the lhcb trigger in run ii · use hlt 1 output for calibration and alignment 10 pib in farm (half...
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The LHCb Trigger in Run II
Rosen Matev, CERN
on behalf of the LHCb collaboration
20th Real Time Conference
2016, Padova
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The LHCb experiment
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The LHCb experiment
~45 kHz bb pairs and ~1 MHz cc pairs
at 13 TeV and L = 4⨉10
32
cm
−2
s
−1
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Online system architecture
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L0 trigger
Timing and
Fast Control
SubdetetorsSubdetetorsSubdetetors
FE electronics
Readout boardsReadout boardsReadout boards
Readout network
Switch Switch Switch Switch
Node
Node
Nodes
Node
Node
Nodes
Node
Node
Nodes
Node
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Nodes
Switch
Node
Node
Nodes
Event filter farmMonitoring
Storage
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LHC
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Event filter farm
● 62 homogeneous sub-farms
● 50880 logical cores
● 800 new nodes in Run II
almost double performance
● Servers alone cost >5 MCHF
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CPU # cores RAM disk
920 x Intel x5650 12 (24) 24 GB 2 TB
100 x AMD 6272 16 (32) 32 GB 2 TB
800 x Intel E5-2630v3 16 (32) 32 GB 4 TB
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Run I trigger
● Hardware trigger (L0)
○ Based on calorimeters and muon
detectors
○ Fixed latency of 4 μs
○ Reduces rate to 1 MHz
● Software trigger (HLT)
○ Runs on event filter farm
○ HLT 1: partial reconstruction
○ HLT 2: full up front reconstruction
○ Events buffered to allow processing
out of fill
○ Output rate 5 kHz
○ Total time budget ~35 ms/event
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Run I: online and offline
● Online
○ Best possible within CPU budget
● Offline
○ Best available performance
regardless of CPU
● Differences
○ Pattern recognition
○ Alignment
○ No hadron identification online
○ Particle selections
● Goal for Run II
○ Run offline reconstruction online
○ Offline quality alignment and
calibrations needed online
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Ex.: B
s
0
→ J/ψ φ lifetime measurement
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Deferred triggering
● Stable beams ~35% of the time
● Buffer events to disk and
process between fills
● Run I
○ Defer 20% of L0 accepted events
○ Effectively 25% more CPU
● Run II
○ Defer 100% of HLT 1 accepted events
○ More efficient use of buffers due to larger real-time reduction
○ Save 100% of events at 150 kHz instead of 20% at 1 MHz
○ Use HLT 1 output for calibration and alignment
○ 10 PiB in farm (half in 2015)
○ Sufficient buffer given LHC’s performance comparable to 2015
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Disk usage models
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Run II trigger
● Hardware trigger (L0)
○ Based on multiplicity, calorimeters
and muon detectors
○ Fixed latency of 4 μs
○ Reduces rate to 1 MHz
○ Higher thresholds in Run II
● Software trigger (HLT)
○ HLT farm nearly doubled
○ HLT split in two applications
■ HLT 1 and HLT 2
○ Events buffered after HLT 1
○ Output rate 12.5 kHz
○ HLT software 40% faster
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HLT 1 overview
10
● Inclusive selections, ~100 kHz
○ Single and two track MVA selections
● Inclusive muon selections, ~40 kHz
○ Single and dimuon selections
○ Additional low pT track reconstruction
● Exclusive selections
○ Lifetime unbiased beauty and charm
selections
○ Selections for alignment
● Low multiplicity trigger for central
exclusive production analyses
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Real-time alignment and calibration
● Alignment and calibration crucial for optimal physics performance
● Alignment per fill
○ Collect suitable data with dedicated HLT 1 selections
○ Run alignment workers on the HLT farm (1 per node)
○ Controller iterates until converged, ~5 min
○ Apply updates of VELO and/or Tracker alignment if needed
○ RICH mirror alignment and muon alignment for monitoring
○ ECAL gain calibration
● Calibration per 1 h run:
○ RICH and Outer Tracker t
0
○ Available ~1 min after
collection of data
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HLT 2 reconstruction
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Run I Run II
HLT 1 time ~20 ms ~35 ms
HLT 1 rate ~80 kHz ~150 kHz
HLT 2 time ~150 ms ~650 ms
HLT 2 rate ~5 kHz ~12.5 kHz
2012 CPU heat map
2015 CPU heat map
● Full event reconstruction
○ HLT 1 vertices and tracks
○ All charged tracks
○ Neutral particles
○ RICH, Muon and Calo PID
○ Same reconstruction online
and offline
○ 30% speedup achieved
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Real-time analysis -- TURBO stream
● Online = offline
○ Do physics analysis with HLT
reconstructed objects
● TURBO stream
○ Store HLT candidate information
○ Remove most of detector raw data
○ Space required reduced by > 90 %
● Ideal for high-yield analyses
● Very fast turn around (~24 h)
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arXiv:1604.05596
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Analyses with TURBO
● Presented preliminary results one week after data taking!
● Published Run II measurements performed exclusively with Turbo
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TURBO++
● New in 2016
● Persist arbitrary variables like isolation with HLT candidate
● Can save HLT candidate + any reconstructed objects
○ Custom binary serialization in SOA format, LZMA compression per event
○ Event size of 50 kB, including a minimal subset of the raw data
● Can do qualitatively new things on HLT output
○ Entire analysis can be done on trigger output, incl. flavour tagging
○ e.g. in charm spectroscopy: D
∗ → D
0
( K
−
π
+
)π
+
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Turbo
Turbo++
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TURBO++
Successful use of the TURBO++
with the new 2016 data!
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B
s
→ J/ψ
K
+
K
-
D
*+
→ D
0
(K
π)π
+
Λc
+ →
p
K
-
π
+
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Conclusions
● Two times more CPU power in HLT farm
● Full offline-quality reconstruction available online
● Calibration and alignment running online
● TURBO stream – do physics with trigger objects
○ More physics per byte
○ 2015 – save exclusive decays
○ 2016 – can save all reconstructed objects
○ Results already published and more to come
● Critical for the upgrade (Run III)
○ Real-time alignment and calibration
○ Selective persistence of raw and reconstructed data
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Thank you