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Neural networks can converge faster with help from a smarter batch selection strategy.
In this regard, we propose Ada-Boundary, a novel adaptive-batch selection algorithm that constructs an effective mini-batch according to the learning progress of the model.
Our key idea is to present confusing samples what the true label is.
Thus, the samples near the current decision boundary are considered as the most effective to expedite convergence.
Taking advantage of our design, Ada-Boundary maintains its dominance in various degrees of training difficulty.
We demonstrate the advantage of Ada-Boundary by extensive experiments using two convolutional neural networks for three benchmark data sets.
The experiment results show that Ada-Boundary improves the training time by up to 31.7% compared with the state-of-the-art strategy and by up to 33.5% compared with the baseline strategy.
Deep neural networks (DNNs) have achieved remarkable performance in many fields, especially, in computer vision and natural language processing BID13 BID5 .
Nevertheless, as the size of data grows very rapidly, the training step via stochastic gradient descent (SGD) based on mini-batches suffers from extremely high computational cost, which is mainly due to slow convergence.
The common approaches for expediting convergence include some SGD variants BID28 BID11 that maintain individual learning rates for parameters and batch normalization BID9 that stabilizes gradient variance.
Recently, in favor of the fact that not all samples have an equal impact on training, many studies have attempted to design sampling schemes based on the sample importance BID25 BID3 (1) at the training accuracy of 60%.
An easy data set (MNIST) does not have "too hard" sample but "moderately hard" samples colored in gray, whereas a relatively hard data set (CIFAR-10) has many "too hard" samples colored in black.
(b) shows the result of SGD on a hard batch.
The moderately hard samples are informative to update a model, but the too hard samples make the model overfit to themselves.
et al., 2017; BID10 .
Curriculum learning BID0 ) inspired by human's learning is one of the representative methods to speed up the training step by gradually increasing the difficulty level of training samples.
In contrast, deep learning studies focus on giving higher weights to harder samples during the entire training process.
When the model requires a lot of epochs for convergence, it is known to converge faster with the batches of hard samples rather than randomly selected batches BID21 BID17 BID4 .
There are various criteria for judging the hardness of a sample, e.g., the rank of the loss computed from previous epochs BID17 .Here, a natural question arises: Does the "hard" batch selection always speed up DNN training?
Our answer is partially yes: it is helpful only when training an easy data set.
According to our indepth analysis, as demonstrated in FIG1 (a), the hardest samples in a hard data set (e.g., CIFAR-10) were too hard to learn.
They are highly likely to make the decision boundary bias towards themselves, as shown in FIG1 (b) .
On the other hand, in an easy data set (e.g., MNIST), the hardest samples, though they are just moderately hard, provide useful information for training.
In practice, it was reported that hard batch selection succeeded to speed up only when training the easy MNIST data set BID17 BID4 , and our experiments in Section 4.4 also confirmed the previous findings.
This limitation calls for a new sampling scheme that supports both easy and hard data sets.
In this paper, we propose a novel adaptive batch selection strategy, called Ada-Boundary, that accelerates training and is better generalized to hard data sets.
As opposed to existing hard batch selection, Ada-Boundary picks up the samples with the most appropriate difficulty, considering the learning progress of the model.
The samples near the current decision boundary are selected with high probability, as shown in FIG3 (a).
Intuitively speaking, the samples far from the decision boundary are not that helpful since they are either too hard or too easy: those on the incorrect (or correct) side are too hard (or easy).
This is the reason why we regard the samples around the decision boundary, which are moderately hard, as having the appropriate difficulty at the moment.
Overall, the key idea of Ada-Boundary is to use the distance of a sample to the decision boundary for the hardness of the sample.
The beauty of this design is not to require human intervention.
The current decision boundary should be directly influenced by the learning progress of the model.
The decision boundary of a DNN moves towards eliminating the incorrect samples as the training step progresses, so the difficulty of the samples near the decision boundary gradually increases as the model is learned.
Then, the decision boundary keeps updated to identify the confusing samples in the middle of SGD, as illustrated in FIG3 (b) .
This approach is able to accelerate the convergence speed by providing the samples suited to the model at every SGD iteration, while it is less prone to incur an overfitting issue.
