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In Situ Visualization Techniques for High Performance Computing

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In Situ Visualization Techniques for High Performance Computing In Situ Visualization Techniques for High Performance Computing James Kress Department of Computer and Information Science, University of Oregon, Eugene Oregon [email protected] Abstract—Scientific visualization for exascale computing is even exabytes, of data over the course of a simulation. very likely to require in situ processing. Traditional simulation This trend poses a problem for the traditional post- checkpointing and post hoc visualization will likely be unsustain- able on future systems at current trends due to the growing gap processing visualization methodology. The traditional visual- between I/O bandwidth and FLOPS. As a result, the majority ization workflow performs visualization as a post-processing of simulation data may be lost if in situ visualization techniques task, where simulation outputs are read from disk, into the are not deployed. In situ visualization in this paradigm will be memory of a parallel tool where analysis and visualization given unprecedented access to simulation output, potentially being are performed. Visualization is generally I/O bound [10], [11], able to process all relevant simulation output at every simulation and as the relative I/O bandwidth continues to decrease, the time step, allowing for very high temporal fidelity compared to challenges of visualizing increasingly larger data will become traditional post hoc visualization. However, this access poses many more problematic. Post hoc visualization is particularly sensi- challenges in terms of data management, resource management tive to the I/O bottleneck, as data is first written to disk by the and sharing, algorithm development and design, and implemen- simulation, and then read back from disk by the visualization tation approaches. Currently, the community primarily relies on two in situ techniques: tightly coupled (on the same resource routine. as the simulation) and loosely coupled (not sharing resources Given this reality, many large-scale simulation codes are with the simulation). Each of these approaches have positive and attempting to bypass the I/O bottleneck by using in situ negative factors which affect their performance under different visualization and analysis, i.e., processing simulation data simulation, resource, and visualization type constraints. Meaning, when it is generated. Broadly speaking, two paradigms have that for every given visualization task, it is not generally known emerged [12]. First, co-processing, or tightly coupled, meth- which method would give the best performance on every data ods, where the simulation and visualization code run in the type, architecture, and set of resource constraints. Due to the lack of research and development on this topic it is still an open same process using the same resources. Second, concurrent- research problem requiring future research. processing, or loosely coupled, methods, where the simulation transfers data over the network to a separate set of visualization nodes for processing. I INTRODUCTION A key question for in situ analysis is whether there is As leading-edge supercomputers get increasingly powerful, a priori knowledge of which visualizations and analyses to scientific simulations running on these machines are gener- produce. If this a priori knowledge exists, then co-processing ating ever larger volumes of data. However, the increasing techniques are an option, and will avoid network and addi- cost of data movement, in particular moving data to disk, tional resource usage. However, it is not guaranteed that the is increasingly limiting the ability to process, analyze, and required visualizations and analyses are known a priori. That fully comprehend simulation results [1], hampering knowl- is, a domain scientist may need to explore the data in an edge extraction. Specifically, while I/O bandwidths regularly interactive fashion, or even produce unanticipated analyses, increase with each new supercomputer, these increases are well lending itself more towards concurrent-processing methods. below corresponding increases in computational ability and Under these constraints, it becomes clear that both types of data generated. Further, this trend is predicted to persist for in situ visualization have a place in the workflows of future the foreseeable future. simulations, but there are challenges for each that need to be Relative decreases in I/O pose a problem for stakeholders addressed. running on these systems ranging from simulation scientists to In situ processing poses many new challenges to both visualization researchers. To that end, the Advanced Scientific simulation and visualization scientists that were hidden or Computing Research (ASCR) Scientific Grand Challenges less predominant with the post-processing paradigm. A few of Workshop Series produced reports spanning eight different the issues facing in situ include: how the in situ routines are scientific domains (High Energy Physics, Climate, Nuclear integrated with the simulation, how data is translated from the Physics, Fusion, Nuclear Energy, Basic Energy Sciences, Biol- simulation representation to the visualization representation, ogy, National Security) [2], [3], [4], [5], [6], [7], [8], [9], that how resources are allocated between the simulation and the vi- explored the computing challenges, including visualization and sualization, how faults are isolated in the visualization routines, analysis challenges, for codes in each of those eight domains. how to massively scale communication heavy visualization Each report mentioned data movement, storage, and analysis algorithms, and even how to do exploratory visualization in as a major obstacle in the move to exascale. Many of these an in situ world. One avenue of approach that could aid in scientific domains will be required to deal with petabytes, or solving these problems, is the ability to model them under third fastest computer in the world [16]. It contains 18,688 varying computational setups and data loads. This modeling nodes, consumes a total of 9 MW of power, and has a peak work is an exciting area of future research for in situ. performance of 27 PF. However, new upgrades are going to In the remainder of this paper, we survey and explore in drastically cut the number of nodes in the system down to just situ visualization itself, key areas involved in in situ workflows, around 3,500 nodes, a total power consumption of 10 MW, and and identify areas where the research is incomplete, or requires a peak performance of 150 PF. This change highlights that the further study. This paper is organized as follows: trends in high challenges of exascale are already here. Moving from a system performance computing and work in scientific visualization that currently has million-way concurrency towards a new and graphics are duscussed in Section II, the state of in situ system which will have billion-way concurrency will require visualization is discussed in Section III, and an overview of a redesign of not only the simulations and codes running on the limited area of in situ performance modeling is presented this system (focusing on parallelizing the underlying algo- in Section IV. rithms) [17], but also in how data is saved and analyzed [18]. Taking it one step further, Table II shows the expected II BACKGROUND characteristics of an actual machine at exascale. This table In this section, we will cover important areas of background focuses on the system performance versus the system I/O, research for in situ visualization and analysis. First, we look at in order to highlight the data challenge. The system peak trends in high performance computing and their implications performance is expected to rise by a factor of 500, yet the I/O for the future of visualization. Next, we explore the tradi- capacity is only expected to rise by a factor of 20. This means tional scientific visualization and compositing pipelines, and that the current problems faced by simulation codes in terms discuss prevalent scientific visualization tools including current of how frequently they can save data are only going to get research in the area of data models, portable performance, and worse. Take, for example, the leading-edge fusion simulation massive scaling. code XGC1 which saves time steps on average every 100 steps [20]. Moving this code to an exascale system without addressing the data problem is going to mean that time steps II-A High Performance Computing will now only be saved every 1,000 to 10,000 steps. This will drastically increase the likelihood that interesting physics will High Performance Computing (HPC) is a landscape of constant be lost between saves. evolution. This evolution is seen in the composition of the HPC systems themselves, as well as the science that they In situ processing can address this with faster analysis enable. By using these systems, scientists have gained deeper of data streams without having to first send data to disk. understandings in fields ranging from medicine to energy to This means that a higher temporal fidelity of data will be physics to even national security. Computers have seen nearly available for analysis, while even potentially enabling the a 50 billion-fold increase in computing power over the last 70 possibility of interactive steering of the simulation through years [13]. Compared to other technologies, this is virtually the visualization [21]. In situ is an enabling technology and an unprecedented leap, enabling more than ever before, but is discussed further in Section III. bringing with it a vast set of challenges. One of those primary challenges is power. The Department II-B Scientific Visualization of Energy has set a nominal power cap for exascale systems at 20 MW per year. This roughly equates to a yearly energy bill of Visualization is an enabling technology that facilitates insight $20 million dollars. However, reaching this goal is not easy. It into data across many domains. It is an essential tool for would be possible to construct an exascale system today using confirming and communicating trends in data to both the conventional hardware and components, but DARPA estimated domain scientists as well as the general public [22].
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