Event-building and PC farm based Level-3 trigger at the CDF experiment

In the technical design report the event building process at Fermilab's CDF experiment is required to function at an event rate of 300 events/sec. The events are expected to have an average size of 150 kBytes (kB) and are assembled from fragments of 16 readout locations. The fragment size from the different locations varies between 12 kB and 16 kB. Once the events are assembled they are fed into the Level-3 trigger which is based on processors running programs to filter events using the full event information. Computing power on the order of a second on a Pentium II processor is required per event. The architecture design is driven by the cost and is therefore based on commodity components: VME processor modules running VxWorks for the readout, an ATM switch for the event building, and Pentium PCs running Linux as an operation system for the Level-3 event processing. Pentium PCs are also used to receive events from the ATM switch and further distribute them to the processing nodes over multiple 100 Mbps Ethernets. Studies with a prototype of up to 10 VME readout modules and up to 4 receiving PCs are presented. This system is also a candidate for the CMS experiment at CERN.