CMS High Level Trigger performance at 13 TeV

The CMS experiment selects events with a two-level trigger system: the Level-1 trigger (L1) and the High Level Trigger (HLT). The HLT is a farm made of approximately 30k CPU cores that reduces the rate from 100 kHz to about 1 kHz. The HLT has access to the full detector readout and runs a dedicated online event reconstruction to select events. In 2017, LHC instantaneous luminosity during standard operations was about $1.5 \cdot 10^{34}$Hz cm$^{-2}$ with pile-up of 55, well above the design values, and it is expected to exceed $2 \cdot 10^{34}$Hz cm$^{-2}$ in 2018 by increasing the number of proton bunches. In these conditions, the online event selection is very challenging. We present the most recent HLT performance results and the methods used at HLT to cope with a high pile-up environment.