Design of Efficient TLB-based Data Classification Mechanisms in Chip Multiprocessors

Most of the data referenced by sequential and parallel applications running in current chip multiprocessors are referenced by a single thread, i.e., private. Recent proposals leverage this observation to improve many aspects of chip multiprocessors, such as reducing coherence overhead or the access latency to distributed caches. The effectiveness of those proposals depends to a large extent on the amount of detected private data. However, the mechanisms proposed so far either do not consider either thread migration or the private use of data within different application phases, or do entail high overhead. As a result, a considerable amount of private data is not detected. In order to increase the detection of private data, this thesis proposes a TLB-based mechanism that is able to account for both thread migration and private application phases with low overhead. Classification status in the proposed TLB-based classification mechanisms is determined by the presence of the page translation stored in other core's TLBs. The classification schemes are analyzed in multilevel TLB hierarchies, for systems with both private and distributed shared last-level TLBs. This thesis introduces a page classification approach based on inspecting other core's TLBs upon every TLB miss. In particular, the proposed classification approach is based on exchange and count of tokens. Token counting on TLBs is a natural and efficient way for classifying memory pages. It does not require the use of complex and undesirable persistent requests or arbitration, since when two ormore TLBs race for accessing a page, tokens are appropriately distributed classifying the page as shared. However, TLB-based ability to classify private pages is strongly dependent on TLB size, as it relies on the presence of a page translation in the system TLBs. To overcome that, different TLB usage predictors (UP) have been proposed, which allow a page classification unaffected by TLB size. Specifically, this thesis introduces a predictor that obtains system-wide page usage information by either employing a shared last-level TLB structure (SUP) or cooperative TLBs working together (CUP).La mayor parte de los datos referenciados por aplicaciones paralelas y secuenciales que se ejecutan enCMPs actuales son referenciadas por un único hilo, es decir, son privados. Recientemente, algunas propuestas aprovechan esta observación para mejorar muchos aspectos de los CMPs, como por ejemplo reducir el sobrecoste de la coherencia o la latencia de los accesos a cachés distribuidas. La efectividad de estas propuestas depende en gran medida de la cantidad de datos que son considerados privados. Sin embargo, los mecanismos propuestos hasta la fecha no consideran la migración de hilos de ejecución ni las fases de una aplicación. Por tanto, una cantidad considerable de datos privados no se detecta apropiadamente. Con el fin de aumentar la detección de datos privados, proponemos un mecanismo basado en las TLBs, capaz de reclasificar los datos a privado, y que detecta la migración de los hilos de ejecución sin añadir complejidad al sistema. Los mecanismos de clasificación en las TLBs se han analizado en estructuras de varios niveles, incluyendo TLBs privadas y con un último nivel de TLB compartido y distribuido. Esta tesis también presenta un mecanismo de clasificación de páginas basado en la inspección de las TLBs de otros núcleos tras cada fallo de TLB. De forma particular, el mecanismo propuesto se basa en el intercambio y el cuenteo de tokens (testigos). Contar tokens en las TLBs supone una forma natural y eficiente para la clasificación de páginas de memoria. Además, evita el uso de solicitudes persistentes o arbitraje alguno, ya que si dos o más TLBs compiten para acceder a una página, los tokens se distribuyen apropiadamente y la clasifican como compartida. Sin embargo, la habilidad de los mecanismos basados en TLB para clasificar páginas privadas depende del tamaño de las TLBs. La clasificación basada en las TLBs se basa en la presencia de una traducción en las TLBs del sistema. Para evitarlo, se han propuesto diversos predictores de uso en las TLBs (UP), los cuales permiten una clasificación independiente del tamaño de las TLBs. En concreto, esta tesis presenta un sistema mediante el que se obtiene información de uso de página a nivel de sistema con la ayuda de un nivel de TLB compartida (SUP) o mediante TLBs cooperando juntas (CUP).La major part de les dades referenciades per aplicacions paral·leles i seqüencials que s'executen en CMPs actuals són referenciades per un sol fil, és a dir, són privades. Recentment, algunes propostes aprofiten aquesta observació per a millorar molts aspectes dels CMPs, com és reduir el sobrecost de la coherència o la latència d'accés a memòries cau distribuïdes. L'efectivitat d'aquestes propostes depen en gran mesura de la quantitat de dades detectades com a privades. No obstant això, els mecanismes proposats fins a la data no consideren la migració de fils d'execució ni les fases d'una aplicació. Per tant, una quantitat considerable de dades privades no es detecta apropiadament. A fi d'augmentar la detecció de dades privades, aquesta tesi proposa un mecanisme basat en les TLBs, capaç de reclassificar les dades com a privades, i que detecta la migració dels fils d'execució sense afegir complexitat al sistema. Els mecanismes de classificació en les TLBs s'han analitzat en estructures de diversos nivells, incloent-hi sistemes amb TLBs d'últimnivell compartides i distribuïdes. Aquesta tesi presenta un mecanisme de classificació de pàgines basat en inspeccionar les TLBs d'altres nuclis després de cada fallada de TLB. Concretament, el mecanisme proposat es basa en l'intercanvi i el compte de tokens. Comptar tokens en les TLBs suposa una forma natural i eficient per a la classificació de pàgines de memòria. A més, evita l'ús de sol·licituds persistents o arbitratge, ja que si dues o més TLBs competeixen per a accedir a una pàgina, els tokens es distribueixen apropiadament i la classifiquen com a compartida. No obstant això, l'habilitat dels mecanismes basats en TLB per a classificar pàgines privades depenen de la grandària de les TLBs. La classificació basada en les TLBs resta en la presència d'una traducció en les TLBs del sistema. Per a evitar-ho, s'han proposat diversos predictors d'ús en les TLBs (UP), els quals permeten una classificació independent de la grandària de les TLBs. Específicament, aquesta tesi introdueix un predictor que obté informació d'ús de la pàgina a escala de sistema mitjançant un nivell de TLB compartida (SUP) or mitjançant TLBs cooperant juntes (CUP).Esteve García, A. (2017). Design of Efficient TLB-based Data Classification Mechanisms in Chip Multiprocessors [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86136TESI

