The CMS experiment at CERN incorporates one of the highest throughput Data Acquisition (DAQ) systems in High-Energy Physics. Its network throughput will further increase by over an order of magnitude at the High-Luminosity LHC.
The current Run 3 CMS Event Builder receives all the fragments of Level-1 trigger accepted events from the front-end electronics (740 data streams from the CMS subdetectors) and assembles them into complete 2 MB events at a rate of 100 kHz, with a throughput of 1.6 Tbit/s. Its output is handed over to the High Level Trigger (HLT), which runs in a farm consisting of approximately 200 computers equipped with general-purpose GPUs. The HLT selects interesting events at a rate of several kHz. The CMS DAQ will undergo a full upgrade prior to the start of the HL-LHC operation in 2029. The DAQ readout, network, and compute hardware will be entirely replaced to cope with a maximum input rate of 750 kHz and a nominal event size of 8.4 MB.
As with the current system, the Phase-2 event builder will consist of commercial off-the-shelf compute elements interconnected by a high-performance switched network in an all-to-all fashion. The switched network will have to handle an average throughput of about 50 Tb/s. To cope with the increased Level-1 rate, data fragments from individual Level-1 accepts corresponding to one LHC beam revolution (LHC orbit) will be aggregated into "orbit fragments" in the DAQ custom readout electronics. It is these orbit fragments that will then be further aggregated into full orbit data blocks, thus allowing the event builder protocol to work at a constant message rate of about 11 kHz, avoiding excessive message overheads. The final event building will then be delegated to the HLT processes, working each on individual orbit blocks.
To accommodate this new concept of "orbit building", the event builder software components will be entirely redesigned. In this work the key new features of the Phase-2 "event" builder are discussed. A study of new software solutions for the Phase-2 Event Builder and preliminary test benchmarks are presented with related performance results.
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