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Heterogeneous computing come with tremendous potential and is a leading candidate for scientific applications that are becoming more and more complex. Accelerators such as GPUs whose computing momentum is growing faster than ever offer application performance when compute intensive portions of an application are offloaded to them. It is quite evident that future computing architectures are moving towards hybrid systems consisting of multi-GPUs and multicore CPUs. A variety of high-level languages and software tools can simplify programming these systems. Directive-based programming models are being embraced since they not only ease programming complex systems but also abstract low-level details from the programmer. We already know that OpenMP has been making programming CPUs easy and portable. Similarly, a directive-based programming model for accelerators is OpenACC that is gaining popularity since the directives play an important role in developing portable software for GPUs. A combination of OpenMP and OpenACC, a hybrid model, is a plausible solution to port scientific applications to heterogeneous architectures especially when there is more than one GPU on a single node to port an application to. However OpenACC meant for accelerators is yet to provide support for multi-GPUs. But using OpenMP we could conveniently exploit features such as for and section to distribute compute intensive kernels to more than one GPU. We demonstrate the effectiveness of this hybrid approach with some case studies in this paper.

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Accelerators offer the potential to significantly improve the performance of scientific applications when offloading compute intensive portions of programs to the accelerators. However, effectively tapping their full potential is difficult owing to the programmability challenges faced by the users when mapping computation algorithms to the massively parallel architectures such as GPUs.Directive-based programming models offer programmers an option to rapidly create prototype applications by annotating region of code for offloading with hints to the compiler. This is critical to improve the productivity in the production code. In this paper, we study the effectiveness of a high-level directivebased programming model, OpenACC, for parallelizing a seismic migration application called Kirchhoff Migration on GPU architecture. Kirchhoff Migration is a real-world production code in the Oil & Gas industry. Because of its compute intensive property, we focus on the computation part and explore different mechanisms to effectively harness GPU's computation capabilities and memory hierarchy. We also analyze different loop transformation techniques in different OpenACC compilers and compare their performance differences. Compared to one socket (10 CPU cores) on the experimental platform, one GPU achieved a maximum speedup of 20.54x and 6.72x for interpolation and extrapolation kernel functions.

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Existing studies show that using single GPU can lead to obtaining significant performance gains. We should be able to achieve further performance speedup if we use more than one GPU. Heterogeneous processors consisting of multiple CPUs and GPUs offer immense potential and are often considered as a leading candidate for porting complex scientific applications. Unfortunately programming heterogeneous systems requires more effort than what is required for traditional multicore systems. Directive-based programming approaches are being widely adopted since they make it easy to use/port/maintain application code. OpenMP and OpenACC are two popular models used to port applications to accelerators. However, neither of the models provides support for multiple GPUs. A plausible solution is to use combination of OpenMP and OpenACC that forms a hybrid model; however, building this model has its own limitations due to lack of necessary compilers’ support. Moreover, the model also lacks support for direct device-to-device communication. To overcome these limitations, an alternate strategy is to extend OpenACC by proposing and developing extensions that follow a task-based implementation for supporting multiple GPUs. We critically analyze the applicability of the hybrid model approach and evaluate the proposed strategy using several case studies and demonstrate their effectiveness.

A dramatic improvement in energy efficiency is mandatory for sustainable supercomputing and has been identified as a major challenge. Affordable energy solution continues to be of great concern in the development of the next generation of supercomputers. Low power processors, dynamic control of processor frequency and heterogeneous systems are being proposed to mitigate energy costs. However, the entire software stack must be re-examined with respect to its ability to improve efficiency in terms of energy as well as performance.In order to address this need, a better understanding of the energy behavior of applications is essential. In this paper we explore the energy efficiency of some common kernels used in high performance computing on a multi-GPU platform, and compare our results with multicore CPUs. We implement these kernels using optimized libraries like FFTW, CUBLAS and MKL. Our experiments demonstrate a relationship between energy consumption and computation-communication factors of certain application kernels. In general, we observe that the correlation of energy consumption to GPU global memory accesses is 0.73 and power consumption to operations per unit time is 0.84, signifying a strong positive relationship between them. We believe that our results will assist the HPC community in understanding the power/energy behavior of scientific kernels on multi-GPU platforms.