A Linux-PC farm for online event reconstruction at HERA-B

Summary form only given. HERA-B's physics program is dedicated to study CP violation in decays of neutral B mesons. To allow for immediate data analysis and to avoid time-consuming re-processing of data, events will be reconstructed online in the last step of the four level DAQ and trigger system before being logged to tape. The online event reconstruction is performed on a PC farm. To process events at a rate of 50 Hz with processing times of 2-4 sec, up to 200 farm nodes are needed. With an average of 100 kB/event, the input rate to the farm is 5 MB/sec. The farm nodes are grouped into so-called mini-farms with 20 nodes each and are interconnected by standard Fast-Ethernet. In the last few years Linux has become a basic software development platform for online and offline in HEP. In HERA-B, system and control software as well as event reconstruction programs are provided for Linux. In 1998 and early 1999 two test runs have been carried out in preparation of a long physics run, starting summer 1999. Twenty farm nodes were installed and included into the data path. Until mid 1999 the system will be upgraded to 50 nodes. In the contribution to Real-Time 99 concept and implementation of the hard- and software are described and discussed. We focus on experiences which have been gained during the test runs, especially on the performance of the system, having the upcoming first physics run in mind.