Performance Evaluation of Specialized Hardware for Fast Global Operations on Distributed Memory Multicomputers

Workstation cluster multicomputers are increasingly being applied for solving scientific problems that require massive computing power. Parallel Virtual Machine (PVM) is a popular message-passing model used to program these clusters. One of the major performance limiting factors for cluster multicomputers is their inefficiency in performing parallel program operations involving collective communications. These operations include synchronization, global reduction, broadcast/multicast operations and orderly access to shared global variables. Hall has demonstrated that a .secondary network with wide tree topology and centralized coordination processors (COP) could improve the performance of global operations on a variety of distributed architectures [Hall94a]. My hypothesis was that the efficiency of many PVM applications on workstation clusters could be significantly improved by utilizing a COP system for collective communication operations. To test my hypothesis, I interfaced COP system with PVM. The interface software includes a virtual memory-mapped secondary network interface driver, and a function library which allows to use COP system in place of PVM function calls in application programs. My implementation makes it possible to easily port any existing PVM applications to perform fast global operations using the COP system. To evaluate the performance improvements of using a COP system, I measured cost of various PVM global functions, derived the cost of equivalent COP library global functions, and compared the results. To analyze the cost of global operations on overall execution time of applications, I instrumented a complex molecular dynamics PVM application and performed measurements. The measurements were performed for a sample cluster size of 5 and for message sizes up to 16 kilobytes. The comparison of PVM and COP system global operation performance clearly demonstrates that the COP system can speed up a variety of global operations involving small-to-medium sized messages by factors of 5-25. Analysis of the example application for a sample cluster size of 5 show that speedup provided by my global function libraries and the COP system reduces overall execution time for this and similar applications by above 1.5 times. Additionally, the performance improvement seen by applications increases as the cluster size increases, thus providing a scalable solution for performing global operations

Exploring the value of supporting multiple DSM protocols in Hardware DSM Controllers

Journal ArticleThe performance of a hardware distributed shared memory (DSM) system is largely dependent on its architect's ability to reduce the number of remote memory misses that occur. Previous attempts to solve this problem have included measures such as supporting both the CC-NUMA and S-COMA architectures is the same machine and providing a programmable DSM controller that can emulate any DSM mechanism. In this paper we first present the design of a DSM controller that supports multiple DSM protocols in custom hardware, and allows the programmer or compiler to specify on a per-variable basis what protocol to use to keep that variable coherent. This simulated performance of this DSM controller compares favorably with that of conventional single-protocol custom hardware designs, often outperforming the conventional systems by a factor of two. To achieve these promising results, that multi-protocol DSM controller needed to support only two DSM architectures (CC-NUMA and S-COMA) and three coherency protocols (both release and sequentially consistent write invalidate and release consistent write update). This work demonstrates the value of supporting a degree of flexibility in one's DSM controller design and suggests what operations such a flexible DSM controller should support

Cluster Computing Review

In the past decade there has been a dramatic shift from mainframe or ‘host−centric’ computing to a distributed ‘client−server’ approach. In the next few years this trend is likely to continue with further shifts towards ‘network−centric’ computing becoming apparent. All these trends were set in motion by the invention of the mass−reproducible microprocessor by Ted Hoff of Intel some twenty−odd years ago. The present generation of RISC microprocessors are now more than a match for mainframes in terms of cost and performance. The long−foreseen day when collections of RISC microprocessors assembled together as a parallel computer could out perform the vector supercomputers has finally arrived. Such high−performance parallel computers incorporate proprietary interconnection networks allowing low−latency, high bandwidth inter−processor communications. However, for certain types of applications such interconnect optimization is unnecessary and conventional LAN technology is sufficient. This has led to the realization that clusters of high−performance workstations can be realistically used for a variety of applications either to replace mainframes, vector supercomputers and parallel computers or to better manage already installed collections of workstations. Whilst it is clear that ‘cluster computers’ have limitations, many institutions and companies are exploring this option. Software to manage such clusters is at an early stage of development and this report reviews the current state−of−the−art. Cluster computing is a rapidly maturing technology that seems certain to play an important part in the ‘network−centric’ computing future

Software vs. Hardware Shared Memory Implementation: A Case Study

We compare the performance of software-supported shared memory on a general-purpose network to hardware-supported shared memory on a dedicated interconnect. Up to eight processors, our results are based on the execution of a set of application programs on a SGI 4D/480 multiprocessor and on TreadMarks, a distributed shared memory system that runs on a Fore ATM LAN of DECstation-5000/240s. Since the DEC-station and the 4D/480 use the same processor, primary cache, and compiler, the shared-memory implementation is the principal difference between the systems. Our results show that TreadMarks performs comparably to the 4D/480 for applications with moderate amounts of synchronization, but the difference in performance grows as the synchronization frequency increases. For applications that require a large amount of memory bandwidth, TreadMarks can perform better than the SGI 4D/480. Beyond eight processors, our results are based on execution-driven simulation. Specifically, we compare a software implementation on a general-purpose network of uniprocessor nodes, a hardware implementation using a directory-based protocol on a dedicated interconnect, and a combined implementation using software to provide shared memory between multiprocessor nodes with hardware implementing shared memory within a node. For the modest size of the problems that we can simulate, the hardware implementation scales well and the software implementation scales poorly. The combined approach delivers performance close to that of the hardware implementation for applications with small to moderate synchronization rates and good locality. Reductions in communication overhead improve the performance of the software and the combined approach, but synchronization remains a bottleneck

Hermes: DSM por software con granularidad fina

Los sistemas de DSM por software sobre clusters de workstations constituyen una alternativa interesante para el procesamiento paralelo, gracias a su gran potencial para la escalabilidad y excelente relaci on costo/performance. Al encarar el diseño de un sistema de este tipo, los aspectos fundamentales a considerar son: que sea simple y eficiente, que pueda utilizarse sobre una plataforma est andar sin requerimientos onerosos de hardware, que minimice el efecto negativo de false sharing proveniente de la gruesa granularidad de consistencia que acarrea el uso del mecanismo de protecci on de memoria virtual, y que evite las latencias prohibitivas asociadas al tr afico de mensajes sobre protocolos de red est andares durante las etapas de sincronizaci on. En este trabajo presentamos Hermes, un sistema de DSM por software que resuelve la problem atica citada con un enfoque simple y totalmente novedoso, al evitar el costo de mantener un orden parcial de las referencias {con ciertos modelos de consistencia relajados{. Hermes provee adem as control de granularidad fina de complejidad y sobrecarga m nima, que potencia su escalabilidad y brinda a su vez una alta flexibilidad para utilizar el modelo de consistencia que resulte m as apropiado. En lo que respecta a la comunicaci on, proponemos recurrir al uso de interfaces de red mapeadas a memoria virtual.Eje: Procesamiento distribuido y paralelo (PDP)Red de Universidades con Carreras en Informática (RedUNCI