This document discusses modeling the performance of ultra-large-scale systems using layered simulations. It presents a three-layered simulation model consisting of a world view layer, component layer, and physical layer. This model allows performance to be evaluated at different levels of abstraction. Two case studies are described that apply this modeling approach: one identifies a CPU bottleneck in an RSS cloud system, and another compares centralized versus distributed monitoring of a large-scale system. The layered simulation model enables performance to be evaluated earlier in the development process and helps different stakeholders understand performance impacts.