Multi-threaded evolution of the data-logging system of the ATLAS experiment at CERN

The ATLAS experiment observes proton-proton collisions delivered by the LHC accelerator at a centre of mass energy of 7 TeV with a peak luminosity of ~ 10³³ cm^-2s^-1 in 2011. The ATLAS Trigger and Data Acquisition (TDAQ) system selects interesting events on-line in a three-level trigger system in order to store them at a budgeted average rate of ~ 400 Hz for an event size of ~ 1.2 MB. This paper focuses on the TDAQ data-logging system. Its purpose is to receive events from the third level trigger, process them and stream the data into different raw files according to the trigger decision. The system currently in production is based on an essentially single-threaded design that is anticipated not to cope with the increase in event rate and event size foreseen as part of the ATLAS and LHC upgrade programs. This design also severely limits the possibility of performing additional CPU-intensive tasks. Therefore, a novel design able to exploit the full power of multi-core architecture is needed. The main challenge of such a design is the conflict between the largely parallel nature of the data-logging event processing and the constraint of sequential file writing. In this paper, we present a thread-pool based implementation of the TDAQ data-logging software. We report here on the functionality and performance of the new system and on our development experience.