HPC systems have seen impressive growth in terms of performance over a period of many years. Soon the next milestone is expected to be reached with the deployment of exascale systems in 2021. In this paper, we provide an overview of the exascale challenges from a computer architecture's perspective and explore technological and other constraints. The analysis of upcoming architectural options and emerging technologies allow for setting expectations for application developers, which will have to

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... cope with heterogeneous architectures, increasingly diverse compute technologies as well as deeper memory and storage hierarchies. Finally, needs resulting from changing science and engineering workflows will be discussed, which need to be addressed by making HPC systems available as part of more open e-infrastructures that provide also other compute and storage services.