ATLAS trigger algorithms for general purpose graphics processor units

We present ATLAS Trigger algorithms developed to exploit General Purpose Graphics Processor Units (GPGPU). ATLAS is a particle physics experiment located on the LHC collider at CERN. The ATLAS Trigger system has two levels, hardware-based Level 1 and the High Level Trigger implemented in software running on a farm of commodity CPU. Performing the trigger event selection within the available farm resources presents a significant challenge that will increase following future LHC upgrades. GPGPUs are being evaluated as a potential solution for trigger algorithms acceleration. Key factors determining the potential benefit of this new technology are the relative execution speed-up, the number of GPUs required and the relative financial cost of the selected GPU. We have developed a trigger demonstrator which includes algorithms for reconstructing tracks in the Inner Detector and Muon Spectrometer and clusters of energy deposited in the Calorimeter. A “client-server” architecture is used to integrate the GPU-accelerated code running on the server side with multiple clients running the remaining serial code on the CPUs. We give a description of the algorithms implemented and present measurements that compare the performance of various algorithm configurations.