We have conducted extensive experiments to demonstrate the superiority of Ada-Boundary.
Two popular convolutional neural network (CNN) 1 models are trained using three benchmark data sets.
Compared to random batch selection, Ada-Boundary significantly reduces the execution time by 14.0-33.5%.
At the same time, it provides a relative improvement of test error by 7.34-14.8% in the final epoch.
Moreover, compared to the state-of-the-art hard batch selection BID17 , Ada-Boundary achieves the execution time smaller by 18.0% and the test error smaller by 13.7% in the CIFAR-10 data set.2 Ada-Boundary COMPONENTS The main challenge for Ada-Boundary is to evaluate how close a sample is to the decision boundary.
In this section, we introduce a novel distance measure and present a method of computing the sampling probability based on the measure.
To evaluate the sample's distance to the decision boundary, we note that the softmax distribution, which is the output of the softmax layer in neural networks, clearly distinguishes how confidently the learner predicts and whether the prediction is right or wrong, as demonstrated in FIG5 .
If the prediction probability of the true label is the highest, the prediction is correct; otherwise, incorrect.
If the highest probability dominates the distribution, the model's confidence is strong; otherwise, weak.
Let h(y|x i ; θ t ) be the softmax distribution of a given sample x i over y ∈ {1, 2, . . .
, k} labels, where θ t is the parameter of a neural network at time t.
Then, the distance from a sample x i with the true label y i to the decision boundary of the neural network with θ t is defined by the directional distance function in Eq. (1).
More specifically, the function consists of two terms related to the direction and magnitude of the distance, determined by the model's correctness and confidence, respectively.
The correctness is determined by verifying whether the label with the highest probability matches the true label y i , and the confidence is computed by the standard deviation of the softmax distribution.
Intuitively, the standard deviation is a nice indicator of the confidence because the value gets closer to zero when the learner confuses.
DISPLAYFORM0 One might argue that the cross-entropy loss, H(p, q) = −p(x i ) log(q(x i )) where p(x i ) and q(x i ) are the true and softmax distributions for x i , can be adopted for the distance function.
However, because p(x i ) is formulated as a one-hot true label vector, the cross-entropy loss cannot capture the prediction probability for false labels, which is an important factor of confusing samples.
Another advantage is that our distance function is bounded as opposed to the loss.
For k labels, the maximum value of std (h(y|x i DISPLAYFORM1 The rank-based approach introduced by BID17 is a common way to make the sampling probability of being selected for the next mini-batch.
This approach sorts the samples by a certain importance measure in descending order, and exponentially decays the sampling probability of a given sample according to its rank.
Let N denote the total number of samples.
Then, each r-th ranked sample is selected with the probability p(r) which drops by a factor of exp (log(s e )/N ).Here, s e is the selection pressure parameter that affects the probability gap between the most and the least important samples.
When normalized to sum up to 1.0, the probability of the r-th ranked sample's being selected is defined by Eq. (3).
DISPLAYFORM2 In the existing rank-based approach, the rank of a sample is determined by |dist(x i , y i ; θ t )| in ascending order, because it is inversely proportional to the sample importance.
However, if the mass of the true sample distribution is skewed to one side (e.g., easy side) as shown in FIG6 , the mini-batch samples are selected with high probability from the skewed side rather than around the decision boundary where |dist(x i , y i ; θ t )| is very small.
This problem was attributed to unconditionally fixed probability to a given rank.
In other words, the samples with similar ranks are selected with similar probabilities regardless of the magnitude of the distance values.
To incorporate the impact of the distance into batch selection, we adopt the quantization method BID6 BID2 ) and use the quantization index q instead of the rank r. Let Δ be the quantization step size and d be the output of the function dist(x i , y i ; θ t ) of a given sample x i .
Then, the index q is obtained by the quantizer Q(d) as in Eq. (4).
The quantization index gets larger as a sample moves away from the decision boundary.
In addition, the difference between two indexes reflects the difference in the actual distances.
DISPLAYFORM3 In Eq. (4), we set Δ to be k −1 √ k − 1/N such that the index q is bounded to N (the total number of samples) by Eq. (2).
The sampling probability of a given sample x i with the true label y i is defined as Eq. (5).
