Experience with highly-parallel software for the storage system of the ATLAS Experiment at CERN

The ATLAS experiment is observing proton-proton collisions delivered by the LHC accelerator. 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 rate of several hundred Hz. This paper focuses on the TDAQ data-logging system and in particular on the implementation and performance of a novel parallel software design. In this respect, the main challenge presented by the data-logging workload is the conflict between the largely parallel nature of the event processing, especially the recently introduced event compression, and the constraint of sequential file writing and checksum evaluation. This is further complicated by the necessity of operating in a fully data-driven mode, to cope with continuously evolving trigger and detector configurations. In this paper we report on the design of the new ATLAS on-line storage software. In particular we will discuss our development experience using recent concurrency-oriented libraries. Finally we will show the new system performance with respect to the old, single-threaded software design.