Development of a phase-II track trigger based on GPUs for the CMS experiment

The High Luminosity LHC (HL-LHC) is a project to increase the luminosity of the Large Hadron Collider to 5 · 1034 cm-2 s-1. The CMS experiment at CERN is planning a major upgrade in order to cope with an expected average number of overlapping collisions per bunch crossing of 140. A key element of this upgrade will be the introduction of tracker information at the very first stages of the trigger system for which several possible hardware implementations are under study. In particular the adoption of Graphics Processing Units in the first level of the trigger system is currently being investigated in several HEP experiments. Graphics Processing Units (GPUs) are massively parallel architectures that can be programmed using extensions to the standard C and C++ languages. In a synchronous system they have been proven to be highly reliable and to show a deterministic time response even in presence of branch divergences. These two features allow GPUs to be well suited to run pattern recognition algorithms on detector data in a trigger environment. Our discussion of an implementation of a track trigger system based on GPUs will include a description of the framework developed for moving data from and to multiple GPUs using GPUDirect and executing pattern recognition algorithms.