As shown in FIG6 , our quantization-based method provides a well-balanced distribution, even if the true sample distribution is skewed.
DISPLAYFORM4 3 Ada-Boundary ALGORITHM 3.1 MAIN PROPOSED ALGORITHM Algorithm 1 describes the overall procedure of Ada-Boundary.
The input to the algorithm consists of the samples of size N (i.e., training data set), the mini-batch size b, the selection pressure s e , and the threshold γ used to decide the warm-up period.
In the early stages of training, since the quantization index for each sample is not confirmed yet, the algorithm requires the warm-up period during γ epochs.
Randomly selected mini-batch samples are used to warm-up (Lines 6-7), and their quantization indexes are updated (Lines 11-16).
After the warm-up epochs, the algorithm computes the sampling probability of each sample by Eq. (5) and selects mini-batch samples based on the probability (Lines 8-10).
Then, the quantization indexes are updated in the same way (Lines 11-16).
Here, we compute the indexes using the model with θ t+1 after every SGD step rather than every epoch, in order to reflect the latest state of the model; besides, we asynchronously update the indexes of the samples only included in the mini-batch, to avoid the forward propagation of the entire samples which induces a high computational cost.
For a more sophisticated analysis of sampling strategies, we modify a few lines of Algorithm 1 to present three heuristic sampling strategies, which are detailed in Appendix A. (i) Ada-Easy is designed to show the effect of easy samples on training, so it focuses on the samples far from the decision boundary to the positive direction.
DISPLAYFORM5 (ii) Ada-Hard is similar to the existing hard batch strategy BID17 , but it uses our distance function instead of the loss.
That is, Ada-Hard focuses on the samples far from the decision boundary to the negative direction, which is the opposite of Ada-Easy. (iii) Ada-Uniform is designed to select the samples for a wide range of difficulty, so it samples uniformly over the distance range regardless of the sample distribution.
FIG7 shows the distributions of mini-batch samples drawn by these three variants.
The distribution of Ada-Easy is skewed to the easy side, that of Ada-Hard is skewed to the hard side, and that of Ada-Uniform tends to be uniform.
To avoid additional inference steps of Ada-Boundary (Line 14 in Algorithm 1), we present a historybased variant, called Ada-Boundary(History).
It updates the qunatization indexes using the previous model with θ t .
See Appendix B for the detailed algorithm and experiment results.
In this section, all the experiments were performed on three benchmark data sets: MNIST 2 of handwritten digits (LeCun, 1998) with 60,000 training and 10,000 testing images; Fashion-MNIST 3 of various clothing BID26 with 60,000 training and 10,000 testing images; and CIFAR-10 4 of a subset of 80 million categorical images BID12 with 50,000 training and 10,000 testing images.
We did not apply any data augmentation and pre-processing procedures.
A simple model LeNet-5 (LeCun et al., 2015) was used for two easy data sets, MNIST and Fasion-MNIST.
A complex model WideResNet-16-8 (Zagoruyko and Komodakis, 2016) was used for a relatively difficult data set, CIFAR-10.
Batch normalization BID9 was applied to both models.
As for hyper-parameters, we used a learning rate of 0.01 and a batch size of 128; the training epoch was set to be 50 for LeNet-5 and 70 for WideResNet-16-8, which is early stopping to clearly show the difference in convergence speed.
Regarding those specific to our algorithm, we set the selection pressure s e to be 100, which is the best value found from s e = {10, 100, 1000} on the three data sets, and set the warm-up threshold γ to be 10.
Technically, a small γ was enough to warm-up, but to reduce the performance variance caused by randomly initialized parameters, we used the larger γ and shared model parameters for all strategies during the warm-up period.
Due to the lack of space, the experimental results using DenseNet (L = 25, k = 12) BID8 on two hard data sets, CIFAR-100 4 and Tiny-ImageNet 5 , are discussed in Appendix C together with the impact of the selection pressure s e .
We compared Ada-Boundary with not only random batch selection but also four different adaptive batch selections.
Random batch selection selects the next batch uniformly at random from the entire data set.
One of four adaptive selections is the state-of-the-art strategy that selects hard samples based on the loss-rank, which is called online batch selection BID17 , and the remainders, Ada-Easy, Ada-Hard, and Ada-Uniform, are the three variants introduced in Section 3.2.
