Sun Dec 8th through Sat the 14th, 2019 at Vancouver Convention Center
Summary: This paper develops a new method for adaptively sampling training examples during stochastic optimization. It is known that the optimal distribution that minimizes the nuclear norm of the covariance of the gradient estimate is one where the the probability of sampling an example is proportional to the magnitude of the gradient of the loss on that example. Sampling according to this distribution is of course impractical, because computing this distribution is as expensive as computing the full gradient and requires O(N) time per iteration. To get around this, prior work either maintains a fixed distribution across all iterations or makes strong assumptions on the distribution of gradients of different training examples (e.g.: the gradients of training examples of the same class are similar). This paper proposes a method that can adaptively sample from different distributions every iteration and requires little assumptions on the distribution of gradients, and yet requires the same per-iteration cost as SGD. This is an important contribution, since conventional wisdom suggests that it is not possible to maintain a distribution over individual training examples from which to sample with a per-iteration cost of less than O(N). (This is referred to as the chicken-and-egg loop in the paper.) This paper shows this is in fact possible for some objective functions and therefore introduces a new class of approaches that can adaptively reduce the variance of gradient estimates without requiring an increase in the per-iteration computational cost. My main suggestion is to more precisely characterize conditions under which the variance of the proposed gradient estimate is smaller than the variance of gradient estimate under uniform sampling. Specifically, p_i should should be upper or lower bounded with the p_1 (true positive probability) and p_2 (false positive probability) associated with the family of hash functions. Then, in the expression for the nuclear norm of the covariance of the gradient estimate, each term should fall into one of three cases: when the gradient magnitude >= t, when it is <= ct, where 0 < c < 1, or when it is between ct and t. Applying the definition of the hash family should give an upper bound on the nuclear norm of the covariance. This should reveal the dependence on p_1 and p_2; one can further optimize over the hyperparameters K and L, which would make dependence of per-iteration cost on p_1, p_2 and the distribution of gradient magnitudes explicit. This should also give sufficient conditions on the distribution of gradient magnitudes (in terms of the proportion of training examples whose gradient magnitudes fall in [0, ct], (ct, t) and [t, inf)) such that the proposed method achieves a lower variance than the uniform sampling scheme. If we then consider the sufficient conditions to keep the nuclear norm of the covariance upper bounded by a constant, this might reveal a dependence on N, and it would be interesting to see what the dependence is. In Sect. 2.3, a few quantities in the expressions for the gradient and the nuclear norm of the covariance are random variables, which makes the expressions random. For example, l is random, because it depends on the sampling over hash tables. Similarly, |S| is random, because it depends on which hash table with a non-empty bucket containing the current value of the parameters is the first to be selected. The randomness for these quantities should be marginalized out to arrive at deterministic expressions, or if this is analytically difficult, high-probability bounds should be derived: to eliminate the randomness in l, you can derive a result of the form "with probability of 1 - \delta, the nuclear norm is upper bounded by some expression, where the probability is over the randomness of sampling of hash tables", and to eliminate the randomness in |S|, it might suffice to consider the worst case, i.e. the largest possible bucket across all parameter values/queries and across all hash tables. Also, the definition of S doesn't seem right, because x doesn't appear anywhere in the expressions for the gradient estimate or the nuclear norm of the covariance. There are more recent nearest neighbor search algorithms that outperform LSH, e.g. DCI [a]. I suspect a sampling version could also be derived, and because MIPS can be reduced nearest neighbor search, such an approach could also be used in the setting considered in this paper. In fact, I believe it could be especially useful here, because DCI does not perform discretization/space partitioning, and so there wouldn’t be the issue of empty buckets. It would be interesting to see how the proposed approach compares to such a baseline. [a] Li & Malik, "Fast k-nearest neighbour search via prioritized DCI", ICML 2017 L232: "by setting small values of K and l" - one does not have the freedom to set l to a particular value, because it is a random variable. What you mean is that by setting K