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The ATLAS High Level Trigger SteeringJournée de réflexion – Sept. 14th 2007
Till EifertDPNC – ATLAS group
• 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
H
L
T
75 kHz
~3 kHz
~200 Hz
40 MHz
RoI data
LVL1 Acc.
ROD ROD ROD
2.5 s
Event Builder
EB
~4 GB/s
ROS
ROB ROB ROB
120 GB/s
Calo/Mu TrigDet Other detectors
1 PB/s
Event Filter
EFPEFP
EFP EFN
~300 MB/s
LVL2
L2P
L2SV
L2NL2PL2P
ROIB
LVL2 Acc.
RoI’s
Event Size ~1.5 MB
LVL1Calorimeter
TriggerMuon
Trigger
CTP
Pipelines2.5 s
EF Acc.
(Region of Interest)
RoI requests
2007 Sept 14 The ATLAS High Level Trigger Steering 2
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 The ATLAS High Level Trigger Steering 3 3
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 14 The ATLAS High Level Trigger Steering 4
Generic name
e5 e5_PT e10 g10 2e10 e20_XE12 XE12
LVL1 EM3
PS
EM8 2EM8 EM18_XE12 XE12
PS
LVL2 e5 PT e10 g10 2e10 e20_xe12 PT
EF e5 PT e10 g10 2e10 e20_xe12 PT
Sample trigger menu
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)
Chain: represents several steps, several algorithms at each step
2007 Sept 14 The ATLAS High Level Trigger Steering 5
Steering concepts
Sequence
Fex Hypo TE’TE
Step
TE
Chain
step
step
step
predecessor
successor
LVL1 item
LVL2chain
EFchain
Generic name Trigger Elemente10
L2_e10
EF_e10
L1_EM8
L2_e10cl
L2_e10tr
L2_e10
L1_EM8
EF_e10
L2_e10clL1_EM8
L2_e10cl L2_e10tr
L2_e10L2_e10tr
L2_e10
clustering
tracking
combine
TETE
2007 Sept 14 The ATLAS High Level Trigger Steering 6
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 ET or pT threshold
Other types of algorithm available for more complex logic.
Sequence
TE’TE Fex Hypo
2007 Sept 14 The ATLAS High Level Trigger Steering 7
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
e10 2e10 g10
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 14 The ATLAS High Level Trigger Steering 8
Trigger Aware AnalysisTriggerDBAll 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 ESD AOD TAG files
Configures Store
sde
code
d
Trig
ger
Men
u
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 14 The ATLAS High Level Trigger Steering 9
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 14 The ATLAS High Level Trigger Steering 10
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 14 The ATLAS High Level Trigger Steering 12
CachingTwo sequences: same fex, different hypo:
Hypo_
e5L2_e5clL1_EM3
Hypo_
e10
L2_e10clL1_EM8
Cluste
r find
erClus
ter f
inder
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 14 The ATLAS High Level Trigger Steering 13
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 14 The ATLAS High Level Trigger Steering 14
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