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

2. 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 The ATLAS Trigger System HLTHLT 75 kHz ~3 kHz ~200 Hz 40 MHz RoI data LVL1 Acc. ROD 2.5  s Event Builder EB ~4 GB/s ROS ROB 120 GB/s Calo/Mu TrigDet Other detectors 1 PB/s Event Filter EFP EFN ~300 MB/s LVL2 L2P L2SV L2N L2P ROIB LVL2 Acc. RoI’s Event Size ~1.5 MB LVL1 Calorimeter Trigger Muon Trigger CTP Pipelines 2.5  s EF Acc. (Region of Interest) RoI requests 2007 Sept 14The ATLAS High Level Trigger Steering2

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 15The ATLAS High Level Trigger Steering3 3

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 2007 Sept 14The ATLAS High Level Trigger Steering4

5. Generic name e5e5_PTe10g102e10e20_XE12XE12 LVL1EM3 PS EM82EM8EM18_XE12XE12 PS LVL2e5PTe10g102e10e20_xe12PT EFe5PTe10g102e10e20_xe12PT Sample trigger menu From draft 10 31 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) Chain: represents several steps, several algorithms at each step 2007 Sept 14The ATLAS High Level Trigger Steering5

6. Steering concepts Sequence Fex Hypo TE’TE Step TE Chain step predecessor successor LVL1 item LVL2 chain EF chain Generic name Trigger Element e10 L2_e10 EF_e10 L1_EM8 L2_e10cl L2_e10tr L2_e10 L1_EM8 EF_e10 L2_e10cl L1_EM8 L2_e10cl L2_e10tr L2_e10 L2_e10tr L2_e10 clustering tracking combine TE 2007 Sept 14The ATLAS High Level Trigger Steering6

7. Trigger algorithms 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 E T or p T threshold Other types of algorithm available for more complex logic. Sequence TE’TE Fex Hypo 2007 Sept 14The ATLAS High Level Trigger Steering7

8. Steering logic ■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 steps chains e102e10g10 sequences 1x L1_EM8 RoI 3x L1_EM8 RoI configuration event Trigger Elements (TE) L2_e10cl L2_e10tr L2_e10 L2_g10 L1_EM8 2007 Sept 14The ATLAS High Level Trigger Steering8

9. Trigger Aware Analysis TriggerDB All configuration data online DB (COOL) Preparation Data taking Reconstruction Trigger aware analysis 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 Persisted in ESDAOD TAG files Configures Stores decoded Trigger Menu LVL1 + HLT result (encode trigger decision) Decoded Trigger Menu Check trigger decision if desired trigger passed Trigger efficiency studies down to algorithm level (subset only) 2007 Sept 14The ATLAS High Level Trigger Steering9

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 2007 Sept 14The ATLAS High Level Trigger Steering10

11. Backup slides

12. HLT steering - overhead time Average time to process an event Framework is only small fraction of total Times consistent with target Steering has been run live online during cosmic data runs 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. 2007 Sept 14The ATLAS High Level Trigger Steering12

13. Caching Two sequences: same fex, different hypo: Hypo_e5 L2_e5cl L1_EM3 Hypo_e10 L2_e10cl L1_EM8 Cluster finder 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 2007 Sept 14The ATLAS High Level Trigger Steering13

14. Benefits of caching Time to process an event Useful gain in speed Simplifies configuration Times consistent with target 50 ms with caching (default) 270 ms without caching 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. 2007 Sept 14The ATLAS High Level Trigger Steering14