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Analytical modeling is becoming an increasingly important technique used in the design of chip multiprocessors. Most such models assume multi-programmed workload mixes and either ignore or oversimplify the behavior of multi-threaded applications. In particular, data sharing observed in multi-threaded applications, and its impact on chip design decisions, has not been well characterized in prior analytical modeling work. In this work we describe why data sharing behavior is hard to capture in an analytical model, and study why, and by how much, past attempts have fallen short. We propose a new methodology to measure the impact of data sharing, which quantifies the reduction in on-chip cache miss rates attributable solely to the presence of data sharing. We then extend an existing analytical performance model for a many-core chip by incorporating into it the impact of data sharing in contemporary multi-threaded workloads. We use this analytical model to explore the chip design space for a hypothetical many-core chip of the future. We find that the optimal design point is substantially different when the impact of data sharing is modeled compared to when it is not. Data sharing can enable reassigning a significant fraction of the total chip area (up to 16%, per our model of a future many-core) from cache resources to core resources, which, in turn, improves the overall chip throughput (by up to 58%).