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Title: Combining adaptive algorithms and hypergradient method: a performance and robustness study

Paper content:
Wilson et al. (2017) showed that, when the stepsize schedule is properly designed, stochastic gradient generalizes better than ADAM (Kingma & Ba, 2014).
In light of recent work on hypergradient methods (Baydin et al., 2018), we revisit these claims to see if such methods close the gap between the most popular optimizers.
As a byproduct, we analyze the true benefit of these hypergradient methods compared to more classical schedules, such as the fixed decay of Wilson et al. (2017).
In particular, we observe they are of marginal help since their performance varies significantly when tuning their hyperparameters.
Finally, as robustness is a critical quality of an optimizer, we provide a sensitivity analysis of these gradient based optimizers to assess how challenging their tuning is.
Many new algorithms have been proposed in recent years for the minimization of unconstrained nonconvex functions such as the loss used in deep neural networks.
A critical parameter of all these methods is the stepsize.
A poor choice for that stepsize can either lead to very slow training or divergence.
Worse, in many cases, the stepsize leading to the fastest minimization is the largest one achieving convergence, making the search difficult.
The dramatic effect of that tuning motivated the development of a range of optimization methods trying to integrate temporal metrics to adapt the stepsize for each parameter during optimization BID4 BID14 BID11 BID6 .
In particular, ADAM BID6 has become the default choice for many researchers and practitioners.
One reason for this success is that these methods use a stepsize that is approximately normalized, that is the optimal stepsize potentially varies less across datasets and architectures than the optimal stepsize for non-adaptive methods.
However, BID12 analyzed in depth the impact of adaptive methods on both training and generalization and showed that stochastic gradient methods with a carefully tuned stepsize could reach a lower generalization error than methods optimizing one stepsize per parameter.
In order to achieve this result, a well tuned learning rate along with a suitable decaying schedule was required.
A recent approach for tuning the learning rate online was proposed by BID2 .
This method, called hypergradient descent (HD), does not require setting a decay schedule ahead of time.
One may thus wonder if, by automatically tuning the stepsize, such a technique would remove the last remaining advantage of adaptive methods, i.e. easier tuning.
Our work relates this technique to the recent criticism made about the adaptive gradient methods BID12 and reconsider the value of these methods compared to their non-adaptive counterparts.
More precisely, this paper aims at extending the analysis of BID12 in the following ways:• How competitive is the recent online hypergradient scheme proposed by BID2 compared to the offline scheme of BID12 ?
• Does this online scheme change the conclusions of BID12 ?
• Does this online scheme remove the need for fine-tuning the optimizer's hyperparameters, thereby removing the advantage of ADAM over stochastic gradient with momentum?
•
What is the sensitivity of the learning rate schedule to a suboptimal choice of the hyperparameters?The last point is often overlooked in the study of optimization methods.
While investigating which training conditions bring the best performance led to significant progress in the field, the effort needed to have an optimizer perform at its best should be taken into account when evaluating the performance.
Consider the following question: given a desired level of performance and limited computational ressources, which optimization method should be prefered and how should it be tuned?
By this work, we would like to emphasize the value of tuning for gradient based methods, and what it can reveal about them.
We now review several techniques for stepsize selection.
These techniques may either try to find a single global stepsize or one stepsize for each parameter in the model.
A first class of methods are the adaptive methods.
To compute the parameter updates, these methods multiply the gradient with a matrix.
When this matrix is diagonal, this is equivalent to using a distinctive stepsize per parameter.
One of the first such method is Newton's method, multiplying the gradient with the inverse of the Hessian to minimize the local quadratic approximation to the loss.
Similarly, Amari (1998) leveraged the Fisher information matrix to precondition the gradient.
Later, BID7 used the covariance of the gradients to move less in directions of high uncertainty.
A diagonal version of that technique, which adapts the learning rate on a per parameter basis, ADAGRAD, was proposed by BID4 .
Subsequently, many works sought to improve upon ADAGRAD, such as RMSprop BID11 , ADADELTA BID14 and ADAM BID6 .
Other methods BID9 adapt the stepsize to minimize an estimate of the expected loss.
