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Multi-threaded commercial workloads implement many important Internet services. Consequently, these workloads are increasingly used to evaluate the performance of uniprocessor and multiprocessor system designs. This paper identifies performance variability as a potentially major challenge for architectural simulation studies using these workloads. Variability refers to the differences between multiple estimates of a workload's performance. Time variability occurs when a workload exhibits different characteristics during different phases of a single run. Space variability occurs when small variations in timing cause runs starting from the same initial condition to follow widely different execution paths. Variability is a well-known phenomenon in real systems, but is nearly universally ignored in simulation experiments. In a central result of this paper we show that variability in multi-threaded commercial workloads can …