The software based High Level Trigger (HLT) of CMS reduces the data readout rate from 100kHz (obtained from Level 1 trigger) to around 2kHz. It makes use of all detector subsystems and runs a streamlined version of CMS reconstruction. Run-1 and Run-2 of the LHC saw the reconstruction algorithm run on a CPU farm (~30000 CPUs in 2018). 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 HCAL, 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.
|Consider for long presentation||Yes|