To mep or not to mep
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To MEP or not to MEP. Niko Neufeld RTTC meeting, October 26th 2004. The LHCb transport format: MEPs. The online transport format is MEPs: https://edms.cern.ch/document/499933.1

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To MEP or not to MEP

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To MEP or not to MEP

Niko Neufeld

RTTC meeting, October 26th 2004


The LHCb transport format: MEPs

  • The online transport format is MEPs:https://edms.cern.ch/document/499933.1

  • MEP means that event fragments from several consecutive triggers are merged into one MEP and send as a single IPv4 packet

  • MEPs are used in HLT and L1 to reduce the overhead incurred by sending small packets over the network

  • The number of fragments in a MEP is called the packing factor (PF) - it is a constant (during a run), adjustable global parameter of the system

  • The nominal PFs for L1 and HLT are 25 and 10 respectively

Niko Neufeld

CERN, PH


Sending Events in the RTTC

  • In the RTTC we will have a single source of data (a disk-server) sending to at least one sub-farm (most likely three)

  • This means we could send completely build events to the SFC and the SFC would just “forward” them

  • I think we should not opt for this strategy

Niko Neufeld

CERN, PH


Fully realistic event-builder in action with all timing relevant overheads, packet handling issues

No special version of event-builder code

Possibility to much more realistic robustness tests (missing/corrupted MEP from single source will be the most likely error in the final system)

Need to reformat raw data into MEPs: not difficult, study in progress, will present results in next meeting

Need real event-builder:

no problem: will be tested and ready by end of this year (c.f. Benjamin’s talk)

sub-farms will not run at 120 MB/s --> even a less performing SFC can do the job

The case for MEPs

Niko Neufeld

CERN, PH


“To MEP or not to MEP, that is the question here”

  • I think, “’Tis MEPness, but there is a method in it” - or leaving Shakespeare aside:

  • For the price of a few technical details (re-formatting of raw-data) we get a fully realistic system - we should go for MEPs

Niko Neufeld

CERN, PH


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