  BERT-based information retrieval models are expensive, in both time (query
latency) and computational resources (energy, hardware cost), making many of
these models impractical especially under resource constraints. The reliance on
a query encoder that only performs tokenization and on the pre-processing of
passage representations at indexing, has allowed the recently proposed TILDE
method to overcome the high query latency issue typical of BERT-based models.
This however is at the expense of a lower effectiveness compared to other
BERT-based re-rankers and dense retrievers. In addition, the original TILDE
method is characterised by indexes with a very high memory footprint, as it
expands each passage into the size of the BERT vocabulary. In this paper, we
propose TILDEv2, a new model that stems from the original TILDE but that
addresses its limitations. TILDEv2 relies on contextualized exact term matching
with expanded passages. This requires to only store in the index the score of
tokens that appear in the expanded passages (rather than all the vocabulary),
thus producing indexes that are 99% smaller than those of TILDE. This matching
mechanism also improves ranking effectiveness by 24%, without adding to the
query latency. This makes TILDEv2 the state-of-the-art passage re-ranking
method for CPU-only environments, capable of maintaining query latency below
100ms on commodity hardware.