Despite learning a stepsize per parameter, adaptive methods still require a global stepsize, which might change during the course of optimization.
We distinguish two classes of stepsize schedules: offline schedules, which are set ahead of time, and online schedules, which depend on the optimization trajectory.
Offline schedules define before the optimization, for each iteration t, the value of the stepsize α t .
The seminal work of BID8 advocated for a schedule α t = O(1/t), a rate often too slow in practice.
Another decay schedule proposed by BID3 is to halve the stepsize every time t = 2 k .
The overall rate remains O(1/t) but this method seems to work better in practice.
A more aggressive schedule was proposed by BID12 where the stepsize is halved every fixed number of iterations.
While this leads to an exponential decay of the stepsize, precluding the method from reaching a local optimum from any initialization, this led them to the best results.
In our work, we will call lr-decay the schedule where the stepsize is divided by 2 every 25 epochs.
In contrast to offline schedules, online schedules tune the stepsize based on the dynamics of the optimization.
Though extremely appealing, these methods have had little success, mostly because of their brittleness.
Recently, however, BID2 rediscovered the hypergradient update rule for the stepsize, which was originally suggested by BID0 .
The optimisation of the stepsize can be framed as an iterative process that increases the stepsize when the last two gradients are aligned, suggesting that an acceleration is possible, and decreases it otherwise.
It requires no extra gradient computation and only needs a copy of the current gradient to be held in memory for the following iteration.
More precisely, assume we want to minimize a parametric function f : R d → R with d the number of parameters.
Let us denote θ t the parameter iterate at timestep t. A gradient descent update rule can be written: DISPLAYFORM0 and α is the learning rate.
The goal is to move α t−1 toward the value that minimizes the expected objective function value at the next iteration, E[f (u(Θ t , α))].
Taking the derivative with respect to α, we end up, under suitable assumptions, with the following learning rate update rule DISPLAYFORM1 is the noisy gradient of f .
The authors observed that HD decreases the sensitivity to α 0 , the initial learning rate value, and that tuning only β is sufficient.
In the following section, we will see whether this lower sensitivity comes with the ability to recover nearly optimal optimization trajectory, or if it would still benefit from some tuning.
Now, we focus on comparing hypergradient to the fixed decay scheme of BID12 .
We show that, i) hypergradient can significantly benefit from a fine tuning of its hyperparameters, and by doing so, ii) it replicates, in most cases, the training performance of the best offline learing rate adaptation scheme and, that iii) stochastic gradient with hypergradient generalizes better than ADAM, extending BID12 conclusions.
We consider three gradient methods: SGD, SGD with Nesterov momentum BID10 (SGDN) and ADAM.
For each of these methods, we tried both hypergradient (SGD-HD, SGDN-HD, ADAM-HD) and the offline fixed decay (SGD-decay, SGDN-decay, ADAM-decay).We study the performance of these six algorithms on three tasks: MNIST classification task using a feedforward neural network with two hidden layers of 1000 hidden units each, CIFAR-10 classification task using the VGG16 architecture from the Torch blog BID13 , and CIFAR-10 classification task using ResNet18 BID5 .A grid search was performed to optimize the learning rate for each of these methods.
The same grid of values was used for SGD and SGDN.
For ADAM, another grid of comparable size was used.
The scale of each grid was spread around the scale that is commonly known to be suitable for each of these optimizers.
For the hypergradient method (HD), the grid search was conducted over both the initial learning rate α 0 and the hypergradient learning rate β.
However, for the learning rate fixeddecay (lr-decay), we kept the values of the decay factor and the decay frequency used in BID12 , which they have already optimized.
The list of hyperparameters tried may be found in the appendix.
For momentum methods, we used µ = 0.9.
For Adam, we used the default setting with β 1 = 0.9, β 2 = 0.999 and = 10 −8 .
L2 regularization with a coefficient of 10 −4 was used for all our experiments.
Each experiement was conducted five times, using the same random initialization for all optimizers each time.
CIFAR-10 was trained for 250 epochs and MNIST for 200 epochs.
We now present our results.
Before comparing hypergradient and lr-decay, we give a sense of how much hypergradient can boost the optimizers we are considering.