small, l is small with high probability. L233: "if several terms in the summation satisfy |S| / (p_i * N) <= 1" - should this inequality be strict? Also, the proof for this statement should be shown. Minor issues: Eqn. 4 and L123: notation is confusing - angle brackets are typically used to denote inner products rather than concatenation. I would suggest changing them to round brackets. L135: "due to monotonicity" - monotonicity is not enough; you need f to be strictly increasing. If we consider a strictly decreasing function or a constant function (which is non-decreasing), this obviously would not work. L144: "There are few more technical subtleties" - these subtleties should be explained. Also, it should be "a few". Alg. 1: K does not appear in the pseudocode and its meaning is not explained. I assume it's the K from LSH. Also, the notation for collision probability, cp, is a bit confusing, since at first glance it seems to mean some constant some probability p multiplied by some constant c. I would recommend changing it to a single letter. L146: "T is the corresponding feature expansion transformation" - it should be noted that the number of features after expansion grows quadratically in the original dimensionality of the training examples and parameters. Whether or not this introduces complications should be discussed. L186-187: "we can always choose K small enough so that empty buckets are rare". It should be noted that this comes at the cost of having more false positives, i.e. instances where the gradient is actually small in magnitude but gets sampled fairly frequently, because essentially the partitioning of the space is made coarser when K is small. L201: "It is still sub-linear in N (not constant)" - I am not sure if this is a strong argument because making the sampling time constant rather than sub-linear could conceptually make the variance of the gradient estimate increase as N increases, which would lead to slower convergence/more iterations. If the authors believe otherwise, I would suggest analyzing the the variance of the gradient estimate as N increases and showing that the variance is constant in N, perhaps under certain assumptions on the distribution of gradients (as mentioned above). L245: "linear regression and deep learning models" - should clarify whether the deep learning model in question is for regression or classification.
Overall I think the work is interesting but there are problems which make it lower than the NeurIPs standard. - I think the experimental results are not convincing. The major contribution of the paper is s fast sampling scheme, mostly derived from regression problem. So the verify the effectiveness, it should work well on all different types of gradient based training. Comparing only to SGD is not valid enough to claim the scheme works. Comparing on more different types of ML task is also necessary. In particular, BERT is well known for unstable fine-tuning stage so comparing on fine-tuning downstream tasks are not convincing. If the task is to compare different methods on pre-training stage of BERT and have a significant gain, it will be more impressive. - From the point of sampling scheme based on the LSH similarity search, I think it makes sense to compare all different methods in  and  instead of just LSH. Discussing all sort of methods make the present work more complete and have more research value. https://arxiv.org/abs/1806.04189 https://papers.nips.cc/paper/7129-a-greedy-approach-for-budgeted-maximum-inner-product-search.pdf
The idea of incorporating hashing techniques for speeding up adaptive sampling in SGD is interesting. However, if my understanding is correct, the proposed scheme heavily relies on the requirement that the gradient takes the form as a nonlinear transformation of the inner product between the parameter (or augmented by some constant) and a per observation-dependent vector. Therefore, the only example in the paper is linear regression (although the authors mentioned in a short paragraph that the method can also be applied to logistic regression. This raises the concern of how useful this method is for solving general problems. In addition, some arguments in the paper do not seem correct. For example, in line 124, the method assumes each feature vector x_i to be normalized. This does not seem possible since in linear regression, the design can only be normalized per dimension of the feature, not per data point. In addition, the calculation in Theorem 2 just displays the resulting variance, and it is clear when this variance would be smaller than the variance with uniform sampling. I noticed that in the supplement, lemma 1 provides a sufficient condition for so. However, the condition is still very complicated and not easy to verify. Last but not least, I tried to read some proofs in supplementary material, but failed due to bad proofreading. For example, the proof of Lemma 1 refers to equation (16) and (17), but there are only 11 labelled equations in the supplement and 5 in the main paper.