Application of compiler-assisted multiple instruction rollback recovery to speculative execution

Speculative execution is a method to increase instruction level parallelism which can be exploited by both super-scalar and VLIW architectures. The key to a successful general speculation strategy is a repair mechanism to handle mispredicted branches and accurate reporting of exceptions for speculated instructions. Multiple instruction rollback is a technique developed for recovery from transient processor failure. Many of the difficulties encountered during recovery from branch misprediction or from instruction re-execution due to exception in a speculative execution architecture are similar to those encountered during multiple instruction rollback. The applicability of a recently developed compiler-assisted multiple instruction rollback scheme to aid in speculative execution repair is investigated. Extensions to the compiler-assisted scheme to support branch and exception repair are presented along with performance measurements across ten application programs

Low Power Processor Architectures and Contemporary Techniques for Power Optimization – A Review

The technological evolution has increased the number of transistors for a given die area significantly and increased the switching speed from few MHz to GHz range. Such inversely proportional decline in size and boost in performance consequently demands shrinking of supply voltage and effective power dissipation in chips with millions of transistors. This has triggered substantial amount of research in power reduction techniques into almost every aspect of the chip and particularly the processor cores contained in the chip. This paper presents an overview of techniques for achieving the power efficiency mainly at the processor core level but also visits related domains such as buses and memories. There are various processor parameters and features such as supply voltage, clock frequency, cache and pipelining which can be optimized to reduce the power consumption of the processor. This paper discusses various ways in which these parameters can be optimized. Also, emerging power efficient processor architectures are overviewed and research activities are discussed which should help reader identify how these factors in a processor contribute to power consumption. Some of these concepts have been already established whereas others are still active research areas. © 2009 ACADEMY PUBLISHER

Recursos anchos: una técnica de bajo coste para explotar paralelismo agresivo en códigos numéricos

Els bucles son la part que més temps consumeix en les aplicacions numèriques. El rendiment dels bucles està limitat tant pels recursos oferts per l'arquitectura com per les recurrències del bucle en la computació. Per executar més operacions per cicle, els processadors actuals es dissenyen amb graus creixents de replicació de recursos (tècnica de replicació) para ports de memòria i unitats funcionals. En canvi, el gran cost en termes d'àrea i temps de cicle d'aquesta tècnica limita tenir alts graus de replicació: alts valors en temps de cicle contraresten els guanys deguts al decrement en el nombre de cicles, mentre que alts valors en l'àrea requerida poden portar a configuracions impossibles d'implementar. Una alternativa a la replicació de recursos, és fer los més amples (tècnica que anomenem "widening"), i que ha estat usada en alguns dissenys recents. Amb aquesta tècnica, l'amplitud dels recursos s'amplia, fent una mateixa operació sobre múltiples dades. Per altra banda, alguns microprocessadors escalars de propòsit general han estat implementats amb unitats de coma flotants que implementen la instrucció sumar i multiplicar unificada (tècnica de fusió), el que redueix la latència de la operació combinada, tanmateix com el nombre de recursos utilitzats. A aquest treball s'avaluen un ampli conjunt d'alternatives de disseny de processadors VLIW que combinen les tres tècniques. S'efectua una projecció tecnològica de les noves generacions de processadors per predir les possibles alternatives implementables. Com a conclusió, demostrem que tenint en compte el cost, combinar certs graus de replicació i "widening" als recursos hardware és més efectiu que aplicar únicament replicació. Així mateix, confirmem que fer servir unitats que fusionen multiplicació i suma pot tenir un impacte molt significatiu en l'increment de rendiment en futures arquitectures de processadors a un cost molt raonable.Loops are the main time-consuming part of numerical applications. The performance of the loops is limited either by the resources offered by the architecture or by recurrences in the computation. To execute more operations per cycle, current processors are designed with growing degrees of resource replication (replication technique) for memory ports and functional units. However, the high cost in terms of area and cycle time of this technique precludes the use of high degrees of replication. High values for the cycle time may clearly offset any gain in terms of number of execution cycles. High values for the area may lead to an unimplementable configuration. An alternative to resource replication is resource widening (widening technique), which has also been used in some recent designs in which the width of the resources is increased (i.e., a single operation is performed over multiple data). Moreover, several general-purpose superscalar microprocessors have been implemented with multiply-add fused floating point units (fusion technique), which reduces the latency of the combined operation and the number of resources used. On this thesis, we evaluate a broad set of VLIW processor design alternatives that combine the three techniques. We perform a technological projection for the next processor generations in order to foresee the possible implementable alternatives. From this study, we conclude that if the cost is taken into account, combining certain degrees of replication and widening in the hardware resources is more effective than applying only replication. Also, we confirm that multiply-add fused units will have a significant impact in raising the performance of future processor architectures with a reasonable increase in cost

