Gorbunov, S.

Lindenstruth, V.

Rohr, D.

GSI Repository

ALICE HLT TPC GPU Tracker∗D. Rohr1, S. Gorbunov1, V. Lindenstruth1 for the ALICE Collaboration1Frankfurt Institute for Advanced Studies, Frankfurt, GermanyThe Alice High Level TriggerThe Large Hadron Collider (LHC) at the European Cen-ter for Particle Physics (CERN) is today’s most powerfulparticle collider built to search for rare particles such as theHiggs boson and to study properties of dense hot medium.Alice is one of its four major detectors designed specifi-cally for the second purpose. The ALICE High Level Trig-ger (HLT) is a compute farm of about 250 nodes and it isthe first point where all data from the various subdetectorsare available together. The HLT is capable of a full onlineevent reconstruction and uses data compression techniquesin order to reduce the data rate stored to the tapes to a fea-sible level.The Alice HLT TPC TrackerReconstruction of the trajectories (tracking) of particlesmeasured in the Time Projection Chamber (TPC) is onecritical part of event reconstruction and requires signifi-cant compute resources. The HLT implements a paralleltracking algorithm that can make use of GPUs. It is basedon the cellular automation and the Kalman filter [1]. Thetracker employs a pipeline which ensures continuous GPUutilization. Results of the GPU and the CPU version ofthe tracker match exactly except for artifacts caused by dif-ferent rounding due to non-associative floating point arith-metic [5] [2] [3]. A direct comparison of the GPU trackerto a hexa-core CPU demonstrates a speedup factor of aboutthree [4].The HLT compute nodes are not exclusively used fortracking but also for other tasks such as cluster transforma-tion and vertexing. Hence, not all CPU cores are availablefor tracking. The below figure shows the speedup of boththe GPU and the CPU implementation of the HLT trackerrunning on four CPU cores and, in the case of the GPUtracker, also on the GPU. The HLT tracker is 12 to 15 timesfaster than the offline version and the GPU tracker is fasterby another order of magnitude [2] (see figure 1).If the CPU tracker employs all available cores of onenode, tracking takes about as long as tracking on the GPU.Hence, plugging a GPU in an existing node saves an entirenode - and the required additional infrastructure. Overall,the GPU tracker enables TPC tracking with only about onetenth the investment which would be required otherwise.During the Pb-Pb run in 2010 the GPU tracker wasfirst tested under real conditions. This led to some mi-nor improvements, e.g. with respect to memory consump-tion. Some other improvements coped with irreproducibleresults caused by concurrency, which - even though they∗Work supported by GSI, BMBF, HGS-HIRe 0 20 40 60 80 100 120 140 160 1800.5*106 1*106 1.5*106 2*106Speedup Factor (compared to Offline)Number of ClustersHLT GPU TrackerHLT GPU Tracker ComponentHLT CPU Tracker ComponentFigure 1: HLT Tracker Speedup.did not affect physical observables - complicated QA ef-forts. Later on, the GPU tracker was operated during thelead run in 2011, and it ran stably in 24/7 operation duringthe full proton phase of 2012. For the current proton-leadrun the HLT was upgraded with more GPUs to cope withthe increased rate.At the moment, the HLT is equipped with about 60 FermiGPUs achieving a maximum data rate of more than 200 Hzin central lead-lead collisions.References[1] S. Gorbunov, D. Rohr, et al. ALICE HLT high speed trackingon GPU, IEEE Transactions on Nuclear Science 58 (2011)1845[2] D. Rohr, S. Gorbunov, et al. ALICE HLT TPC Tracking ofPb-Pb Events on GPUs, Journal of Physics: Conference Se-ries (2012, Vol. 396, p. 12044)[3] D. Rohr ALICE TPC Online Tracker on GPUs for Heavy-Ion Events, Proceedings of 13th International Workshop onCellular Nanoscale Networks and their Applications (2012,pp. 298303)[4] S. Gorbunov, M. Kretz, D. Rohr, ALICE TPC Online Track-ing on GPU, GSI Scientific Report 2009[5] D. Rohr, ALICE TPC Tracking on GPU for Pb-Pb Run inDecember 2010, GSI Scientific Report 2010IT-14 GSI SCIENTIFIC REPORT 2012506

ALICE HLT TPC GPU Tracker

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ALICE HLT TPC GPU Tracker

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