Compiler optimization to improve data locality for processor multithreading

Over the last decade processor speed has increased dramatically, whereas the speed of the memory subsystem improved at a modest rate. Due to the increase in the cache miss latency (in terms of the processor cycle), processors stall on cache misses for a significant portion of its execution time. Multithreaded processors has been proposed in the literature to reduce the processor stall time due to cache misses. Although multithreading improves processor utilization, it may also increase cache miss rates, because in a multithreaded processor multiple threads share the same cache, which effectively reduces the cache size available to each individual thread. Increased processor utilization and the increase in the cache miss rate demands higher memory bandwidth. A novel compiler optimization method has been presented in this paper that improves data locality for each of the threads and enhances data sharing among the threads. The method is based on loop transformation theory and optimizes both spatial and temporal data locality. The created threads exhibit high level of intra-thread and inter-thread data locality which effectively reduces both the data cache miss rates and the total execution time of numerically intensive computation running on a multithreaded processor

Hierarchical multithreading: programming model and system software

This paper addresses the underlying sources of performance degradation (e.g. latency, overhead, and starvation) and the difficulties of programmer productivity (e.g. explicit locality management and scheduling, performance tuning, fragmented memory, and synchronous global barriers) to dramatically enhance the broad effectiveness of parallel processing for high end computing. We are developing a hierarchical threaded virtual machine (HTVM) that defines a dynamic, multithreaded execution model and programming model, providing an architecture abstraction for HEC system software and tools development. We are working on a prototype language, LITL-X (pronounced "little-X") for latency intrinsic-tolerant language, which provides the application programmers with a powerful set of semantic constructs to organize parallel computations in a way that hides/manages latency and limits the effects of overhead. This is quite different from locality management, although the intent of both strategies is to minimize the effect of latency on the efficiency of computation. We work on a dynamic compilation and runtime model to achieve efficient LITL-X program execution. Several adaptive optimizations were studied. A methodology of incorporating domain-specific knowledge in program optimization was studied. Finally, we plan to implement our method in an experimental testbed for a HEC architecture and perform a qualitative and quantitative evaluation on selected applications

Distributed-Memory Breadth-First Search on Massive Graphs

This chapter studies the problem of traversing large graphs using the breadth-first search order on distributed-memory supercomputers. We consider both the traditional level-synchronous top-down algorithm as well as the recently discovered direction optimizing algorithm. We analyze the performance and scalability trade-offs in using different local data structures such as CSR and DCSC, enabling in-node multithreading, and graph decompositions such as 1D and 2D decomposition.Comment: arXiv admin note: text overlap with arXiv:1104.451

Mitosis based speculative multithreaded architectures

In the last decade, industry made a right-hand turn and shifted towards multi-core processor designs, also known as Chip-Multi-Processors (CMPs), in order to provide further performance improvements under a reasonable power budget, design complexity, and validation cost. Over the years, several processor vendors have come out with multi-core chips in their product lines and they have become mainstream, with the number of cores increasing in each processor generation. Multi-core processors improve the performance of applications by exploiting Thread Level Parallelism (TLP) while the Instruction Level Parallelism (ILP) exploited by each individual core is limited. These architectures are very efficient when multiple threads are available for execution. However, single-thread sections of code (single-thread applications and serial sections of parallel applications) pose important constraints on the benefits achieved by parallel execution, as pointed out by Amdahl’s law. Parallel programming, even with the help of recently proposed techniques like transactional memory, has proven to be a very challenging task. On the other hand, automatically partitioning applications into threads may be a straightforward task in regular applications, but becomes much harder for irregular programs, where compilers usually fail to discover sufficient TLP. In this scenario, two main directions have been followed in the research community to take benefit of multi-core platforms: Speculative Multithreading (SpMT) and Non-Speculative Clustered architectures. The former splits a sequential application into speculative threads, while the later partitions the instructions among the cores based on data-dependences but avoid large degree of speculation. Despite the large amount of research on both these approaches, the proposed techniques so far have shown marginal performance improvements. In this thesis we propose novel schemes to speed-up sequential or lightly threaded applications in multi-core processors that effectively address the main unresolved challenges of previous approaches. In particular, we propose a SpMT architecture, called Mitosis, that leverages a powerful software value prediction technique to manage inter-thread dependences, based on pre-computation slices (p-slices). Thanks to the accuracy and low cost of this technique, Mitosis is able to effectively parallelize applications even in the presence of frequent dependences among threads. We also propose a novel architecture, called Anaphase, that combines the best of SpMT schemes and clustered architectures. Anaphase effectively exploits ILP, TLP and Memory Level Parallelism (MLP), thanks to its unique finegrain thread decomposition algorithm that adapts to the available parallelism in the application

Mitosis based speculative multithreaded architectures

In the last decade, industry made a right-hand turn and shifted towards multi-core processor designs, also known as Chip-Multi-Processors (CMPs), in order to provide further performance improvements under a reasonable power budget, design complexity, and validation cost. Over the years, several processor vendors have come out with multi-core chips in their product lines and they have become mainstream, with the number of cores increasing in each processor generation. Multi-core processors improve the performance of applications by exploiting Thread Level Parallelism (TLP) while the Instruction Level Parallelism (ILP) exploited by each individual core is limited. These architectures are very efficient when multiple threads are available for execution. However, single-thread sections of code (single-thread applications and serial sections of parallel applications) pose important constraints on the benefits achieved by parallel execution, as pointed out by Amdahl’s law. Parallel programming, even with the help of recently proposed techniques like transactional memory, has proven to be a very challenging task. On the other hand, automatically partitioning applications into threads may be a straightforward task in regular applications, but becomes much harder for irregular programs, where compilers usually fail to discover sufficient TLP. In this scenario, two main directions have been followed in the research community to take benefit of multi-core platforms: Speculative Multithreading (SpMT) and Non-Speculative Clustered architectures. The former splits a sequential application into speculative threads, while the later partitions the instructions among the cores based on data-dependences but avoid large degree of speculation. Despite the large amount of research on both these approaches, the proposed techniques so far have shown marginal performance improvements. In this thesis we propose novel schemes to speed-up sequential or lightly threaded applications in multi-core processors that effectively address the main unresolved challenges of previous approaches. In particular, we propose a SpMT architecture, called Mitosis, that leverages a powerful software value prediction technique to manage inter-thread dependences, based on pre-computation slices (p-slices). Thanks to the accuracy and low cost of this technique, Mitosis is able to effectively parallelize applications even in the presence of frequent dependences among threads. We also propose a novel architecture, called Anaphase, that combines the best of SpMT schemes and clustered architectures. Anaphase effectively exploits ILP, TLP and Memory Level Parallelism (MLP), thanks to its unique finegrain thread decomposition algorithm that adapts to the available parallelism in the application.Postprint (published version