All the algorithms were implemented using TensorFlow 6 and executed using a single NVIDIA Tesla V100 GPU on DGX-1.
For reproducibility, we provide the source code at https://github.
com/anonymized.
To measure the performance gain over the baseline (random batch selection) as well as the state-ofart (online batch selection), we used the following three metrics.
We repeated every test five times for robustness and reported the average.
The wall-clock training time is discussed in Appendix D. (ii) Gain epo : Reduction in number of epochs to obtain the same error (%).
In FIG9 (a), the test error of 1.014 · 10 2 achieved at the 50th epoch by random batch selection can be achieved only at the 29th epoch by Ada-Boundary.
Thus, Gain err was (50 − 29)/50 × 100 = 42.0%.(iii) Gain tim : Reduction in running time to obtain the same error (%).
In FIG9 FIG9 shows the convergence curves of training loss and test error for five batch selection strategies on three data sets, when we used the SGD optimizer for training.
In order to improve legibility, only the curves for the baseline and proposed strategies are dark colored; thus, the three metrics in the figure were calculated against the baseline strategy, random batch selection.
Owing to the lack of space, we discuss the results with the momentum optimizer in Appendix E. Ada-Easy was excluded in FIG9 because its convergence speed was much slower than other strategies.
That is, easy samples did not contribute to expedite training.
We conduct convergence analysis of the five batch selection strategies for the same number of epochs, as follows:
• MNIST FIG9 (a)): All adaptive batch selections achieved faster convergence speed compared with random batch selection.
Ada-Boundary, Ada-Hard, and online batch selection showed similar performance.
Ada-Uniform was the fastest at the beginning, but its training loss and test error increased sharply in the middle of the training or testing procedures.• Fashion-MNIST FIG9 ): Ada-Boundary showed the fastest convergence speed in both training loss and test error.
In contrast, after warm-up epochs, the training loss of the other adaptive batch selections increased temporarily, and their test error at the final epoch became similar to that of random batch selection.• CIFAR-10 FIG9 ): Ada-Boundary and Ada-Hard showed the fastest convergence on training loss, but in test error, the convergence speed of Ada-Hard was much slower than that of Ada-Boundary.
This means that focusing on hard samples results in the overfitting to "too hard" samples, which is indicated by a larger difference between the converged training loss (error) and the converged test error.
Also, the slow convergence speed of online batch selection in test error is explained by the same reason.
In summary, in the easiest MNIST data set, all adaptive batch selections accelerated their convergence speed compared with random batch selection.
However, as the training difficulty (complexity) increased from MNIST to Fashion-MNIST and further to CIFAR-10, only Ada-Boundary converged significantly (by Gain err ) faster than random batch selection.
We clarify the quantitative performance gains of Ada-Boundary over random batch and online batch selections in TAB3 .
Ada-Boundary significantly outperforms both strategies, as already shown in FIG9 .
There is only one exception in MNIST, because online batch selection is known to work well with an easy data set BID17 .
The noticeable advantage of AdaBoundary is to reduce the training time significantly by up to around 30%, which is really important for huge, complex data sets.
There have been numerous attempts to understand which samples contribute the most during training.
Curriculum learning BID0 , inspired by the perceived way that humans and animals learn, first takes easy samples and then gradually increases the difficulty of samples in a manual manner.
Self-paced learning BID14 uses the prediction error to determine the easiness of samples in order to alleviate the limitation of curriculum learning.
They regard that the importance is determined by how easy the samples are.
However, easiness is not sufficient to decide when a sample should be introduced to a learner BID4 .Recently, BID24 used Bayesian optimization to optimize a curriculum for training dense, distributed word representations.
BID20 emphasized that the right curriculum not only has to arrange data samples in the order of difficulty, but also introduces a small number of samples that are dissimilar to the previously seen samples.
BID23 proposed a hard-example mining algorithm to eliminate several heuristics and hyper-parameters commonly used to select hard examples.
However, these algorithms are designed to support only a designated task, such as natural language processing or region-based object detection.
The neural data filter proposed by BID3 is orthogonal to our work because it aims at filtering the redundant samples from streaming data.
As mentioned earlier, Ada-Boundary in general follows the philosophy of curriculum learning.
