The Internet has made database management systems and Web servers integral parts of today’s business and communications infrastructure. These and other commercial transaction-processing applications work with critical personal and business data—storing it, providing access to it, and manipulating it. As dependence on these applications increases, so does the need for them to run reliably and efficiently. Our group at the University of Wisconsin (www.cs.wisc.edu/multifacet/) researches innovative ways to improve the performance of the multiprocessor servers that run these important commercial applications. Execution-driven simulation is a design evaluation tool that models system hardware. These simulations capture actual program behavior and detailed system interactions. They are more flexible and less expensive than hardware prototypes, and they model important system details more accurately than analytic modeling does. However, the combination of large systems and demanding workloads is difficult to simulate, especially on the inexpensive machines available to most researchers. Commercial workloads, unlike simpler workloads, rely heavily on operating system services such as input/output, process scheduling, and interprocess communication. To run commercial workloads correctly, simulators must model these services. In addition, multiprocessor servers introduce the challenges of interactions among processors, large main memories, and many disks. To make effective use of limited simulation resources, researchers must balance three goals: • developing a representative approximation of large workloads, • achieving tractable simulation times, and • simulating a sufficient level of timing detail. We developed a simulation methodology to achieve these goals. Our methodology uses multiple simulations, pays careful attention to scaling effects on workload behavior, and extends VirtutechAB’s Simics full-system functional simulator with detailed timing models.