The CMS Online Filter Farm

N. Marinelli et al.

IASA and University of Athens

The future LHC collider will provide the CMS experiment with O (10^9) p-p collisions per second by colliding bunches of protons at a frequency of 40 MHz. In the CMS experiment a maximum of 100KHz Level 1 trigger rate is foreseen, while data at a mean maximum rate of 100Hz can actually be stored. Raw data from the various CMS sub-detector read-out systems, are assembled into complete events by the Event Builder. The Online Filter Farm, consisting of about 1000 commodity processors (Filter Units), processes these event to reduce the rate to storage to the design figure.A Filter Farm Framework manages the collection and dispatching of event data from the Event Builder. A Filter Task performs data un-formatting, physics reconstruction and selection and makes the final accept/reject decision to achieve the final rejection factor by accepting only events with interesting physics content. The reconstruction and selection algorithms used to accomplish this task are the same used for off-line analysis, allowing for consistent development and optimization of the required algorithms. A monitoring system is used to obtain online feedback concerning the quality of the data being collected, while control processors and system monitoring, are managed by the central DAQ control system. In this document the general hardware and software design of the CMS Filter Farm is described and the present status and plans for future developments are reported. Recent results obtained with a prototype version are shown.