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Section: Scientific Foundations

Keywords : numerical simulation, computational steering, scientific visualization, interaction, coupling.

Computational steering for parallel and distributed numerical simulations

Thanks to the constant evolution of computational capacity, numerical simulations are becoming more and more complex; it is not uncommon to couple different models in different distributed codes running on supercomputer or heterogeneous grids. Nowadays, such simulations are typically running in batch mode, and the analysis of the results is then performed on a local workstation as a post-processing step, which implies to preliminary collect all the simulation output files. In most research fields, the post-processing step is realized thanks to 3D scientific visualization techniques. In the batch approach, there is a lack of control over the in-progress computations, and it might drastically decrease the profitability of the computational resources (repeated tests with different input files separated by excessively long waiting periods).

For years, the scientific computing community has expressed the need of new computational steering tools. The computational steering is an alternative approach to the typical simulation work-flow of performing computation and visualization sequentially. It mainly consists in coupling a remote simulation with a graphics system through the network in order to provide scientists with on-line visualization and interactive steering. On-line visualization appears very useful to monitor the evolution of the simulation by rendering the current results. It also allows us to validate the simulation codes and to detect conceptual or programming errors before the completion of a long-running application. Interactive steering allows the researcher to change the parameters of the simulation without stopping it. As the on-line visualization provides an immediate visual feedback on the effect of a parameter change, the scientist gains additional insight in the simulation, regarding to the cause-effect relationship. Such a tool might help the scientist to better grasp the complexity of the underlying models and to drive more rapidly the simulation into the right direction . Basically, a computational steering environment can be defined as a communication infrastructure, coupling a simulation with a remote user interface, called steering system. This interface usually provides on-line visualization and user interaction.

Over the last decade, many steering environments have been developed. They distinguish themselves by their simulation integration process. A first solution is the problem solving environment (PSE) approach, like in SCIRun. This approach allows the scientist to construct a steering application according to a visual programming model. As an opposite, the majority of the steering environments, such as the well-known CUMULVS, are based on the annotation of the application source-code. This latter approach allows fine grain steering functionalities and can achieve better run-time performances.

Even though most of existing computational steering environments, such as CUMULVS, DAQV or gViz support parallel simulations, they are limited to sequential visualization systems. This leads to an important bottleneck and increased rendering time. In the gViz project, the IRIS Explorer visualization system has been extended to run the different modules (simulation, visualization, rendering) in a distributed fashion on the Grid, but the visualization and the rendering modules are still sequential. Recent works in the Uintah PSE (Problem Solving Environment) has addressed the problem of massively parallel computation connected to a remote parallel visualization module, but this latter module is only running on a shared-memory machine. Therefore, it would be particularly valuable for the scientist if a steering environment would be able to perform parallel visualization using a PC-based graphics cluster.

In the EPSN project, we intend to explore this latter purpose, called M× N computational steering . More precisely, the EPSN environment (Environment for the Steering of Parallel Numerical Simulations, see section  5.2 ) enables to interconnect parallel simulations with visualization systems, that can be parallel as well. In other words, we want to provide a framework that can benefit from immersive virtual reality technology (e.g. tiled display wall) and that might help scientists to better grasp the complexity of real-life simulations. Such a coupling between parallel numerical simulations and parallel visualization systems raises crucial issues we investigate in the EPSN project:

Parallel coordination of steering operations.

The on-line visualization and the computational steering of parallel simulations come up against a serious coherence problem. Indeed, data distributed over parallel processes must be accessed carefully to ensure they are presented to the visualization system in a meaningful way. To overcome this problem, we introduce a Hierarchical Task Model (HTM) that represents the control-flow of the simulation, too often considered in other approaches as a ``single-loop'' program. Thanks to this representation, we schedule in parallel the user interaction on the simulation processes and satisfy the temporal coherence.

Parallel data redistribution.

The efficient redistribution of data is a crucial issue to achieve an efficient coupling between simulation and visualization codes. Most of the previous works have limited their studies to structured data with regular distributions (e.g dense arrays with simple block-cyclic distributions). However, computational steering applications (or multi-physics simulations) frequently handle more irregular data, like particle sets, unstructured meshes or hierarchical grids. In such a context, the data transfer involves to switch from one distribution to another, and thus to ``redistribute'' the data. For this, we introduce a description model of distributed data, based on the notion of ``complex object''. Thanks to this model, we propose redistribution algorithms that generate symbolic messages, independently from a particular communication layer. We distinguish two main approaches: the spatial approach and the placement approach.


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