The Region of Interest Strategy for the ATLAS Second Level Trigger

1
The Region of Interest Strategy for the ATLAS
Second Level Trigger

• Véronique Boisvert
• CERN
• On behalf of the ATLAS Trigger/DAQ High Level
  Trigger Group

CHEP March 25th 2003
2
Outline

• Motivation
• Introduction
• The RegionSelector Tool
• Implementation
• Timing Results
• Conclusion

3
Motivation
Need fast, reliable, easy to make Trigger
Algorithms
4
Introduction

• bunch crossing rate 40MHz
• 108 electronic channels
• 1.6 MB/event

11m
23m
5
Introduction Trigger Architecture
6
Introduction Regions of Interest

• RoI Mechanism
• Typically a few ROI / event
• Only few of event data required!
• But more complex system

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Sequence of Data Access
See Architecture of ATLAS High Level Trigger
Event Selection Software tomorrow

• data store fetches requested collections only
  and caches them for subsequent requests
• unit of retrieval from the DAQ is 1
  ReadOutBuffer
• possibility to do data preparation

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Collections and Identifiers

• See Use of a generic identification scheme
  connecting events and detector description in the
  ATLAS experiment on Thursday
• Need to optimize collection granularity
• Trade off between
• judicious navigation for algorithms
• Minimize data access requests
• Use of offline identifiers
• In process of validating design of using
  offline code into online environment
• Facilitate development of algorithms
• Study boundary between Level2 and EF
• Allow performance studies for physics analyses
• Needed for Inner Detector Regions
• No Level 1 hardware identifier available

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The Region Selector Tool

• Algorithms at Level2 have access to Level1 RoI
  regions
• Tool Region in detector ? Data collection
  identifiers
• Requirements
• Fast!
• Different geometrical region descriptions
• Cone
• Region following a track
• Etc.
• Code to be run in multi-threaded environment

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Implementation The RegionSelector

• At initialization
• Fill 2 maps for each layer
• Map lt ?, set ltidentifiersgtgt
• Map lt identifier, vectorlt?min, ?maxgtgt
• At Execution time we want identifiers within lt
  fmin, fmaxgt and lt?min, ?maxgt
• Loop over the layers
• Loop over f, get identifiers
• Loop over the identifiers and check ? interval
• Return list of identifiers
• No duplicate identifier ensured

EtaPhiMap
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Implementation data access granularity
Preliminary
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Implementation Geometrical Regions
CONE
Implemented for outer most detectors TRT, LAr,
Tile and Muon

• Simple origin h, Dh, f, Df

WEDGE OF CHEESE
Implemented for inner most detectors Pixel and SCT

• Complicated origin, Dz, h, Dh(r), f, Df
• Realistic to account for LHC beam spread in z

HELICAL ROAD
Future development

• Complicated origin, 1/r (min), 1/r (max),tan q,
  f (min), f(max), h(min), h(max)
• Trajectory-oriented (for uniform B field)

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Timing Measurements
Dh
Improvements forthcoming
Preliminary
Using 1 GHz Pentium 3 Numbers in ms
4 GHz scaled (div 3)
DF
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Timing Measurements

• In Process of timing algorithms as well
• Offline way
• Fully timing instrumented TestBeds
• Running a Level2 LAr Calorimeter algorithm making
  use of Region Selector
• Preliminary numbers O(1 ms)
• RegionSelector 0.04-0.07 ms
• Small fraction of the algorithm time!
• ltlt 10 ms

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Conclusion

• The ATLAS High Level Trigger relies on the Region
  of Interest Mechanism to access only relevant
  part of the detector
• The RegionSelector is the software tool used to
  extract the needed data collection identifiers
  based on some arbitrary geometrical regions
• The timing measurements show a very minimal
  overhead on the algorithms coming from this tool
• The RegionSelector tool will also be used for
  offline reconstruction of the data