LHCb Event Building System

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The task of the LHCb Event Building system is the collection of the data fragments from the Readout Units into an entry node in the Event Filter Farm (the Sub Farm Controller). The transport of the fragments is achieved by means of a large Gigabit Ethernet switching network. The rate of incoming data fragments is the nominal Level 1 trigger accept rate of 40 kHz (upgradeable to 100 kHz). The fragments (some 60 per event) have to be concatenated into one contiguous event at the destination, to be presented to the higher level triggers (Level 2 and 3). 

In the event building project we study transport of the fragment thought the switching network, the behavior of the switches under load, the scaling and behavior of the network as a whole and the various possibilities of the construction of (sub-)events at the various stages during data flow. 

Data Flow

The data flow of the LHCb event building system is unidirectional from the sources (Readout Units) to the destinations (Sub-farm Controllers). There is no synchronization between sources. Every source sends data as soon as it is ready to do so. The destination assignment is static and based on the event number. Data are thus pushed through the system at every stage. The only feed-back in the system is some back-pressure in the network to cope with temporary, localized network contention.


The protocol is adapted to the two basic features of the event building system: 1) pure push-through, 2) high frequency. It is a custom very light protocol implemented on top of Ethernet. It consists of a header and eventually of a trailing error block to collect  errors (mostly time-outs), which occurred during the transport.

Fragment Assembly

At the time 3 possibilities to concatenate the event fragments into one contiguous event are under study:

  1. Smart NICs
  2. Network Processors
  3. The host CPU of the Subfarmcontroller




Links to other sites and useful information

Net processor makers race toward 10-Gbit/s goal  (from EETimes), NPUs seek out software (EETimes), 
The 10 Commandments
- essential reading ;-) ,

"Carrier Sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications" (aka Ethernet :-)) IEEE Standard 802.3 (2000 version)
"PCI  Local Bus Specification Revision 2.2, Dec. 1998" [pdf]
"Tigon/PCI Ethernet Controller Revision 1.04", Databook [pdf], "Tigon Firmware 12.3.11" [pdf]
"IA-32 Intel Architecture Software Developer's Manual", Volume 1 [pdf],  Volume 2 [pdf], Volume 3 [pdf]


This page last edited by Niko Neufeld on November 29, 2001.