Fat-tree topologies have been widely adopted as the communication network in data centers in the past decade. Nowadays, high-performance computing (HPC) system designers are considering using fat-tree as the interconnection network for the next generation supercomputers. For extreme-scale computing systems like the data centers and supercomputers, the performance is highly dependent on the interconnection networks. In this paper, we present FatTreeSim, a PDES-based toolkit consisting of a

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... scalable fat-tree network model, with the goal of better understanding the design constraints of fat-tree networking architectures in data centers and HPC systems, as well as evaluating the applications running on top of the network. FatTreeSim is designed to model and simulate large-scale fat-tree networks up to millions of nodes with protocol-level fidelity. We have conducted extensive experiments to validate and demonstrate the accuracy, scalability and usability of FatTreeSim. On Argonne Leadership Computing Facility's Blue Gene/Q system, Mira, FatTreeSim is capable of achieving a peak event rate of 305 M/s for a 524,288-node fat-tree model with a total of 567 billion committed events. The strong scaling experiments use up to 32,768 cores and show a near linear scalability. Comparing with a small-scale physical system in Emulab, FatTreeSim can accurately model the latency in the same fat-tree network with less than 10% error rate for most cases. Finally, we demonstrate FatTreeSim's usability through a case study in which FatTreeSim serves as the network module of the YARNsim system, and the error rates for all test cases are less than 13.7%.