  Resource scheduling in infrastructure as a service (IaaS) is one of the keys
for large-scale Cloud applications. Extensive research on all issues in real
environment is extremely difficult because it requires developers to consider
network infrastructure and the environment, which may be beyond the control. In
addition, the network conditions cannot be controlled or predicted. Performance
evaluations of workload models and Cloud provisioning algorithms in a
repeatable manner under different configurations are difficult. Therefore,
simulators are developed. To understand and apply better the state-of-the-art
of cloud computing simulators, and to improve them, we study four known
open-source simulators. They are compared in terms of architecture, modeling
elements, simulation process, performance metrics and scalability in
performance. Finally, a few challenging issues as future research trends are
outlined.
