Efficient resources assignment schemes for clustered multithreaded processors

New feature sizes provide larger number of transistors per chip that architects could use in order to further exploit instruction level parallelism. However, these technologies bring also new challenges that complicate conventional monolithic processor designs. On the one hand, exploiting instruction level parallelism is leading us to diminishing returns and therefore exploiting other sources of parallelism like thread level parallelism is needed in order to keep raising performance with a reasonable hardware complexity. On the other hand, clustering architectures have been widely studied in order to reduce the inherent complexity of current monolithic processors. This paper studies the synergies and trade-offs between two concepts, clustering and simultaneous multithreading (SMT), in order to understand the reasons why conventional SMT resource assignment schemes are not so effective in clustered processors. These trade-offs are used to propose a novel resource assignment scheme that gets and average speed up of 17.6% versus Icount improving fairness in 24%.Peer ReviewedPostprint (published version

Introducing runahead threads

Simultaneous Multithreading processors share their resources among multiple threads in order to improve performance. However, a resource control policy is needed to avoid resource conflicts and prevent some threads from monopolizing them. On the contrary, resource conflicts would cause other threads to suffer from resource starvation degrading the overall performance. This situation is especially sensitive for memory bounded threads, because they hold an important amount of resources while long latency accesses are being served. Several fetch policies and resource control techniques have been proposed to overcome these problems by limiting the per-thread resource utilization. Nevertheless, this limitation is harmful for memory bounded threads because it restricts the memory level parallelism available that hides the long latency memory accesses. In this paper, we propose Runahead threads on SMT scenarios as a valuable solution for both exploiting the memory-level parallelism and reducing the resource contention. This approach switches a memory-bounded eager resource thread to a speculative light thread, avoiding critical resource blocking among multiple threads. Furthermore, it improves the thread-level parallelism by removing long-latency memory operations from the instruction window, releasing busy resources. We compare an SMT architecture using Runahead threads (SMTRA) to both state-of-the-art static fetch and dynamic resource control policies. Our results show that the SMTRA combination performs better, in terms of throughput and fairness, than any of the other policies.Postprint (published version

Runahead threads to improve SMT performance

In this paper, we propose Runahead Threads (RaT) as a valuable solution for both reducing resource contention and exploiting memory-level parallelism in Simultaneous Multithreaded (SMT) processors. Our technique converts a resource intensive memory-bound thread to a speculative light thread under long-latency blocking memory operations. These speculative threads prefetch data and instructions with minimal resources, reducing critical resource conflicts between threads. We compare an SMT architecture using RaT to both state-of-the-art static fetch policies and dynamic resource control policies. In terms of throughput and fairness, our results show that RaT performs better than any other policy. The proposed mechanism improves average throughput by 37% regarding previous static fetch policies and by 28% compared to previous dynamic resource scheduling mechanisms. RaT also improves fairness by 36% and 30% respectively. In addition, the proposed mechanism permits register file size reduction of up to 60% in a SMT processor without performance degradation.Peer ReviewedPostprint (published version

DLP+TLP processors for the next generation of media workloads

Future media workloads will require about two levels of magnitude the performance achieved by current general purpose processors. High uni-threaded performance will be needed to accomplish real-time constraints together with huge computational throughput, as next generation of media workloads will be eminently multithreaded (MPEG-4/MPEG-7). In order to fulfil the challenge of providing both good uni-threaded performance and throughput, we propose to join the simultaneous multithreading execution paradigm (SMT) together with the ability to execute media-oriented streaming /spl mu/-SIMD instructions. This paper evaluates the performance of two different aggressive SMT processors: one with conventional /spl mu/-SIMD extensions (such as MMX) and one with longer streaming vector /spl mu/-SIMD extensions. We will show that future media workloads are, in fact, dominated by the scalar performance. The combination of SMT plus streaming vector /spl mu/-SIMD helps alleviate the performance bottleneck of the integer unit. SMT allowsPeer ReviewedPostprint (published version

