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Evaluation of GPU-based track-triggering for the CMS detector at CERN’s HL-LHC

Hannes Mohr
Faculty for Physics, Karlsruhe Institute of Technology
Karlsruhe Institute of Technology, 2016

@article{mohr2016evaluation,

   title={Evaluation of GPU-based track-triggering for the CMS detector at CERN’s HL-LHC},

   author={Mohr, Hannes},

   year={2016}

}

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In this work we present an evaluation of GPUs as a possible L1 Track Trigger for the High Luminosity LHC, effective after Long Shutdown 3 around 2025. The novelty lies in presenting an implementation based on calculations done entirely in software, in contrast to currently discussed solutions relying on specialized hardware, such as FPGAs and ASICs. Our solution relies on using GPUs for the calculation instead, offering floating point calculations as well as flexibility and adaptability. Normally the involved data transfer latencies make GPUs unfeasible for use in low latency environments. To this end we use a data transfer scheme based on RDMA technology. This mitigates the normally involved overheads. We based our efforts on previous work by the collaboration of the KIT and the English track trigger group [An FPGA-based track finder for the L1 trigger of the CMS experiment at the high luminosity LHC] whose algorithm was implemented on FPGAs. In addition to the Hough transformation used regularly, we present our own version of the algorithm based on a hexagonal layout of the binned parameter space. With comparable computational latency and workload, the approach produces significantly less fake track candidates than the traditionally used method. This comes at a cost of efficiency of around 1 percent. This work focuses on the track finding part of the proposed L1 Track Trigger and only looks at the result of a least squares fit to make an estimate of the performance of said seeding step. We furthermore present our results in terms of overall latency of this novel approach. While not yet competitive, our implementation has surpassed initial expectations and are on the same order of magnitude as the FPGA approach in terms of latencies. Some caveats apply at the moment. Ultimately, more recent technology, not yet available to us in the current discussion will have to be tested and benchmarked to come to a more complete assessment of the feasibility of GPUs as a means of track triggering at the High-Luminosity-LHC’s CMS experiment.
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