We will later compare the best performing configurations to lr-decay.
Baydin et al. (2018) claim that hypergradient "significantly reduces the need of the manual tuning of the initial learning rate for these commonly used algorithms".
For an arbitraty initial learning rate, hypergradient "consistently brings the loss trajectory closer to the optimal one".
As can be seen in FIG0 , a slightly better tuning improves drastically the performance.
Thus, even if hypergradient decreases the sensitivity to the inital learning rate for a fixed value of β, tuning both parameters remains essential.
After having optimized hypergradient, we now compare its best configuration to lr-decay as optimized in BID12 .Hypergradient can replicate a close training perfomance to lr-decay.
FIG2 shows the training loss of hypergradient and lr-decay for the conducted experiments.
When training on CIFAR-10, we observe that SGD-decay and SGDN-decay provide consistantly the best performance.
We note that hypergradient is outperformed by lr-decay for ResNet18, but it is still able to reach a comparable performance on VGG net.
In 5 out of 9 settings, hypergradient performs comparably to lr-decay.
In the 4 others, it performs significantly worse.
Hypergradient generalizes worse than lr-decay.
FIG3 shows the test loss for the experiments of FIG2 .
We observe that hypergradient has a worse generalization performance than lr-decay, except SGDN-HD that is the best generalizing method for VGG net.
It is interesting to see that, by using the optimization dynamics in an online fashion, one can recover the training performance of a carefully tuned decay schedule. (2017) , that SGD-decay and SGDN-decay tend to generalize better than ADAM-decay for the VGG network.
The same observation can be made for ResNet18.
This conclusion is also valid for the hypergradient online scheme.
Indeed, ADAM-HD is consistently outperformed by SGD-HD and SGDN-HD.
So, hypergradient is also providing stochastic gradient with a learning rate schedule that helps it generalize better than ADAM.
This recurrent pattern about adaptive gradient methods ability to generalize seems to suggest that they are less prone to benefit from learning rate decay schedules, whether offline or online; thus limiting their competitiveness.
This makes the hypergradient another learning rate adaptation technique that invites us to reconsider the use of ADAM, as an adaptive gradient algorithm, to train neural networks.
The main practical message here is that it's preferable to combine the stochastic gradient with a well designed learning rate decay.
Momentum is also adviced to complete this combination.
So far, we have seen what these gradients methods with online and offline learning rate schedules can achieve.
Tuning their hyperparameters revealed the potential of some and limitations of others.
In the next section, we give an analysis of the sensitivity of such tuning with respect to the initial stepsize and describe the effect of the described learning rate adaptation methods.
In order to better understand how difficult setting the right stepsize can be, we plot the performance of each optimizer as a function of its learning rate 1 , where performance is defined as the minimal cross-entropy loss, averaged over the 5 runs, reached by each method.
We show that ADAM with a constant stepsize is as difficult to tune as stochastic gradient.
Interestingly, we found that, while lr-decay allows SGD and AGDN to generalize better than ADAM, it also makes the latter easier to tune than the former.
In this section, we focus our sensitivity analysis on the CIFAR-10 classification problem using VGG and ResNet18 architectures, since they pmake the differences in the behaviour of the optimizers we are studying explicit.
In this section, SGD, SGDN and ADAM will be used to denote the constant stepsize version of these gradient methods.
To better compare the sensitivity of each optimizer in the neighbourhood of its best learning rate, we plotted the ratio to the best learning rate on the x-axis.
Hence, the minimum value is always reached for a ratio of 1.
FIG4 and 5 represent the training and test errors over the grid of inital learning rates for VGG net and ResNet18 respectively.
On these figures, the gradient methods were used with a constant stepsize.
For VGG net, FIG4 shows the performance of SGD and SGDN worsens faster when increasing the learning rate than when decreasing it.
This observation suggest that one should aim at the highest step size that does not diverge, an observation which matches the theory on quadratic bowls.
ADAM shows a similar U-shaped dependency of the performance with respect to the initial learning rate, but its behaviour is more symmetric.
In general, these observations raise the problem of how fine-grained should the set of learning rate values one is selecting from be.
