Heterogeneous cluster architectures and applications

Why such a topic for a special issue? Which is the reason for fostering recent work on cluster architectures that do not fit into the widely established categories? Are unconventional applications especially interesting so that they are worth a special issue? Do these disruptive pieces of work deserve such a focus in a journal? From the point of view of the guest editors of this special issue, the progress of technology can be seen as a two-step process. One of the steps is performing research in order to achieve a better version of the designs we are currently enjoying. In this regard, for instance, when an interconnection network is enhanced so that its bandwidth is doubled, datacenters can deliver a new level of performance to their customers. In a similar way, new generations of processors help to significantly reduce energy consumption of the servers used in such datacenter. Also, installing new versions of accelerators in such servers allows applications to experience important reductions in their execution time at the time that the overall power consumption of the facility is not impacted. All these new developments require an enormous amount of research. For example, faster interconnects require to glue together research on microelectronics, physics, communication protocols, encoding schemes, and thermal issues, to name only a few. The creation of new generations of processors and accelerators also involves large amounts of research from a variety of areas, thus making that a big team of researchers is required in order to bring all those ideas to market. Nevertheless, if those developments are observed from a very broad and long-term perspective, they could be thought to be incremental, despite of their clear significance and despite that they enable new levels of performance. Hence, which should be the non-incremental developments? Our understanding of this fundamental problem is that the non-incremental developments would be those based on ideas that could cause surprise to those researchers initially listening to them. For instance, the very first time that several computers were interconnected in order to create a cluster of computers that cooperatively work together in order to solve a problem, at that point in time an unconventional idea appeared. From a long-term perspective, all the later developments around clusters are incremental; despite those developments are the result of many hours of work and huge amounts of effort from very smart researchers that probably devoted their lives to create those improved versions of the technology behind that initial idea of putting together a few computers into a cluster so that they collaborate in order to solve a problem. One more example of these non-incremental developments could be the use of Graphics Processing Unit (GPU) or any other accelerator in general, in order to reduce the execution time of applications. In this regard, new versions of these accelerators are required so that technology makes progress. Furthermore, these new versions of these accelerators are the result of big investments in time as well as funding resources, which involve hundreds of trained researchers. However, the qualitative difference was performed when someone had the initial idea of attaching an accelerator to the system. That was the point in time that really impacted technology and made it evolve because that allowed a performance increase of several orders of magnitude for the datacenters. Hence, all these non-incremental developments are the other step of the two-step process described previously regarding the progress of technology. Actually, in this two-step process, these non-incremental ideas would be the first step, and the second step would be composed of all those