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The ATLAS High Level Trigger Steering Journée de réflexion – Sept. 14 th 2007

The ATLAS High Level Trigger Steering Journée de réflexion – Sept. 14 th 2007. Till Eifert DPNC – ATLAS group. The ATLAS Trigger System. LVL1. Muon Trigger. Calorimeter Trigger. ROD. ROD. ROD. CTP. RoI’s. ROB. ROB. ROB. ROIB. L2SV. L2P. L2P. L2P. EB. EFN. EFP. EFP. EFP.

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The ATLAS High Level Trigger Steering Journée de réflexion – Sept. 14 th 2007

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  1. The ATLAS High Level Trigger SteeringJournée de réflexion – Sept. 14th 2007 Till Eifert DPNC – ATLAS group

  2. The ATLAS Trigger System LVL1 Muon Trigger Calorimeter Trigger ROD ROD ROD CTP RoI’s ROB ROB ROB ROIB L2SV L2P L2P L2P EB EFN EFP EFP EFP Calo/Mu TrigDet • Level 1 • Hardware based • Coarse granularity calorimeter and muons only • High Level Trigger (HLT) • Level 2 and Event Filter • Software based • Mostly commodity hardware (PC + Ethernet) • Level 2 (L2) • Data requested from ROBs over network • Full detector granularity in RoIs • Special fast algorithms • Event Filter (EF) • Seeded by L2 • Potential full event access • Full detector granularity • Offline algorithms Other detectors 40 MHz 1 PB/s Pipelines 2.5 ms 2.5s LVL1 Acc. 75 kHz 120 GB/s (Region of Interest) H L T RoI requests ROS LVL2 RoI data L2N ~4 GB/s LVL2 Acc. Event Builder ~3 kHz Event Filter EF Acc. ~200 Hz Event Size ~1.5 MB ~300 MB/s The ATLAS High Level Trigger Steering

  3. Key features of ATLAS trigger strategy • HLT uses Regions of Interest • Based on L1 triggers • Reduce data bandwidth at L2 • Reduce processing time • Early rejection • Three level trigger • Steps within L2 and EF • Reduce processing time • Reduce decision latency Regions of Interest Till Eifert -- 2007 Sept 15 3 The ATLAS High Level Trigger Steering

  4. HLT steering - in a nutshell • RoI mechanism • Each trigger level or sub-step is seeded by the result of the previous one • Early rejection • Drop event as soon as it cannot pass the trigger • Minimise average processing time • Fast • Leave most time for event-selection algorithms • Flexible • Enable/disable triggers • Construct complex menus from simple building blocks • Instrumented for monitoring • Work in both online and offline s/w environments • Online data taking • Offline development/debugging and simulation The ATLAS High Level Trigger Steering

  5. Sample trigger menu Chain: represents several steps, several algorithms at each step From draft 1031 start-up menu. It contains e,g,mu,tau,j,xe,te,je, single, multiple and combined triggers, various thresholds, some with pre-scale (PS) and/or pass through (PT) The ATLAS High Level Trigger Steering

  6. Steering concepts TE TE TE Chain L2_e10 Generic name Step predecessor Trigger Element L1_EM8 e10 L2_e10cl LVL1 item step L2_e10tr L2_e10 L1_EM8 LVL2 chain step Sequence L2_e10 EF chain Hypo Fex TE TE’ step EF_e10 L2_e10cl L1_EM8 clustering L2_e10cl tracking L2_e10tr successor L2_e10tr combine L2_e10 EF_e10 The ATLAS High Level Trigger Steering

  7. Trigger algorithms Sequence Hypo Fex TE TE’ • Typical feature extraction (Fex) algorithm: • Seeded by previous step or RoI • Retrieves detector data • Finds “feature” e.g. cluster, track • Updates RoI position • Runs once per RoI • Typical hypothesis (Hypo) algorithm: • Follows Fex algorithm • Compares features to hypothesis • Marks TE’ as valid or not • Runs once per threshold Example Hypotheses: Calo cluster: Cut on cluster shape parameters Electron: cut on cluster-track matching variables Most cases: apply ET or pT threshold Other types of algorithm available for more complex logic. The ATLAS High Level Trigger Steering

  8. Steering logic configuration chains steps • Static configuration + dynamic event state • First create initial TEs from L1 RoIs • One per threshold per RoI • At EF, from L2 output instead • Activate relevant chains • Loop over steps • Loop over active chains • Loop over TEs (event) that match step requirements (config) • Run sequence that links TE from prev. step in chain to required TE • Result (depends on algorithms): TE is active or not • If insufficient active TEs remain, deactivate chain • If no active chains remain, end loop over steps • Apply pre-scale, pass-through and reject/accept event e10 2e10 g10 L1_EM8 L2_e10cl L2_e10tr sequences L2_e10 L2_g10 event 1x L1_EM8 RoI 3x L1_EM8 RoI Trigger Elements (TE) The ATLAS High Level Trigger Steering

  9. Trigger Aware Analysis TriggerDBAll configuration data Preparation Stores decoded Trigger Menu Configures online DB (COOL) Data taking Decoded Trigger Menu LVL1 + HLT result (encode trigger decision) • Trigger Decision class • Configuration: Chains (name, version), prescales, pass through rates • Trigger chain result: pass/fail?, passed through?, prescaled?, last successful signature? • Navigation: which ROI / trigger element satisfied the trigger selection ? • Trigger EDM: redefine (tighten) trigger selection Reconstruction ESD AOD Persisted in TAG files (subset only) Trigger aware analysis • Check trigger decision if desired trigger passed • Trigger efficiency studies down to algorithm level The ATLAS High Level Trigger Steering

  10. Conclusions • The HLT steering implements the key features of the HLT event selection strategy: • Region of Interest/seeding • Steps/early rejection • Complex menus have been built up • Caching saves time and simplifies config • Time overhead cmp to trigger algos is modest • Well instrumented for monitoring • Already used in cosmic runs and tech. runs The ATLAS High Level Trigger Steering

  11. Backup slides

  12. HLT steering - overhead time • Framework is only small fraction of total • Times consistent with target 3 GHz Xeon CPU Code is not yet optimised: expect to reduce steering time.. Inclusive e/ menu with low thresholds for very low lumi. 400 top events: atypically busy; average 16 RoIs (~4 EM) per event rather than the usual ~2. Average time to process an event Steering has been run live online during cosmic data runs The ATLAS High Level Trigger Steering

  13. Caching Two sequences: same fex, different hypo: L1_EM3 Cluster finder L2_e5cl Hypo_e5 L1_EM8 Cluster finder Hypo_e10 L2_e10cl 1) Steering will run fex only once per RoI; Second time, results are taken from cache 2) Same sequence in different chains is also cached  Implicit caching when same item appears multiple times in configuration The ATLAS High Level Trigger Steering

  14. Benefits of caching • Useful gain in speed • Simplifies configuration • Times consistent with target 3 GHz Xeon CPU Code is not yet optimised: expect to reduce steering time.. Inclusive e/ menu with low thresholds for very low lumi. 400 top events: atypically busy; average 16 RoIs (~4 EM) per event rather than the usual ~2. Time to process an event 50 ms with caching (default) 270 ms without caching The ATLAS High Level Trigger Steering

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