Contention elimination by replication of sequential sections in distributed shared memory programs

In shared memory programs contention often occurs at the transition between a sequential and a parallel section of the code. As all threads start executing the parallel section, they often access data just modified by the thread that executed the sequential section, causing a flurry of data requests to converge on that processor.We address this problem in a software distributed shared memory system by replicating the execution of the sequential sections on all processors. Communication during this replicated sequential execution is reduced by using multicast.We have implemented replicated sequential execution with multicast support in OpenMP/NOW, a version of of OpenMP that runs on networks of workstations. We do not rely on compile-time data analysis, and therefore we can handle irregular and pointer-based applications. We show significant improvement for two pointer-based applications that suffer from severe contention without replicated sequential execution

Asynchronous Validity Resolution in Sequentially Consistent Shared Virtual Memory

Shared Virtual Memory (SVM) is an effort to provide a mechanism for a distributed system, such as a cluster, to execute shared memory parallel programs. Unfortunately, SVM has performance problems due to its underlying distributed architecture. Recent developments have increased performance of SVM by reducing communication. Unfortunately this performance gain was only possible by increasing programming complexity and by restricting the types of programs allowed to execute in the system. Validity resolution is the process of resolving the validity of a memory object such as a page. Current SVM systems use synchronous or deferred validity resolution techniques in which user processing is blocked during the validity resolution process. This is the case even when resolving validity of false shared variables. False-sharing occurs when two or more processes access unrelated variables stored within the same shared block of memory and at least one of the processes is writing. False sharing unnecessarily reduces overall performance of SVM systems?because user processing is blocked during validity resolution although no actual data dependencies exist. This thesis presents Asynchronous Validity Resolution (AVR), a new approach to SVM which reduces the performance losses associated with false sharing while maintaining the ease of programming found with regular shared memory parallel programming methodology. Asynchronous validity resolution allows concurrent user process execution and data validity resolution. AVR is evaluated by com-paring performance of an application suite using both an AVR sequentially con-sistent SVM system and a traditional sequentially consistent (SC) SVM system. The results show that AVR can increase performance over traditional sequentially consistent SVM for programs which exhibit false sharing. Although AVR outperforms regular SC by as much as 26%, performance of AVR is dependent on the number of false-sharing vs. true-sharing accesses, the number of pages in the program’s working set, the amount of user computation that completes per page request, and the internodal round-trip message time in the system. Overall, the results show that AVR could be an important member of the arsenal of tools available to parallel programmers

Using Multicast and Multithreading to Reduce Communication in Software DSM Systems

This paper examines the performance benefits of employing multicast communication and application-level multithreading in the Brazos software distributed shared memory (DSM) system. Application-level multithreading in Brazos allows programs to transparently take advantage of available local multiprocessing. Brazos uses multicast communication to reduce the number of consistency-related messages, and employs two adaptive mechanisms that reduce the detrimental side effects of using multicast communication. We compare three software DSM systems running on identical hardware: (1) a single-threaded point-to-point system, (2) a multithreaded point-to-point system, and (3) Brazos, which incorporates both multithreading and multicast communication. For the six applications studied, multicast and multithreading improve speedup on eight processors by an average of 38%

Efficient Home-Based protocols for reducing asynchronous communication in shared virtual memory systems

En la presente tesis se realiza una evaluación exhaustiva de ls Sistemas de Memoria Distribuida conocidos como Sistemas de Memoria Virtual Compartida. Este tipo de sistemas posee características que los hacen especialmente atractivos, como son su relativo bajo costo, alta portabilidad y paradigma de progración de memoria compartida. La evaluación consta de dos partes. En la primera se detallan las bases de diseño y el estado del arte de la investigación sobre este tipo de sistemas. En la segunda, se estudia el comportamiento de un conjunto representativo de cargas paralelas respecto a tres ejes de caracterización estrechamente relacionados con las prestaciones en estos sistemas. Mientras que la primera parte apunta la hipótesis de que la comunicación asíncrona es una de las principales causas de pérdida de prestaciones en los Sistemas de Memoria Virtual Compartida, la segunda no sólo la confirma, sino que ofrece un detallado análisis de las cargas del que se obteiene información sobre la potencial comunicación asíncrona atendiendo a diferentes parámetros del sistema. El resultado de la evaluación se utiliza para proponer dos nuevos protocolos para el funcionamiento de estos sistemas que utiliza un mínimo de recursos de hardware, alcanzando prestaciones similares e incluso superiores en algunos casos a sistemas que utilizan circuitos hardware de propósito específico para reducir la comunicación asíncrona. En particular, uno de los protocolos propuestos es comparado con una reconocida técnica hardware para reducir la comunicación asíncrona, obteniendo resultados satisfactorios y complementarios a la técnica comparada. Todos los modelos y técnicas usados en este trabajo han sido implementados y evalados utilizando un nuevo entorno de simulación desarollado en el contexto de este trabajo.Petit Martí, SV. (2003). Efficient Home-Based protocols for reducing asynchronous communication in shared virtual memory systems [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/2908Palanci