  The proliferation of commercial cloud computing providers has generated
significant interest in the scientific computing community. Much recent
research has attempted to determine the benefits and drawbacks of cloud
computing for scientific applications. Although clouds have many attractive
features, such as virtualization, on-demand provisioning, and "pay as you go"
usage-based pricing, it is not clear whether they are able to deliver the
performance required for scientific applications at a reasonable price. In this
paper we examine the performance and cost of clouds from the perspective of
scientific workflow applications. We use three characteristic workflows to
compare the performance of a commercial cloud with that of a typical HPC
system, and we analyze the various costs associated with running those
workflows in the cloud. We find that the performance of clouds is not
unreasonable given the hardware resources provided, and that performance
comparable to HPC systems can be achieved given similar resources. We also find
that the cost of running workflows on a commercial cloud can be reduced by
storing data in the cloud rather than transferring it from outside.