More closely related to the adaptive batch selection, BID17 keep the history of losses for previously seen samples, and compute the sampling probability based on the loss rank.
The sample probability to be selected for the next mini-batch is exponentially decayed with its rank.
This allows the samples with low ranks (i.e., high losses) are considered more frequently for the next mini-batch.
Gao and Jojic (2017)'s work is similar to BID17 's work except that gradient norms are used instead of losses to compute the probability.
In contrast to curriculum learning, both methods focus on only hard samples for training.
Also, they ignore the difference in actual losses or gradient norms by transforming the values to ranks.
We have empirically verified that Ada-Boundary outperforms online batch selection BID17 , which is regarded as the state-of-the-art of this category.
Similar to our work, BID1 claimed that the uncertain samples should be preferred during training, but their main contribution lies on training more accurate and robust model by choosing samples with high prediction variances.
In contrast, our main contribution lies on training faster using confusing samples near the decision boundary.
For the completeness of the survey, we mention the work to accelerate the optimization process of conventional algorithms based on importance sampling.
BID19 re-weight the obtained gradients by the inverses of their sampling probabilities to reduce the variance.
BID22 biased the sampling to the Lipschitz constant to quickly find the solution of a strongly-convex optimization problem arising from the training of conditional random fields.
In this paper, we proposed a novel adaptive batch selection algorithm, Ada-Boundary, that presents the most appropriate samples according to the learning progress of the model.
Toward this goal, we defined the distance from a sample to the decision boundary and introduced a quantization method for selecting the samples near the boundary with high probability.
We performed extensive experiments using two CNN models for three benchmark data sets.
The results showed that Ada-Boundary significantly accelerated the training process as well as was better generalized in hard data sets.
When training an easy data set, Ada-Boundary showed a fast convergence comparable to that of the state-of-the-art algorithm; when training relatively hard data sets, only Ada-Boundary converged significantly faster than random batch selection.
The most exciting benefit of Ada-Boundary is to save the time needed for the training of a DNN.
It becomes more important as the size and complexity of data becomes higher, and can be boosted with recent advance of hardware technologies.
Our immediate future work is to apply Ada-Boundary to other types of DNNs such as the recurrent neural networks (RNN) BID18 and the long short-term memory (LSTM) BID7 , which have a neural structure completely different from the CNN.
In addition, we plan to investigate the relationship between the power of a DNN and the improvement of Ada-Boundary.
For Ada-Easy which prefers easy samples to hard samples, q should be small for the sample located deep in the positive direction.
For Ada-Hard, q should be small for the sample located deep in the negative direction.
Thus, Ada-Easy and Ada-Hard can be implemented by modifying the quantizers Q(d) in Line 16 of Algorithm 1.
When we set Δ = k −1 √ k − 1/N to make the index q bound to N , the quantizers of Ada-Easy and Ada-Hard are defined as Eqs. (6) and (7) , respectively.
DISPLAYFORM0 Ada-Uniform can be implemented by using F −1 (x) to compute the sampling probability in Line 9 of Algorithm 1, where F (x) is the empirical sample distribution according to the sample's distance to the decision boundary.
C Ada-Boundary ON TWO HARD DATA SETS As a practical paper, we include the experimental results on two more challenging data sets: CIFAR-100 composed of 100 image classes with 50, 000 training and 10, 000 testing images; Tiny-ImageNet composed of 200 image classes with 100, 000 training and 10, 000 testing images.
All images in Tiny-ImageNet were resized to 32 × 32 images.
One of the state-of-the-art model DenseNet (L=25, k=12) BID8 was used for two hard data sets with momentum optimizer.
Regarding algorithm parameters, we used a learning rate of 0.1 and a batch size of 128; The training epoch and warm-up threshold γ were set to be 90 and 10, respectively.
We repeated every test five times for robustness and reported the average.
The selection pressure s e determines how strongly the boundary samples are selected.
The greater the s e , the greater the sampling probability of the boundary sample, so more boundary samples were chosen for the next mini-batch.
On the other hand, the less s e makes Ada-Boundary closer to random batch selection.
FIG14 shows the convergence curves of Ada-Boundary with varying s e on two hard data sets.