Locality-oblivious cache organization leveraging single-cycle multi-hop NoCs

Locality has always been a critical factor in on-chip data placement on CMPs as accessing further-away caches has in the past been more costly than accessing nearby ones. Substantial research on locality-aware designs have thus focused on keeping a copy of the data private. However, this complicatesthe problem of data tracking and search/invalidation; tracking the state of a line at all on-chip caches at a directory or performing full-chip broadcasts are both non-scalable and extremely expensive solutions. In this paper, we make the case for Locality-Oblivious Cache Organization (LOCO), a CMP cache organization that leverages the on-chip network to create virtual single-cycle paths between distant caches, thus redefining the notion of locality. LOCO is a clustered cache organization, supporting both homogeneous and heterogeneous cluster sizes, and provides near single-cycle accesses to data anywhere within the cluster, just like a private cache. Globally, LOCO dynamically creates a virtual mesh connecting all the clusters, and performs an efficient global data search and migration over this virtual mesh, without having to resort to full-chip broadcasts or perform expensive directory lookups. Trace-driven and full system simulations running SPLASH-2 and PARSEC benchmarks show that LOCO improves application run time by up to 44.5% over baseline private and shared cache.Semiconductor Research CorporationUnited States. Defense Advanced Research Projects Agency (Semiconductor Technology Advanced Research Network

Multicore-Aware Reuse Distance Analysis

This paper presents and validates methods to extend reuse distance analysis of application locality characteristics to shared-memory multicore platforms by accounting for invalidation-based cache-coherence and inter-core cache sharing. Existing reuse distance analysis methods track the number of distinct addresses referenced between reuses of the same address by a given thread, but do not model the effects of data references by other threads. This paper shows several methods to keep reuse stacks consistent so that they account for invalidations and cache sharing, either as references arise in a simulated execution or at synchronization points. These methods are evaluated against a Simics-based coherent cache simulator running several OpenMP and transaction-based benchmarks. The results show that adding multicore-awareness substantially improves the ability of reuse distance analysis to model cache behavior, reducing the error in miss ratio prediction (relative to cache simulation for a specific cache size) by an average of 69% for per-core caches and an average of 84% for shared caches

Characterization and modeling of atomic memory operations in arm based architectures

