Performance of the High-Level Trigger System at CMS in LHC Run-2

The CMS experiment at the LHC selects events with a two-level trigger system, the Level-1 (L1) trigger, and the high-level trigger (HLT). The HLT reduces the rate from 100 to about 1 kHz and has access to the full detector readout and runs a streamlined version of the offline event reconstruction. During LHC Run-2, the peak instantaneous luminosity reached values up to 2.0×10
³⁴
cm
^-2
s
^-1
, posing a challenge to the online event selection. An overview of the HLT system at CMS, the online physics object reconstruction, and the main triggers using those physics objects in 2016-2018 proton collision data-taking period is presented. The performance of a representative set of physics triggers is also discussed.