  We propose and compare methods for gradient-based domain adaptation of
self-attentive neural machine translation models. We demonstrate that a large
proportion of model parameters can be frozen during adaptation with minimal or
no reduction in translation quality by encouraging structured sparsity in the
set of offset tensors during learning via group lasso regularization. We
evaluate this technique for both batch and incremental adaptation across
multiple data sets and language pairs. Our system architecture - combining a
state-of-the-art self-attentive model with compact domain adaptation - provides
high quality personalized machine translation that is both space and time
efficient.
