This paper summarises the operational experience and improvements of the ATLAS hierarchical multi-tier computing infrastructure in the past year leading to taking and processing of the first collisions in 2009 and 2010. Special focus will be given to the Tier-0 which is responsible, among other things, for a prompt processing of the raw data coming from the online DAQ system and is thus a critical part of the chain. We will give an overview of the Tier-0 architecture, and improvements based on the operational experience. Emphasis will be put on the new developments, namely the Task Management System opening Tier-0 to expert users and Web 2.0 monitoring and management suite. We then overview the achieved performances with the distributed computing system, discuss observed data access patterns over the grid and describe how we used this information to improve analysis rates.