  Predicting the performance of various infrastructure design options in
complex federated infrastructures with computing sites distributed over a wide
area network that support a plethora of users and workflows, such as the
Worldwide LHC Computing Grid (WLCG), is not trivial. Due to the complexity and
size of these infrastructures, it is not feasible to deploy experimental
test-beds at large scales merely for the purpose of comparing and evaluating
alternate designs. An alternative is to study the behaviours of these systems
using simulation. This approach has been used successfully in the past to
identify efficient and practical infrastructure designs for High Energy Physics
(HEP). A prominent example is the Monarc simulation framework, which was used
to study the initial structure of the WLCG. New simulation capabilities are
needed to simulate large-scale heterogeneous computing systems with complex
networks, data access and caching patterns. A modern tool to simulate HEP
workloads that execute on distributed computing infrastructures based on the
SimGrid and WRENCH simulation frameworks is outlined. Studies of its accuracy
and scalability are presented using HEP as a case-study. Hypothetical
adjustments to prevailing computing architectures in HEP are studied providing
insights into the dynamics of a part of the WLCG and candidates for
improvements.
