LHCb Event Filter Farm

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The trigger system of the LHCb detector has several levels. The level 0 and level 1 triggers are implemented in hardware and investigate only a small part of the data collected by the LHCb Detector. After these first two trigger levels, the whole detector is read out by the Data Acquisition System and the data needs to be passed through the read-out network to the LHCb Event Filter Farm, where the additional trigger levels 2 and 3 are performed in software. These trigger levels include the entire data collected by the detector in their evaluation of an event.

Figure 1 - The LHCb DAQ system architecture

More specifically, after event-fragments have been collected by the Read-out Units (RU), they are passed through the Read-out Network (RN) in such a way that all data belonging to one particular event reaches one specific Sub-Farm Controller (SFC).

There will be ~ 25-30 sub-farms, each containing a Sub-Farm Controller (SFC) and ~ 10-20 PCs or "nodes". Any data traffic from or to the nodes has to go through and is controlled by the SFC of the sub-farm.

A SFC therefore receives multiple segments of data belonging to the same event. It assembles them and reorders them forming an event data-structure, which now contains all the data collected about the actual event. This process is called "Event-building".

After a SFC has built a new event it will pass it on to exactly one of its nodes. The node receiving a new event will perform the level 2 and 3 trigger algorithms and, if the event is accepted, it will write back the processed event data to permanent storage. As usual, this data traffic goes through the SFC as well.

Figure 2 - Sub-farm architecture

Figure 2 shows the architecture of the sub-farms. The SFC is connected to a switch, which links it to its nodes. The link between the SFC and the switch is a Gigabit Ethernet connection (unidirectional nominal throughput: 1000 Mbit/s). The links between the switch and each of the sub-farms are fast Ethernet connection (nominal throughput per link: 100 Mbit/s).

Each SFC should support the forwarding of events at ~ 80 Mbyte/s (640 Mbit/s). Notice that this implies an incoming data traffic of 80 Mbyte/s and an outgoing traffic of 80 Mbyte/s at the same time.





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This page was last edited by Wulf Thannhaeuser on September 24, 2002.