Overview of the ATLAS data acquisition system operating at the TeV scale

This paper focuses on the operation of the ATLAS data acquisition system during the first months of 2010. ATLAS is one of the two multipurpose detectors at the Large Hadron Collider (LHC), which provides proton-proton collisions at the unprecedented centre-of-mass energy of 7 TeV. The ATLAS data acquisition system is based on O(2k) processing nodes, interconnected by a multi-layer Gigabit Ethernet network. About 20k applications will provide the needed capabilities in terms of run control, event selection, data flow, local storage and data monitoring. The whole data acquisition system has been successfully commissioned during the last two years with cosmic ray and calibration data and it turned out to be robust and reliable. Nevertheless, the continuous operation with beams, the concurrent trigger commissioning, and the understanding of detector and physics performance will pose new challenges. The flexibility of the data acquisition infrastructure will be probed and exploited, in order to comply with the consequent unpredictable working conditions in terms of data-flow, monitoring and configuration requirements. Concerning the latter in particular, the data acquisition efficiency will have to be kept under control, profiting by the special tools and techniques especially put in place. The goal is to minimise both downtime and dead-time, allowing for runtime reconfiguration of the data acquisition and sub-detectors systems as well as for automatic error handling and recovery.