Journal paper accepted at ACM Transactions on Information Systems [1].

Abstract. Learning-to-Rank models based on additive ensembles of regression trees have been proven to be very effective for scoring query results returned by large-scale Web search engines. Unfortunately, the computational cost of scoring thousands of candidate documents by traversing large ensembles of trees is high. Thus, several works have investigated solutions aimed at improving the efficiency of document scoring by exploiting advanced features of modern CPUs and memory hierarchies. In this paper, we present QuickScorer, a new algorithm that adopts a novel cache-efficient representation of a given tree ensemble, it performs an interleaved traversal by means of fast bitwise operations, and also supports ensembles of oblivious trees. An extensive and detailed test assessment is conducted on two standard Learning-to-Rank datasets and on a novel very-large dataset we made publicly available for conducting significant efficiency tests. The experiments show unprecedented speedups over the best state-of-the-art baselines ranging from 1.9x to 6.6x. The analysis of low-level profiling traces shows that QuickScorer efficiency is due to its cache-aware approach both in terms of data layout and access patterns, and to a control flow that entails very low branch mis-prediction rates.