Exploring Processor and Memory Architectures for Multimedia

Multimedia has become one of the cornerstones of our 21st century society and, when combined with mobility, has enabled a tremendous evolution of our society. However, joining these two concepts introduces many technical challenges. These range from having sufficient performance for handling multimedia content to having the battery stamina for acceptable mobile usage. When taking a projection of where we are heading, we see these issues becoming ever more challenging by increased mobility as well as advancements in multimedia content, such as introduction of stereoscopic 3D and augmented reality. The increased performance needs for handling multimedia come not only from an ongoing step-up in resolution going from QVGA (320x240) to Full HD (1920x1080) a 27x increase in less than half a decade. On top of this, there is also codec evolution (MPEG-2 to H.264 AVC) that adds to the computational load increase. To meet these performance challenges there has been processing and memory architecture advances (SIMD, out-of-order superscalarity, multicore processing and heterogeneous multilevel memories) in the mobile domain, in conjunction with ever increasing operating frequencies (200MHz to 2GHz) and on-chip memory sizes (128KB to 2-3MB). At the same time there is an increase in requirements for mobility, placing higher demands on battery-powered systems despite the steady increase in battery capacity (500 to 2000mAh). This leaves negative net result in-terms of battery capacity versus performance advances. In order to make optimal use of these architectural advances and to meet the power limitations in mobile systems, there is a need for taking an overall approach on how to best utilize these systems. The right trade-off between performance and power is crucial. On top of these constraints, the flexibility aspects of the system need to be addressed. All this makes it very important to reach the right architectural balance in the system. The first goal for this thesis is to examine multimedia applications and propose a flexible solution that can meet the architectural requirements in a mobile system. Secondly, propose an automated methodology of optimally mapping multimedia data and instructions to a heterogeneous multilevel memory subsystem. The proposed methodology uses constraint programming for solving a multidimensional optimization problem. Results from this work indicate that using today’s most advanced mobile processor technology together with a multi-level heterogeneous on-chip memory subsystem can meet the performance requirements for handling multimedia. By utilizing the automated optimal memory mapping method presented in this thesis lower total power consumption can be achieved, whilst performance for multimedia applications is improved, by employing enhanced memory management. This is achieved through reduced external accesses and better reuse of memory objects. This automatic method shows high accuracy, up to 90%, for predicting multimedia memory accesses for a given architecture

Exploiting Adaptive Techniques to Improve Processor Energy Efficiency

Rapid device-miniaturization keeps on inducing challenges in building energy efficient microprocessors. As the size of the transistors continuously decreasing, more uncertainties emerge in their operations. On the other hand, integrating more and more transistors on a single chip accentuates the need to lower its supply-voltage. This dissertation investigates one of the primary device uncertainties - timing error, in microprocessor performance bottleneck in NTC era. Then it proposes various innovative techniques to exploit these opportunities to maintain processor energy efficiency, in the context of emerging challenges. Evaluated with the cross-layer methodology, the proposed approaches achieve substantial improvements in processor energy efficiency, compared to other start-of-art techniques