The use of Graphics Processing Units (GPUs) for high-performance computing has gained growing momentum in recent years. Unfortunately, GPU-programming platforms like Compute Unified Device Architecture (CUDA) are complex, user unfriendly, and increase the complexity of developing high-performance parallel applications. In addition, runtime systems that execute those applications often fail to fully utilize the parallelism of modern CPU-GPU systems. Typically, parallel kernels run entirely on the most powerful device available, leaving other devices idle. These observations sparked research in two directions: (1) high-level approaches to software development for GPUs, which strike a balance between performance and ease of programming; and (2) task partitioning to fully utilize the available devices. In this paper, we propose a framework, called PSkel, that provides a single high-level abstraction for stencil programming on heterogeneous CPU-GPU systems, while allowing the programmer to partition and assign data and computation to both CPU and GPU. Our current implementation uses parallel skeletons to transparently leverage Intel Threading Building Blocks (Intel Corporation, Santa Clara, CA, USA) and NVIDIA CUDA (Nvidia Corporation, Santa Clara, CA, USA). In our experiments, we observed that parallel applications with task partitioning can improve average performance by up to 76% and 28% compared with CPU-only and GPU-only parallel applications, respectively.A common approach to address the CPU-GPU programming complexity is the use of algorithmic skeletons. Parallel skeletons model and abstract common parallel programming patterns (computation and coordination phases), thereby enabling the programmer to focus on algorithm design, rather than on runtime system details. Among existing parallel skeletons, the stencil pattern is critical in many scientific computing domains, including image and signal processing and computational fluid dynamics [3,4]. The large body of recent work targeting GPU implementations of high-performance stencil computations stresses the importance of that pattern [5][6][7][8].Another important aspect of CPU-GPU platforms is that their runtime systems generally fail to exploit the platform's full potential for parallel processing. Specifically, the runtime systems do not partition the work (computations and data) of parallel applications across CPUs and GPUs to increase their utilization. For that reason, many existing frameworks have runtime systems that enable either static or dynamic task partitioning [5,[9][10][11][12][13]. However, those frameworks either fail to provide high-level abstractions, support only multi-GPU systems, or do not partition tasks to both CPU and GPU simultaneously. The aforementioned observations prompt for systems that can both exploit task partitioning efficiently and provide high-level abstractions for CPU-GPU programming.In this paper, we propose and evaluate PSkel (Parallel Skeletons), a framework for stencil programming in heterogeneous CPU-GPU systems. PSkel ...

Summary The stencil pattern is important in many scientific and engineering domains, spurring great interest from researchers and industry. In recent years, various optimizations have been proposed for parallel stencil applications running on graphics processing units (GPUs). In particular, tiling is a technique that can significantly enhance application performance by improving data locality and by reducing the volume of communication between host memory and GPU. In addition, tiling enables stencil applications to process inputs that are larger than the physical GPU memory. However, implementing tiling efficiently is complex, time‐consuming, and error‐prone. In this paper, we propose transparently optimized automatic stencil tiling (TOAST), an automatic tiling mechanism for iterative stencil computations running on GPUs; TOAST has 3 main benefits: (1) It incorporates an optimization model that seeks to maximize data reuse within tiles while respecting the amount of dynamically available GPU memory; (2) it offers a virtualized GPU memory for stencil computations, allowing for large input data; and (3) it performs optimal tiling transparently to the developer of the parallel stencil application. The current implementation of TOAST augments the PSkel framework with an internal solver based on genetic algorithms. Our experimental results show that TOAST improves the performance of iterative stencil applications by up to 13 ×  compared with their multithreaded (central processing unit–based) optimized versions and up to 48 ×  compared with a naive tiling approach on GPU. The TOAST mechanism is able to automatically achieve a low percentual overhead of data management compared with actual stencil computation.

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