A wide valley means that fewer values are sufficient to find a well performing configuration while a tighter one suggests that one requires to select from a finer range of values.
The architecture influences the tuning.
In FIG5 , we see that all the three gradient methods exhibit a wider valley around the best learning rate value, which implies that it is easier to tune the learning rate for a ResNet18 than for a VGG16.
This importance of the architecture choice in the tuning of the learning rate is often overlooked in analysis comparing such architectures.
The lr-decay scheme improved the training and generalization performance of gradient methods.
Here, so that to complete our understanding of such learning rate adaptation approach, we investigate the way it changes the tuning sensitivity.
Figure 6: The effect of lr-decay on the learning rate sensitivity for VGG net and ResNet18 on CIFAR-10.
lr-decay makes the tuning of ADAM easier.
ResNet18 makes the gradient methods less sensitive to the stepsize choice.
Figures 6 shows the sensitivity of each gradient method (constant stepsize) along with its lr-decay version.
For all these experiments, we can see that lr-decay moves the best learning rate value to a higher one (there are less values on the right of the best stepsize of lr-decay).
Concerning the VGG architecture (and 6a), for SGD and SGDN, lr-decay improves over their performance with the best constant stepsize, but at the expense of a much sharper slope at the right of its own new best learning rate.
In addition, for ADAM, lr-decay decreases the sentivity at the right of best stepsize which, makes tuning less challenging for ADAM-decay than SGD/SGDN-decay.
Thus, the generalization potential that lr-decay brings to stochastic gradient BID12 comes at the expense of a trickier tuning.
Moreover, for smaller scales of the learning rate lr-decay is not penalizing the performance.
It even allows some improvements.
For ResNet18 (6b), lr-decay shows even more robustness to a suboptimal choice of the learning rate.
The performance remains quite at the same level for a larger range of the learning rate scale.
lr-decay seems to significantly decrease the sensitivity for ADAM, by flattening its U-shaped sensitivity curve, which confirms that ADAM-decay requires less tuning effort.
Figure 7 shows the sensitivity of hypergradient methods to the stepsize α 0 , for different values of the hypergradient stepsize β, for a VGG net trained on the CIFAR-10 dataset.
Figure 7: The effect of hypergradient on the learning rate sensitivity for VGG net and ResNet18 on CIFAR-10.
For a given β parameter, Hypergradient demonstrates some robustness to the initial learning rate.
However to reach higher performance, tuning β is crucial.
As claimed by BID2 , hypergradient methods have a low sensitivity to the choice of the stepsize α 0 , for a fixed hypergradient learning rate β.
Indeed, on Figure 7 , we see that, for several settings of β, the performance is nearly constant over the 5 stepsizes that we tried.
We'd like to note, however, that some other β settings demonstrate high sensitivity on some range of (the reduced) α 0 value.
For example, the train performance of SGD-HD with β = 10 −5 pis constant for stepsizes that are 100 times smaller than its best step size, but it drastically improves above that stepsize scale and outperforms all the other configurations.
An interesting point to add here, is that stochastic gradient with nesterov momentum (SGDN) happens to be, not only the gradient method that benefits the most from hypergradient in terms of performance and generalization (see section 3), but also the one that is less sensitive to the tuning compared to stochastic gradient and Adam.
We studied the impact of hypergradient methods on common optimizers and observed that it does not perform better than the fixed exponential decay proposed by BID12 .
Further, while hypergradient is designed to simplify the tuning of the stepsize, it can still greatly benefit from a fine tuning of its hyperparameters.
Finally, similar to the conclusions reached by BID12 , SGD and SGDN combined with a tuned hypergradient perform better than ADAM with the same method.
This study raises several questions.
First, is it possible to derive an automatic stepsize tuner that works consistently well across datasets and architectures?
Second, what would an optimizer tuned for robustness look like ?
In any case, our results suggest that the current adaptive methods wouldn't be the best candidates to build on such an optimizer.
One would rather augment the stochastic gradient with more promising learning rate schedules.

assistant: text: We provide a study trying to see how the recent online learning rate adaptation extends the conclusion made by Wilson et al. 2018 about adaptive gradient methods, along with comparison and sensitivity analysis.