Dynamically controlled resource allocation in SMT processors

SMT processors increase performance by executing instructions from several threads simultaneously. These threads use the resources of the processor better by sharing them but, at the same time, threads are competing for these resources. The way critical resources are distributed among threads determines the final performance. Currently, processor resources are distributed among threads as determined by the fetch policy that decides which threads enter the processor to compete for resources. However, current fetch policies only use indirect indicators of resource usage in their decision, which can lead to resource monopolization by a single thread or to resource waste when no thread can use them. Both situations can harm performance and happen, for example, after an L2 cache miss. In this paper, we introduce the concept of dynamic resource control in SMT processors. Using this concept, we propose a novel resource allocation policy for SMT processors. This policy directly monitors the usage of resources by each thread and guarantees that all threads get their fair share of the critical shared resources, avoiding monopolization. We also define a mechanism to allow a thread to borrow resources from another thread if that thread does not require them, thereby reducing resource under-use. Simulation results show that our dynamic resource allocation policy outperforms a static resource allocation policy by 8%, on average. It also improves the best dynamic resource-conscious fetch policies like FLUSH++ by 4%, on average, using the harmonic mean as a metric. This indicates that our policy does not obtain the ILP boost by unfairly running high ILP threads over slow memory-bounded threads. Instead, it achieves a better throughput-fairness balance.Peer ReviewedPostprint (published version

Performance Enhancement of Multicore Architecture

Multicore processors integrate several cores on a single chip. The fixed architecture of multicore platforms often fails to accommodate the inherent diverse requirements of different applications. The permanent need to enhance the performance of multicore architecture motivates the development of a dynamic architecture. To address this issue, this paper presents new algorithms for thread selection in fetch stage. Moreover, this paper presents three new fetch stage policies, EACH_LOOP_FETCH, INC-FETCH, and WZ-FETCH, based on Ordinary Least Square (OLS) regression statistic method. These new fetch policies differ on thread selection time which is represented by instructions’ count and window size. Furthermore, the simulation multicore tool, , is adapted to cope with multicore processor dynamic design by adding a dynamic feature in the policy of thread selection in fetch stage. SPLASH2, parallel scientific workloads, has been used to validate the proposed adaptation for multi2sim. Intensive simulated experiments have been conducted and the obtained results show that remarkable performance enhancements have been achieved in terms of execution time and number of instructions per second produces less broadcast operations compared to the typical algorithm

Out-of-Order Retirement of Instructions in Superscalar, Multithreaded, and Multicore Processors

Los procesadores superescalares actuales utilizan un reorder buffer (ROB) para contabilizar las instrucciones en vuelo. El ROB se implementa como una cola FIFO first in first out en la que las instrucciones se insertan en orden de programa después de ser decodificadas, y de la que se extraen también en orden de programa en la etapa commit. El uso de esta estructura proporciona un soporte simple para la especulación, las excepciones precisas y la reclamación de registros. Sin embargo, el hecho de retirar instrucciones en orden puede degradar las prestaciones si una operación de alta latencia está bloqueando la cabecera del ROB. Varias propuestas se han publicado atacando este problema. La mayoría utiliza retirada de instrucciones fuera de orden de forma especulativa, requiriendo almacenar puntos de recuperación (checkpoints) para restaurar un estado válido del procesador ante un fallo de especulación. Normalmente, los checkpoints necesitan implementarse con estructuras hardware costosas, y además requieren un crecimiento de otras estructuras del procesador, lo cual a su vez puede impactar en el tiempo de ciclo de reloj. Este problema afecta a muchos tipos de procesadores actuales, independientemente del número de hilos hardware (threads) y del número de núcleos de cómputo (cores) que incluyan. Esta tesis abarca el estudio de la retirada no especulativa de instrucciones fuera de orden en procesadores superescalares, multithread y multicore.Ubal Tena, R. (2010). Out-of-Order Retirement of Instructions in Superscalar, Multithreaded, and Multicore Processors [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/8535Palanci