Run-3 Commissioning of CMS Online HLT reconstruction using GPUs

The software-based High-Level Trigger (HLT) of CMS reduces the data readout rate from 100 kHz (obtained from Level 1 trigger) to around 5 kHz. It makes use of all detector subsystems and runs a streamlined version of CMS reconstruction. Run-2 of the LHC saw the reconstruction algorithms run on a CPU farm. But the need to have increased computational power as we approach the high luminosity phase of LHC demands the use of Graphical Processing Units (GPUs) to reign in the cost, size and power consumption of the HLT farm. Parallelization of the reconstruction algorithms, on top of the multi-threading functionality introduced in Run2, allowed parts of the Hadronic Calorimeter (HCAL), Electromagnetic Calorimeter (ECAL) and pixel reconstruction to be offloaded to NVIDIA GPUs. In order to ensure the reproducibility of physics results on any machine, the HLT configuration was designed to run seamlessly with and without GPUs, that is, the algorithms were automatically offloaded to a GPU when one was available and otherwise fell back to running on the CPU. This contribution will describe the development of GPU-based algorithms for the HLT and the challenges they presented, along with the comprehensive validation and commissioning activity undertaken by CMS to ensure the successful operations of the new HLT farm.