The ATLAS experiment at the Large Hadron Collider employs a two-level trigger
system to record data at an average rate of 1 kHz from physics collisions,
starting from an initial bunch crossing rate of 40 MHz. During the LHC Run 2
(2015-2018), the ATLAS trigger system operated successfully with excellent
performance and flexibility by adapting to the various run conditions
encountered and has been vital for the ATLAS Run-2 physics programme. For
proton-proton running, approximately 1500 individual event selections were
included in a trigger menu which specified the physics signatures and selection
algorithms used for the data-taking, and the allocated event rate and
bandwidth. The trigger menu must reflect the physics goals for a given data
collection period, taking into account the instantaneous luminosity of the LHC
and limitations from the ATLAS detector readout, online processing farm, and
offline storage. This document discusses the operation of the ATLAS trigger
system during the nominal proton-proton data collection in Run 2 with examples
of special data-taking runs. Aspects of software validation, evolution of the
trigger selection algorithms during Run 2, monitoring of the trigger system and
data quality as well as trigger configuration are presented.