Efficient fine-grain synchronization is a classic computer architecture challenge that has been profusely addressed in the past. Load Link and Store Conditional (LL/SC) became one of the few solutions to this problem and today it is still part of the State-of-the-art. However, as the core count keeps growing many Instruction Set Architectures (ISA) start to support other synchronization instructions that scale better like Atomic Memory Operations (AMO). In this work we present a characterization of LL/SC and AMO instructions in two current Arm-based server machines. Furthermore, Arm has released its Network-on-Chip (NoC) specification enabling different hardware implementations of how AMO are executed in a multicore. Since the adoption of this new standard is still in its first stages, we have modeled six different AMO policies to explore the hardware design trade offs. We find out that there is no single implementation that outperforms the rest. Therefore, we have designed a hardware solution to dynamically select the best configuration obtaining up to 1.15x speed-ups on relevant benchmarks from the Splash-3 benchmark suite

Exploiting memory allocations in clusterized many-core architectures

Power-efficient architectures have become the most important feature required for future embedded systems. Modern designs, like those released on mobile devices, reveal that clusterization is the way to improve energy efficiency. However, such architectures are still limited by the memory subsystem (i.e., memory latency problems). This work investigates an alternative approach that exploits on-chip data locality to a large extent, through distributed shared memory systems that permit efficient reuse of on-chip mapped data in clusterized many-core architectures. First, this work reviews the current literature on memory allocations and explore the limitations of cluster-based many-core architectures. Then, several memory allocations are introduced and benchmarked scalability, performance and energy-wise, compared to the conventional centralized shared memory solution to reveal which memory allocation is the most appropriate for future mobile architectures. Our results show that distributed shared memory allocations bring performance gains and opportunities to reduce energy consumption

Locality-aware data replication in the Last-Level Cache

Next generation multicores will process massive data with varying degree of locality. Harnessing on-chip data locality to optimize the utilization of cache and network resources is of fundamental importance. We propose a locality-aware selective data replication protocol for the last-level cache (LLC). Our goal is to lower memory access latency and energy by replicating only high locality cache lines in the LLC slice of the requesting core, while simultaneously keeping the off-chip miss rate low. Our approach relies on low overhead yet highly accurate in-hardware run-time classification of data locality at the cache line granularity, and only allows replication for cache lines with high reuse. Furthermore, our classifier captures the LLC pressure at the existing replica locations and adapts its replication decision accordingly. The locality tracking mechanism is decoupled from the sharer tracking structures that cause scalability concerns in traditional coherence protocols. Moreover, the complexity of our protocol is low since no additional coherence states are created. On a set of parallel benchmarks, our protocol reduces the overall energy by 16%, 14%, 13% and 21% and the completion time by 4%, 9%, 6% and 13% when compared to the previously proposed Victim Replication, Adaptive Selective Replication, Reactive-NUCA and Static-NUCA LLC management schemes

Efficient Cache Coherence on Manycore Optical Networks

Ever since industry has turned to parallelism instead of frequency scaling to improve processor performance, multicore processors have continued to scale to larger and larger numbers of cores. Some believe that multicores will have 1000 cores or more by the middle of the next decade. However, their promise of increased performance will only be reached if their inherent scaling challenges are overcome. One such major scaling challenge is the viability of efficient cache coherence with large numbers of cores. Meanwhile, recent advances in nanophotonic device manufacturing are making CMOS-integrated optics a realityâ interconnect technology which can provide significantly more bandwidth at lower power than conventional electrical analogs. The contributions of this paper are two-fold. (1) It presents ATAC, a new manycore architecture that augments an electrical mesh network with an optical network that performs highly efficient broadcasts. (2) It introduces ACKwise, a novel directory-based cache coherence protocol that provides high performance and scalability on any large-scale manycore interconnection net- work with broadcast capability. Performance evaluation studies using analytical models show that (i) a 1024-core ATAC chip using ACKwise achieves a speedup of 3.9Ã compared to a similarly-sized pure electrical mesh manycore with a conventional limited directory protocol; (ii) the ATAC chip with ACKwise achieves a speedup of 1.35Ã compared to the electrical mesh chip with ACKwise; and (iii) a pure electrical mesh chip with ACKwise achieves a speedup of 2.9Ã over the same chip using a conventional limited directory protocol