  At the Large Hadron Collider at CERN in Geneva, Switzerland, atomic nuclei
are collided at ultra-relativistic energies. Many final-state particles are
produced in each collision and their properties are measured by the ALICE
detector. The detector signals induced by the produced particles are digitized
leading to data rates that are in excess of 48 GB/$s$. The ALICE High Level
Trigger (HLT) system pioneered the use of FPGA- and GPU-based algorithms to
reconstruct charged-particle trajectories and reduce the data size in real
time. The results of the reconstruction of the collision events, available
online, are used for high level data quality and detector-performance
monitoring and real-time time-dependent detector calibration. The online data
compression techniques developed and used in the ALICE HLT have more than
quadrupled the amount of data that can be stored for offline event processing.
