. AMPs comprise of cores that implement the same ISA, but differ in performance and power characteristics due to varying sizes of micro-architectural resources. As the computational bottleneck of a workload shifts from one resource to another during its course of execution, reassigning it to another core (where it runs more efficiently), can improve the overall power efficiency. Thus achieving high power efficiency in AMPs requires (i) a diverse set of cores that are optimized for various program phases, (ii) runtime analysis to determine the best core to run on, and (iii) low overhead of re-assigning a thread to a different core type. Decisions to swap threads between AMPs are made at coarse grain granularity of millions of instructions, to mitigate the impact of thread migration overhead. But the computational needs of the program rapidly change during the course of its execution. The best core configuration for an application such that, both power consumption and performance are optimized, changes over time rapidly at fine granularity of thousands of instructions. This dissertation explores ways to design core micro-architecture such that high power efficiency could be achieved, if switching overhead could be lowered, enabling fine grain switching. To take advantage of power saving opportunities at fine grain granularity, this thesis explores reconfigurable/morphable architectures where core resources are reconfigured on demand to suit the needs of the executing application. At first, we explore reconfigurable architectures consisting of two kinds of cores: out-of-order (OOO) big cores and in-order (InO) small cores. The big cores provide higher performance while the small cores are more power efficient. In this proposed architecture, OOO core reconfigures into InO core at run time. Our proposed online management scheme decides to switch between these core types such that we obtain significant power benefits without impacting performance. We also observe that, resource requirements of applications can be quite diverse and consequently, resource bottlenecks or excesses can vary considerably. Thus, reconfiguration between just two core modes may not fully exploit power and performance improvement opportunities. We therefore, explore reconfigurable architectures consisting of diverse core types that not limited to big and little cores. A single core can reconfigure into multiple core modes where each mode has unique power and performance characteristics. Workload performance on a particular core mode depends on a large set of processor resources. Some workloads are highly memory intensive, some exhibit large instruction dependency, some experience high rates of branch mis-prediction, while other workloads exhibit large exploitable instruction level parallelism. A diverse set of core modes is needed, that could address shifting resource needs during various program phases of an application. Different trade-offs in power and performance could be achieved by reducing or expanding the size of various resource. Trade-offs for each core mode are also affected by operating voltage and frequency. We therefore, propose joint core resource resizing with dynamic voltage and frequency scaling (DVFS), which is important for applications whose performance is sensitive to changes in frequency. Thus, at fine granularity, the core should adapt to varying instruction window sizes, execution bandwidth and frequency to meet the demands of the workload at run-time to improve power efficiency. Many current processors employ DVFS aggressively to improve power efficiency and maximize performance. This dissertation studies the tradeoff in power efficiency in using fine grain DVFS and reconfigurable architectures mentioned above.We also explore another important problem due to continued scaling of devices which results in higher vulnerability to soft-errors. We consider dynamic core reconfiguration from the perspectives of both power efficiency and vulnerability to soft-errors. An online management scheme is proposed such that core reconfiguration upon a thread switch not only improves power efficiency but also does not increase the vulnerability to soft errors. In summary, we propose in this thesis several solutions for improving power efficiency by integrating heterogeneity within the core. We also address how popular power reduction techniques like DVFS are comparable to our approach. Finally, we address reliability challenges along with improving power efficiency

A Survey of Techniques for Architecting TLBs

“Translation lookaside buffer” (TLB) caches virtual to physical address translation information and is used in systems ranging from embedded devices to high-end servers. Since TLB is accessed very frequently and a TLB miss is extremely costly, prudent management of TLB is important for improving performance and energy efficiency of processors. In this paper, we present a survey of techniques for architecting and managing TLBs. We characterize the techniques across several dimensions to highlight their similarities and distinctions. We believe that this paper will be useful for chip designers, computer architects and system engineers

An automated OpenCL FPGA compilation framework targeting a configurable, VLIW chip multiprocessor

Modern system-on-chips augment their baseline CPU with coprocessors and accelerators to increase overall computational capacity and power efficiency, and thus have evolved into heterogeneous systems. Several languages have been developed to enable this paradigm shift, including CUDA and OpenCL. This thesis discusses a unified compilation environment to enable heterogeneous system design through the use of OpenCL and a customised VLIW chip multiprocessor (CMP) architecture, known as the LE1. An LLVM compilation framework was researched and a prototype developed to enable the execution of OpenCL applications on the LE1 CPU. The framework fully automates the compilation flow and supports work-item coalescing to better utilise the CPU cores and alleviate the effects of thread divergence. This thesis discusses in detail both the software stack and target hardware architecture and evaluates the scalability of the proposed framework on a highly precise cycle-accurate simulator. This is achieved through the execution of 12 benchmarks across 240 different machine configurations, as well as further results utilising an incomplete development branch of the compiler. It is shown that the problems generally scale well with the LE1 architecture, up to eight cores, when the memory system becomes a serious bottleneck. Results demonstrate superlinear performance on certain benchmarks (x9 for the bitonic sort benchmark with 8 dual-issue cores) with further improvements from compiler optimisations (x14 for bitonic with the same configuration

Investigating Single Precision Floating General Matrix Multiply in Heterogeneous Hardware

The fundamental operation of matrix multiplication is ubiquitous across a myriad of disciplines. Yet, the identification of new optimizations for matrix multiplication remains relevant for emerging hardware architectures and heterogeneous systems. Frameworks such as OpenCL enable computation orchestration on existing systems, and its availability using the Intel High Level Synthesis compiler allows users to architect new designs for reconfigurable hardware using C/C++. Using the HARPv2 as a vehicle for exploration, we investigate the utility of several of the most notable matrix multiplication optimizations to better understand the performance portability of OpenCL and the implications for such optimizations on this and future heterogeneous architectures. Our results give targeted insights into the applicability of best practices that were for existing architectures when used on emerging heterogeneous systems

Jenga: Harnessing Heterogeneous Memories through Reconfigurable Cache Hierarchies

Conventional memory systems are organized as a rigid hierarchy, with multiple levels of progressively larger and slower memories. Hierarchy allows a simple, fixed design to benefit a wide range of applications, because working sets settle at the smallest (and fastest) level they fit in. However, rigid hierarchies also cause significant overheads, because each level adds latency and energy even when it does not capture the working set. In emerging systems with heterogeneous memory technologies such as stacked DRAM, these overheads often limit performance and efficiency. We propose Jenga, a reconfigurable cache hierarchy that avoids these pathologies and approaches the performance of a hierarchy optimized for each application. Jenga monitors application behavior and dynamically builds virtual cache hierarchies out of heterogeneous, distributed cache banks. Jenga uses simple hardware support and a novel software runtime to configure virtual cache hierarchies. On a 36-core CMP with a 1 GB stacked-DRAM cache, Jenga outperforms a combination of state-of-the-art techniques by 10% on average and by up to 36%, and does so while saving energy, improving system-wide energy-delay product by 29% on average and by up to 96%