FIAS Scientific Report 2011 - Frankfurt Institute for Advanced Studies ...

The ALICE High Level Trigger
Collaborators: T. Kollegger 1 , T. Alt 1 , S. Gorbunov 1 , S. Kalcher 1 , M. Kretz 1 , M. Langhammer 1 ,
V. Lindenstruth 1 , D. Ram 1 , D. Rohr 1 , O. Smørholm 1 , T. Steinbeck 1
1 Frankfurt Institute for Advanced Studies
ALICE is one of the four main experiments at the Large Hadron Collider (LHC) at the European Center for
Particle Physics CERN, Geneva. Its main goal is to study the properties of the hot and dense medium created
in collisions of heavy ions. In the autumn of 2011, LHC was operating with lead (Pb) ions and achieved
luminosities an order of magnitude higher than in previous years, with up to 6kHz of Pb+Pb collisions. This
new record interaction rates posed a challenge for the read-out of the detector. Each Pb+Pb collision creates
up to 80MB of total data volume from the various sub-detectors of ALICE, with the Time Projection Chamber
(TPC) by far the largest (∼60MB). This data rate by far exceeds the performance of the detector read-out
systems, so events of interest have to be selected, the task of the trigger system. Another bottleneck is the
mass-storage system, whose sustained bandwidth is significantly lower than the maximum detector read-out
bandwidth.
The ALICE High Level Trigger (HLT) was designed to close the gap between the maximum detector read-out
bandwidth and the storage bandwidth. Being the first point in ALICE where the data from all sub-detectors is
available, the HLT reconstructs the full event, i.e. the trajectories of the several thousand particles emitted from
the collission and passing through the detector system. Figure 1 shows a visualization of a fully reconstructed
event.
Figure 1: A central Pb+Pb event as reconstructed
by the ALICE High Level Trigger.
After sucessful operation in 2009 and 2010, the HLT was upgraded in 2011 to deal with the higher data rates.
The particle track reconstruction in the HLT uses graphic cards (GPUs) in addition to the CPUs, the number of
nodes with GPUs was doubled to a total of 64. The output bandwidth of the system was increased by a factor
4, now reaching more than 3GB/s. With these upgrades, the HLT was able to deal with the maximum possible
detector read-out rates of more than 20 GB/s.
A special emphasis in 2011 was the improvment of the physics reconstruction performance. For this the first
processing step of the TPC reconstruction, the identification of charge clusters deposited by charged particles
traversing the TPC gas, has been optimized. After optimization of the FPGA based HLT cluster finder algorithm
and its parameters, detailed studies showed only negligibale differences in the final track reconstruction
parameters compared to using the, by far slower, offline cluster finder.
Having demonstrated this excellent physics performance, ALICE decided to discard the TPC raw data, relying
for the particle reconstruction soley on the HLT cluster finder output. Together with data format optimizations,
entropy-reducing transformations of the cluster data and subsequent lossless Huffman-compression, the HLT
reduced the 20 GB/s input rate to 4 GB/s storage rate, enabling the recording of all events which could be
handeld by the detector read-out system and thus maximizing the physics reach of ALICE.
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