ATLAS High Level Trigger within the multi-threaded software framework AthenaMT

Rafal Bielski

doi:10.1088/1742-6596/1525/1/012031

Abstract

Athena is the software framework used in the ATLAS experiment throughout the data processing path, from the software trigger system through offline event reconstruction to physics analysis. The shift from high-power single-core CPUs to multi-core systems in the computing market means that the throughput capabilities of the framework have become limited by the available memory per process. For Run 2 of the Large Hadron Collider (LHC), ATLAS has exploited a multi-process forking approach with the copy-on-write mechanism to reduce the memory use. To better match the increasing CPU core count and, therefore, the decreasing available memory per core, a multi-threaded framework, AthenaMT, has been designed and is now being implemented. The ATLAS High Level Trigger (HLT) system has been remodelled to fit the new framework and to rely on common solutions between online and offline software to a greater extent than in Run 2. We present the implementation of the new HLT system within the AthenaMT framework, which is going to be used in ATLAS data-taking during Run 3 (2021 onwards) of the LHC. We also report on interfacing the new framework to the current ATLAS Trigger and Data Acquisition (TDAQ) system, which aims to bring increased flexibility whilst needing minimal modifications to the current system. In addition, we show some details of architectural choices which were made to run the HLT selection inside the ATLAS online dataflow, such as the handling of the event loop, returning of the trigger decision and handling of errors.

Highlights

AthenaMT motivation and design Athena [1] is a software framework used in the ATLAS experiment [2] at all stages of event data processing path, from simulation and trigger, through event reconstruction to physics analysis
At the time of the initial design, there was no strong motivation for concurrent event processing in experiments like ATLAS and the architectural choices made for Gaudi and Athena assumed only a single-process and single-thread mode of operation
In Run 2 of the Large Hadron Collider (LHC) [6], the ATLAS experiment software has already suffered from suboptimal use of resources leading to the throughput of simulation and

Summary

Introduction

1. AthenaMT motivation and design Athena [1] is a software framework used in the ATLAS experiment [2] at all stages of event data processing path, from simulation and trigger, through event reconstruction to physics analysis. These include the processing of partial event data (regional reconstruction) and early termination of an execution path if the event is failing the trigger selection. The scheduling of all algorithms in the HLT is assisted by configuration-time Control Flow which defines sequences of Filter → Input Maker → Reconstruction → Hypothesis algorithms.

Results

Conclusion