Evaluation of likelihood functions for data analysis on Graphics Processing Units

Data analysis techniques based on likelihood function calculation play a crucial role in many High Energy Physics measurements. Depending on the complexity of the models used in the analyses, with several free parameters, many independent variables, large data samples, and complex functions, the calculation of the likelihood functions can require a long CPU execution time. In the past, the continuous gain in performance for each single CPU core kept pace with the increase on the complexity of the analyses, maintaining reason- able the execution time of the sequential software applications. Nowadays, the performance for single cores is not increasing as in the past, while the complexity of the analyses has grown significantly in the Large Hadron Collider era. In this context a breakthrough is represented by the increase of the number of computational cores per computational node. This allows to speed up the execution of the applications, redesigning them with parallelization paradigms. The likelihood function evaluation can be parallelized using data and task parallelism, which are suitable for CPUs and GPUs (Graphics Processing Units), respectively. In this paper we show how the likelihood function evaluation has been parallelized on GPUs. We describe the implemented algorithm and we give some performance results when running typical models used in High Energy Physics measurements. In our implementation we achieve a good scaling with respect to the number of events of the data samples.