  The ALICE experiment has undergone a major upgrade for LHC Run 3 and will
collect data at an interaction rate 50 times larger than before. The new
computing scheme for Run 3 replaces the traditionally separate online and
offline frameworks by a unified one, which is called O2. Processing will happen
in two phases. During data taking, a synchronous processing phase performs data
compression, calibration, and quality control on the online computing farm. The
output is stored on an onsite disk buffer. When there is no beam in the LHC,
the same computing farm is used for the asynchronous reprocessing of the data
which yields the final reconstruction output. The O2 project consists of three
main parts:. The Event Processing Nodes (EPN) equipped with GPUs deliver the
bulk of the computing capacity and perform the majority of the reconstruction
and the calibration. The First Level Processors (FLP) receive the data via
optical links from the detectors and perform local processing where it is
needed, which can optionally happen in the user logic of the FPGA based readout
card. Between the FLP and the EPN farms the data is distributed in the network
such that the EPNs receive complete collision data for the processing. The
Physics and Data Processing (PDP) group develops the software framework and the
reconstruction and calibration algorithms. The current O2 setup is capable of
handling in real time the peak data rate foreseen for data taking of Pb--Pb
collisions at 50 kHz interaction rate..
