The CMS High Level Trigger

The High Level Trigger (HLT) system of the CMS experiment will consist of a series of reconstruction and selection algorithms designed to reduce the Level-1 trigger accept rate of 100 kHz to 100 Hz forwarded to permanent storage. The HLT operates on events assembled by an event builder collecting detector data from the CMS front-end system at full granularity and resolution. The HLT algorithms will run on a farm of commodity PCs, the filter farm, with a total expected computational power of 106 SpecInt95. The farm software, responsible for collecting, analyzing, and storing event data, consists of components from the data acquisition and the offline reconstruction domains, extended with the necessary glue components and implementation of interfaces between them. The farm is operated and monitored by the DAQ control system and must provide near-real-time feedback on the performance of the detector and the physics quality of data. In this paper, the architecture of the HLT farm is described, and the design of various software components reviewed. The status of software development is presented, with a focus on the integration issues. The physics and CPU performance of current reconstruction and selection algorithm prototypes is summarized in relation with projected parameters of the farm and taking into account the requirements of the CMS physics program. Results from a prototype test stand and plans for the deployment of the final system are finally discussed.