Performance of the Final Event Builder for the ATLAS Experiment

Event data from proton-proton collisions at the LHC will be selected by the ATLAS experiment by a three level trigger system, which reduces the initial bunch crossing rate of 40 MHz at its first two trigger levels (LVL1+LVL2) to $\sim$3 kHz. At this rate the Event-Builder collects the data from all Read-Out system PCs (ROSs) and provides fully assembled events to the the Event-Filter (EF), which is the third level trigger, to achieve a further rate reduction to $\sim$200 Hz for permanent storage. The Event-Builder is based on a farm of $O(100)$ PCs, interconnected via Gigabit Ethernet to $O(150)$ ROSs. These PCs run Linux and multi-threaded software applications implemented in C++. All the ROSs and one third of the Event-Builder PCs are already installed and commissioned. Performance measurements have been exercised on this initial system, which show promising results that the required final data rates and bandwidth for the ATLAS event builder are in reach.