Dynamic adaptation of sharing granularity in DSM systems

Itzkovitz, Ayal; Niv, N.; Schuster, Assaf

doi:10.1109/icpp.1999.797407

Cited by 5 publications

(4 citation statements)

References 22 publications

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“…Instead of relaxing the consistency, Itzkovitz and Schster [IS99b,IS99a] also use sequential consistency. They detect pages with false sharing and map the conflictive sections of the same page to different virtual pages at runtime, avoiding the false sharing completely.…”

Section: Runtimementioning

confidence: 99%

Efficient openMP over sequentially consistent distributed shared memory systems

Costa¹

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Nowadays clusters are one of the most used platforms in High Performance Computing and most programmers use the Message Passing Interface (MPI) library to program their applications in these distributed platforms getting their maximum performance, although it is a complex task. On the other side, OpenMP has been established as the de facto standard to program applications on shared memory platforms because it is easy to use and obtains good performance without too much effort. So, could it be possible to join both worlds? Could programmers use the easiness of OpenMP in distributed platforms? A lot of researchers think so. And one of the developed ideas is the distributed shared memory (DSM), a software layer on top of a distributed platform giving an abstract shared memory view to the applications. Even though it seems a good solution it also has some inconveniences. The memory coherence between the nodes in the platform is difficult to maintain (complex management, scalability issues, high overhead and others) and the latency of the remote-memory accesses which can be orders of magnitude greater than on a shared bus due to the interconnection network. Therefore this research improves the performance of OpenMP applications being executed on distributed memory platforms using a DSM with sequential consistency evaluating thoroughly the results from the NAS parallel benchmarks. The vast majority of designed DSMs use a relaxed consistency model because it avoids some major problems in the area. In contrast, we use a sequential consistency model because we think that showing these potential problems that otherwise are hidden may allow the finding of some solutions and, therefore, apply them to both models. The main idea behind this work is that both runtimes, the OpenMP and the DSM layer, should cooperate to achieve good performance, otherwise they interfere one each other trashing the final performance of applications. We develop three different contributions to improve the performance of these applications: (a) a technique to avoid false sharing at runtime, (b) a technique to mimic the MPI behaviour, where produced data is forwarded to their consumers and, finally, (c) a mechanism to avoid the network congestion due to the DSM coherence messages. The NAS Parallel Benchmarks are used to test the contributions. The results of this work shows that the false-sharing problem is a relative problem depending on each application. Another result is the importance to move the data flow outside of the critical path and to use techniques that forwards data as early as possible, similar to MPI, benefits the final application performance. Additionally, this data movement is usually concentrated at single points and affects the application performance due to the limited bandwidth of the network. Therefore it is necessary to provide mechanisms that allows the distribution of this data through the computation time using an otherwise idle network. Finally, results shows that the proposed contributions improve the performance of OpenMP applications on this kind of environments.

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Section: Runtimementioning

confidence: 99%

Efficient openMP over sequentially consistent distributed shared memory systems

Costa¹

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“…Sobel achieves a performance gain between 1.5X and 2X using the inspectorexecutor (prefetch) code transformation as shown in Figure 5. 7 The prefetch opti- However, the performance of the hand optimized and the MPI versions are converging with more than 1024 UPC threads. One interesting observation is that the prefetched optimized version is faster than the UPC hand-optimized one due to double buffering.…”

Section: Applications Performancementioning

confidence: 99%

“…These implementations include a form of software prefetching. However, most of the optimized DSM systems are based on the page fault mechanism with page prefetching and often have poor performance on fine-grained communication [7].…”

Section: Prefetchingmentioning

confidence: 99%

“…Distributed Shared Memory (DSM) systems refer to a wide class of software and hardware implementations, in which each node of a cluster has access to shared memory in addition to each node's non-shared private memory. However, most of the DSM systems rely on the page fault mechanism with page prefetching and often have poor performance on finegrained communication [7]. Partitioned Global Address Space (PGAS) [8,9,10,11,12,13,14,15] languages introduce some complexity through the partitioning of data, due to low performance of the Global Address Space languages and software distributed shared memory systems.…”

Section: Introductionmentioning

confidence: 99%

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Optimization techniques for fine-grained communication in PGAS environments

Alvanos

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Partitioned Global Address Space (PGAS) languages promise to deliver improved programmer productivity and good performance in large-scale parallel machines. However, adequate performance for applications that rely on fine-grained communication without compromising their programmability is difficult to achieve. Manual or compiler assistance code optimization is required to avoid fine-grained accesses. The downside of manually applying code transformations is the increased program complexity and hindering of the programmer productivity. On the other hand, compiler optimizations of fine-grained accesses require knowledge of physical data mapping and the use of parallel loop constructs. This thesis presents optimizations for solving the three main challenges of the fine-grain communication: (i) low network communication efficiency; (ii) large number of runtime calls; and (iii) network hotspot creation for the non-uniform distribution of network communication, To solve this problems, the dissertation presents three approaches. First, it presents an improved inspector-executor transformation to improve the network efficiency through runtime aggregation. Second, it presents incremental optimizations to the inspector-executor loop transformation to automatically remove the runtime calls. Finally, the thesis presents a loop scheduling loop transformation for avoiding network hotspots and the oversubscription of nodes. In contrast to previous work that use static coalescing, prefetching, limited privatization, and caching, the solutions presented in this thesis focus cover all the aspect of fine-grained communication, including reducing the number of calls generated by the compiler and minimizing the overhead of the inspector-executor optimization. A performance evaluation with various microbenchmarks and benchmarks, aiming at predicting scaling and absolute performance numbers of a Power 775 machine, indicates that applications with regular accesses can achieve up to 180% of the performance of hand-optimized versions, while in applications with irregular accesses the transformations are expected to yield from 1.12X up to 6.3X speedup. The loop scheduling shows performance gains from 3-25% for NAS FT and bucket-sort benchmarks, and up to 3.4X speedup for the microbenchmarks.

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Transparent adaptation of sharing granularity in MultiView‐based DSM systems

Niv

Schuster

2001

Softw Pract Exp

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In this paper we propose a mechanism that provides distributed shared memory (DSM) systems with a flexible sharing granularity. The size of the shared memory units is dynamically determined by the system during runtime. This size can range from that of a single variable up to the size of the entire shared memory space. During runtime, the DSM transparently adapts the granularity to the memory access pattern of the application in each phase of its execution. This adaptation, called ComposedView, provides efficient data sharing in software DSM while preserving sequential consistency. Neither complex code analysis nor annotation by the programmer or the compiler are required. Our experiments indicate a substantial performance boost (up to 80% speed‐up improvement) when running a large set of applications using our method, compared to running these benchmark applications with the best fixed granularity. Copyright © 2001 John Wiley & Sons, Ltd.

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Dynamic adaptation of sharing granularity in DSM systems

Cited by 5 publications

References 22 publications

Efficient openMP over sequentially consistent distributed shared memory systems

Efficient openMP over sequentially consistent distributed shared memory systems

Optimization techniques for fine-grained communication in PGAS environments

Transparent adaptation of sharing granularity in MultiView‐based DSM systems

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