The Evolution of the ATLAS Computing Model

Despite the all too brief availability of beam-related data, much has been learned about the usage patterns and operational requirements of the ATLAS computing model since Autumn 2007. Bottom-up estimates are now more detailed, and cosmic ray running has exercised much of the model in both duration and volume. Significant revisions have been made in the resource estimates, and in the usage of those resources. In some cases, this represents an optimization while in others it attempts to counter lack of functionality in the available middleware. There are also changes reflecting the emerging roles of the different data formats. The model continues to evolve with a heightened focus on end-user performance, and the state of the art after a major review process over winter 08/09 is presented.