Benchmarking Data Acquisition event building network performance for the ATLAS HL-LHC upgrade

The ATLAS experiment’s data acquisition (DAQ) system will be extensively updated to take full advantage of the High-Luminosity LHC (HL- LHC) upgrade, allowing it to record data at unprecedented rates. The detector will be read out at 1 MHz, generating over 5 TB/s of data. This design poses significant challenges for the Ethernet-based network, which will have to trans- port 20 times more data than during Run 3. The increased data rate, data sizes and number of servers will exacerbate the TCP Incast effect observed in the past, making it impossible to fully exploit the capabilities of the network and limiting the performance of the processing farm. We present exhaustive and systematic experiments to define buffering requirements in network equipment to minimise the effects of TCP Incast and reduce the impact on processing applications. Both deep and shallow buffer switches were stress tested using DAQ traffic patterns in a test environment at approximately 10% of the expected HL-LHC DAQ system size. As the desired HL-LHC system hardware is not currently avail- able and the laboratory size is significantly smaller, the tests aim to extrapolate buffer requirements to the expected operating point. A novel analytical formula and new simulation models have been developed to cross-validate the results. The results of these evaluations will contribute to the decision-making process for the acquisitions of network hardware for the HL-LHC DAQ.