  This work focuses on the problem of hyper-parameter tuning (HPT) for robust
(i.e., adversarially trained) models, shedding light on the new challenges and
opportunities arising during the HPT process for robust models. To this end, we
conduct an extensive experimental study based on 3 popular deep models, in
which we explore exhaustively 9 (discretized) HPs, 2 fidelity dimensions, and 2
attack bounds, for a total of 19208 configurations (corresponding to 50
thousand GPU hours). Through this study, we show that the complexity of the HPT
problem is further exacerbated in adversarial settings due to the need to
independently tune the HPs used during standard and adversarial training:
succeeding in doing so (i.e., adopting different HP settings in both phases)
can lead to a reduction of up to 80% and 43% of the error for clean and
adversarial inputs, respectively. On the other hand, we also identify new
opportunities to reduce the cost of HPT for robust models. Specifically, we
propose to leverage cheap adversarial training methods to obtain inexpensive,
yet highly correlated, estimations of the quality achievable using
state-of-the-art methods. We show that, by exploiting this novel idea in
conjunction with a recent multi-fidelity optimizer (taKG), the efficiency of
the HPT process can be enhanced by up to 2.1x.