To clearly analyze the impact of the selection pressure, we plotted the minimum of training loss and test error with a given epochs.
Overall, the convergence speed of training loss was accelerated as the s e increased from 2 to 16, but that of test error was faster only when the s e was less than a certain value.
The convergence speed of test error was faster than random batch selection, when s e was less than or equal to 4 (CIFAR-100) and 2 (Tiny-ImageNet).
Surprisingly, the overexposure to the boundary samples using the large s e incurred the overfitting issue in hard data sets, whereas the large s e = 100 worked well for our easy or relatively hard data sets as discussed in Section 4.
That is, the selection pressure s e should be chosen more carefully considering the difficulty of the given data set.
We leave this challenge as our future work.
TAB4 shows the performance gains of Ada-Boundary over random batch selection on two hard data sets.
We only quantify the gains of Ada-Boundary(s e = 2) because its performance was the best as shown in FIG14 .
Ada-Boundary(s e = 2) always outperforms random batch selection.
Especially, it reduces the training time significantly by up to around 20%.
Table 3 shows the wall-clock training time for the same number of parameter updates on two hard data sets FIG14 ).
Ada-Boundary(s e = 2) with momentum was 15.2%-16.0% slower than random batch selection.
However, it reduced the running time by 18.0%-21.4% (by Gain tim ) to obtain the same test error of random batch selection.
D WALL-CLOCK TRAINING TIME
The procedures for recomputing sampling probabilities and updating quantization indexes make Ada-Boundary slower than random batch selection.
Table 4 shows the wall-clock training time for the same number of parameter updates (i.e., the same number of epochs) with SGD ( FIG9 ) and momentum ( FIG1 ).
Ada-Boundary with SGD was 12.8%-14.7% and 6.06%-12.2% slower than random batch and online batch selections, respectively.
Ada-Boundary with momentum was 13.1%-14.7% and 6.67%-12.2% slower than random batch and online batch selections, respectively.
Although Ada-Boundary took longer for the same number of updates, Ada-Boundary achieved significant reduction in running time by 7.96%-33.5% (by Gain tim ) to obtain the same test error of random batch selection due to the fast convergence.
TAB3 E EXPERIMENT RESULTS USING MOMENTUM OPTIMIZER E.1 CONVERGENCE ANALYSIS FIG1 shows the convergence curves of training loss and test error for five batch selection strategies on three data sets, when we used the momentum optimizer with setting the momentum to be 0.9.
In the MNIST data set, we limited the number of epochs to be 30 because both training loss and test error were fully converged after 30 epochs.
We repeat the convergence analysis, as follows:• MNIST FIG1 ): Except Ada-Uniform, all adaptive batch selections converged faster than random batch selection.
Online batch selection showed much faster convergence speed than other adaptive batch selections in training loss, but converged similarly with the others in test error owing to the overfitting to hard samples.• Fashion-MNIST FIG1 ): Ada-Boundary showed the fastest convergence speed in test error, although it did not converge faster than online batch selection in training loss.
In contrast, online batch selection was the fastest in training loss, but its convergence in test error was slightly slower than that of random batch selection.
This emphasizes the need to consider the samples with appropriate difficulty rather than hard samples.
The convergence speeds of Ada-Hard and Ada-Uniform in test error were slower than that of random batch selection.• CIFAR-10 FIG1 ): In both training loss and test error, Ada-Boundary and Ada-Hard showed slightly faster convergence speed than random batch selection.
On the other hand, online batch selection converged slightly slower than random batch selection in both cases.
In summary, in the easiest MNIST data set, most of adaptive batch selections accelerated their convergence speed compared with random batch selection.
However, in Fashion-MNIST data set, only Ada-Boundary converged faster than random batch selection.
In a relatively difficult CIFAR-10 data set, Ada-Boundary and Ada-Hard showed comparable convergence speed and then converged faster than random batch selection.
We quantify the performance gains of Ada-Boundary over random batch and online batch selections in TAB7 .
Ada-Boundary always outperforms both strategies, as already shown in FIG1 .
Compared with TAB3 , Gain tim over random batch selection tends to become smaller, whereas Gain tim over online batch selection tends to become larger.

assistant: text: We suggest a smart batch selection technique called Ada-Boundary.