Speaker
Kostas Kordas
(Laboratori Nazionali di Frascati (LNF))
Description
The ATLAS experiment at the LHC will start taking data in 2007. Event data from
proton—proton collisions will be selected in a three level trigger system which
reduces the initial bunch crossing rate of 40 MHz at its first level trigger (LVL1)
to 75 kHz with a fixed latency of 2.5 μs. The second level trigger (LVL2) collects
and analyses Regions of Interest (RoI) identified by LVL1 and reduces the event rate
further to ~3 kHz with an average latency of tens of ms. Subsequently the
EventBuilder collects the data from all readout systems and provides fully assembled
events to the the Event Filter (EF), which is the third level trigger. The EF
analyzes the entirety of the event data to achieve a further rate reduction to ~200
Hz, with a latency of a few seconds. While LVL1 is based on custom hardware, LVL2,
EventBuilder and EF are based on compute farms of O(3000) PCs, interconnected via
Gigabit Ethernet, running Linux and multi-threaded software applications implemented
in C++. This note focuses on the common design and implementation of the High Level
Trigger Infrastructure, RoI Collection and the EventBuilding.
Both LVL2 and EF (collectively called High Level Trigger) use online software for the
control and data collection aspects, but the actual trigger selection is developed
and tested using the offline software environment. A common approach of the LVL2
Processing Unit and the EF Processing Task for the steering, seeding and sequential
processing of the selection algorithms has been developed.
Significant improvements and generalization of the system design, allow for complex
data flow functionality steered by the results of the event selection at LVL2 and EF.
This allows the handling of events in parallel data streams for physics, calibration,
monitoring or debug purposes. The possibility for event duplication, partial Event
Building and data stripping are currently under development. Insight in special
features of the system, such as load balancing of the various compute farms, traffic
shaping and garbage collection will also be given.
The HLT and EventBuilder are being integrated with the LVL1 trigger and the ATLAS
subdetectors and will be operated with cosmic events as part of the commissioning in
the 2nd half of 2006.
Primary authors
H.P. Beck
(Laboratorium fuer Hochenergiephysik)
Joannes Andreas Bogaerts
(CERN)
Kostas Kordas
(Laboratori Nazionali di Frascati (LNF))
Sergey Sushkov
(Instituto de Fisica de Altas Energias (IFAE))
