Multi-threading in the ATLAS High-Level Trigger

Over the next decade of LHC data-taking the instantaneous luminosity will reach up 7.5 times the design value with over 200 interactions per bunch-crossing and will pose unprecedented challenges for the ATLAS trigger system. With the evolution of the CPU market to many-core systems, both the ATLAS offline reconstruction and High-Level Trigger (HLT) software will have to transition from a multi-process to a multithreaded processing paradigm in order not to exhaust the available physical memory of a typical compute node. The new multithreaded ATLAS software framework, AthenaMT, has been designed from the ground up to support both the offline and online use-cases with the aim to further harmonize the offline and trigger algorithms. The latter is crucial both in terms of maintenance effort and to guarantee the high trigger efficiency and rejection factors needed for the next two decades of data-taking. We report on an HLT prototype in which the need for HLT­specific components has been reduced to a minimum while retaining the key aspects of trigger functionality including regional reconstruction and early event rejection. We report on the first experience of migrating trigger algorithms to this new framework and present the next steps towards a full implementation of the ATLAS trigger within AthenaMT.