  Cloud computing has become the ubiquitous computing and storage paradigm. It
is also attractive for scientists, because they do not have to care any more
for their own IT infrastructure, but can outsource it to a Cloud Service
Provider of their choice. However, for the case of High-Performance Computing
(HPC) in a cloud, as it is needed in simulations or for Big Data analysis,
things are getting more intricate, because HPC codes must stay highly
efficient, even when executed by many virtual cores (vCPUs). Older clouds or
new standard clouds can fulfil this only under special precautions, which are
given in this article. The results can be extrapolated to other cloud OSes than
OpenStack and to other codes than OpenFOAM, which were used as examples.
