Paper:
Scaling Gaussian Process Regression with Derivatives
Gaussian processes (GPs) with derivatives are useful in many applications, including Bayesian optimization, implicit surface reconstruction, and terrain reconstruction. Fitting a GP to function values and derivatives at n points in d dimensions requires linear solves and log determinants with an n(d + 1) × n(d + 1) positive definite matrix -leading to prohibitive O(n 3 d 3 ) computations for standard direct methods. We propose iterative solvers using fast O(nd) matrix-vector multiplications (MVMs), together with pivoted Cholesky preconditioning that cuts the iterations to convergence by several orders of magnitude, allowing for fast kernel learning and prediction. Our approaches, together with dimensionality reduction, enables Bayesian optimization with derivatives to scale to high-dimensional problems and large evaluation budgets.

Review:
The paper investigates the application of the recent SKI and SKIP GP methodologies in the case where additional gradient information is available. They derive the necessary mathematical background, discuss numerical strategies to make the computations robust efficient and efficient and evaluate extensively on established synthetic and real benchmark problems.  A weakness of the paper is the somewhat entangled discussion of dimensionality reduction strategies. D-SKI scales with 6^d, which is approximately 60 million for d=10 and roughly half a trillion for d=15. These numbers demonstrate that D-SKIP will not be optional for many problems typically considered with BO. From the presented experiments, it is unclear to me how the "online dimension reduction" would influence the BO algorithm. Additionally, I find the description in lines 205-211 too vague to be able to understand the experimental evaluation. I will not consider this a problem for the submission as long as the authors hold up their promise of providing the experiment code. (As a minor side note: you might want to remind the reader of reference [25] at this point in the text.)  Nevertheless, a strength of the paper is its holistic description and evaluation of the proposed method. The background and the newly extended analysis seem straight forward, so the authors make good use of the page limit to discuss important practical strategies like preconditioning and provide a principled evaluation. Their basic experiments are run on common test functions which could in principle be used for any desired mathematical analysis. I agree with their rationale on terrain and surface reconstruction and the experiments are convincing.  In terms of significance, I am hesitant to provide a clear judgement. Personally, I have not worked on surface reconstruction before, so I don't know how big this application area is. In terms of originality, this seems to be a clear continuation from early work on SKI and SKIP. As a consequence, I am not surprised by this work, but I can imagine that providing this detailed derivation (and, possibly, the code) was still a highly non-trivial issue, in particular with respect to the pre-conditioning evaluation and experiments.  In conclusion: in my opinion, this is a solid piece of research, whose lack of ground-braking novelty is made up by the good presentation and experimental evaluation.  ---- Post-rebuttal update: Thanks for the authors for the clarifications. I very much appreciated your comment regarding the computational complexity of D-SKIP and I encourage the authors to put this information into the paper as well. I am willing to follow your arguments regarding significance and novelty, and I will support this view in the discussion. I have not updated my score of the overall review as this score already reflected my ignorance in terms of significance "in dubio pro reo".