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Title: A Constructive Prediction of the Generalization Error Across Scales

Paper content:
The dependency of the generalization error of neural networks on model and dataset size is of critical importance both in practice and for understanding the theory of neural networks.
Nevertheless, the functional form of this dependency remains elusive.
In this work, we present a functional form which approximates well the generalization error in practice.
Capitalizing on the successful concept of model scaling (e.g., width, depth), we are able to simultaneously construct such a form and specify the exact models which can attain it across model/data scales.
Our construction follows insights obtained from observations conducted over a range of model/data scales, in various model types and datasets, in vision and language tasks.
We show that the form both fits the observations well across scales, and provides accurate predictions from small- to large-scale models and data.
With the success and heightened adoption of neural networks for real world tasks, some questions remain poorly answered.
For a given task and model architecture, how much data would one require to reach a prescribed performance level?
How big a model would be needed?
Addressing such questions is made especially difficult by the mounting evidence that large, deep neural networks trained on large-scale data outperform their smaller counterparts, rendering the training of high performance models prohibitively costly.
Indeed, in the absence of practical answers to the above questions, surrogate approaches have proven useful.
One such common approach is model scaling, where one designs and compares small-scale models, and applies the obtained architectural principles at a larger scale (e.g., Liu et al., 2018; Real et al., 2018; Zoph et al., 2018) .
Despite these heuristics being widely used to various degrees of success, the relation between the performance of a model in the small-and large-scale settings is not well understood.
Hence, exploring the limitations or improving the efficiency of such methods remains subject to trial and error.
In this work we circle back to the fundamental question: what is the (functional) relation between generalization error and model and dataset sizes?
Critically, we capitalize on the concept of model scaling in its strictest form: we consider the case where there is some given scaling policy that completely defines how to scale up a model from small to large scales.
We include in this context all model parameters, such that traversing from one scale (in which all parameters are known) to another requires no additional resources for specifying the model (e.g., architecture search/design).
We empirically explore the behavior of the generalization error over a wide range of datasets and models in vision and language tasks.
While the error landscape seems fairly complex at first glance, we observe the emergence of several key characteristics shared across benchmarks and domains.
Chief among these characteristics is the emergence of regions where power-law behavior approximates the error well both with respect to data size, when holding model size fixed, and vice versa.
Motivated by these observations, we establish criteria which a function approximating the error landscape should meet.
We propose an intuitive candidate for such a function and evaluate its quality, both in explaining the observed error landscapes and in extrapolating from small scale (seen) to large scale (unseen) errors.
Critically, our functional approximation of the error depends on both model and data sizes.
We find that this function leads to a high quality fit and extrapolation.
For instance, the mean and standard deviation of the relative errors are under 2% when fitting across all scales investigated and under 5% when extrapolating from a slimmed-down model (1/16 of the parameters) on a fraction of the training data (1/8 of the examples) on the ImageNet (Russakovsky et al., 2015) and WikiText-103 (Merity et al., 2016) datasets, with similar results for other datasets.
To the best of our knowledge, this is the first work that provides simultaneously:
• A joint functional form of the generalization error landscape-as dependent on both data and model size-with few, interpretable degrees of freedom (section 5).
• Direct and complete specification (via the scaling policy) of the model configuration attaining said generalization error across model and dataset sizes.
• Highly accurate approximation of error measurements across model and data scales via the functional form, evaluated on different models, datasets, and tasks (section 6 ).
• Highly accurate error prediction from small to large model and data (section 7).
We conclude with a discussion of some implications of our findings as a practical and principled tool for understanding network design at small scale and for efficient computation and trade-off design in general.
We hope this work also provides a useful empirical leg to stand on and an invitation to search for a theory of generalization error which accounts for our findings.
In this work, through insights gained by the joint examination of the dependencies of generalization error on both model and data size, we arrive at criteria for functions consistent with the form of the generalization error under a given scaling policy.
We consider one such function and find it to be in very good agreement with the actual behavior of the error landscape.
Indeed, the agreement is strong enough that extrapolation from small to large scale becomes feasible: the function predicts the behavior of the generalization error in practice for the practical case of scaling models and data.
We discuss several example implications of knowing such a functional form.
Small-scale network development: At the core of small fidelity searches is the notion of performance rank comparison between models.
However, small scale and large scale ranks are not assured to be consistent.
If indeed a functional form such as empirically found in this work holds very generally, then in contrast, one can safely assess scaling rank between models at small scale, with the assurance that it remains consistent.
This suggests that one would be well served by searching over scaling policies; a pertinent example of such a success is Tan & Le (2019) .
The functional form also explains the limitation of small-scale search: once reaching the random-guess error level, where the sensitivity to scaling vanishes, the informativeness of ranking diminishes.
Finally, the functional form allows direct usage of differentiable methods for NAS.
Principled design: Knowing the error landscape function facilitates reasoning about the choice of (m, n) attaining a specified error level.
In other words, for any given error level, one can solve Eq. 5 for m, n based on small-scale measurements.
Thus, one can quantitatively answer design questions regarding the expected (in particular, large-scale) relations between m, n, and .
In fact, Eq. 5 provides direct ansewrs to questions such as "how much data would one require to reach a prescribed performance level?" or "how big a model would be needed?"
Imposing constraints is also straightforward.
For instance, consider the following question: "What is the maximal model size possibly needed (useful), when the data is limited in size, n = n lim (for a given model architecture and scaling policy)?"
For a fixed dataset size, model scaling eventually contributes marginally to error reduction and becomes negligible when bm Similarly, The maximal useful amount of data for a limited sized model m lim is:
Moreover, Eq. 5 allows for complex design trade-offs.
Generally, given some design-tradeoff cost function C(m, n, ), one can minimize such cost s.t. Eq. 5.
For example, consider the case of optimizing for efficient computation which has both practical and environmental importance (Schwartz et al., 2019) .
Since the number of FLOPs during training is ∝ m · n (for constant epoch budget), the trade-off cost function may be formulated as C(FLOPS, ) = C(mn, ).
Further, since constant error contour is very well approximated by c = 1 n α + b m β (Eq. 5), dataset and models may be scaled with optimal resource efficiency with no effect on performance by solving for:
The solution gives us the optimal-computational-efficiency ratio of model to data size:
Limitations: We have made a few simplifying assumptions in our choice of approximating function, in particular in how to model the transition from the initial random-guess error level and the union of the random-guess level of the two scenarios (small model with large data and large model with small data).
We leave a more detailed examination of the behavior of the transitions from random-guess error levels and refinements of the functional form to future work.
Critically, the restrictive nature of our scaling framework (all parameters and hyperparameters described by a policy) is both a blessing and a challenge.
The blessing comes in fulfilling the goal of finding simultaneously both the form of the generalization error and the full specification of the model and hyperparameters that attain it across scales.
The challenge is that we have demonstrated in this work only the case of constant hyper-parameters.
We conjecture that the relation between model configuration and hyperparameter choice (Zela et al., 2018) may entail the potential to formulate hyperparameter-scaling policies similar in nature to the model-scaling polices, and that these too fall under the scope of the form we find in this work.
This too will be the subject of future work.
We hope that this work will bring the actual functional form of the generalization error in this practical case of scaling to the fore, both in practice and as an empirical leg to stand on in the quest for its theoretical origins.
Scaling the models' width is performed by multiplying the number of channels in each convolutional layer and the width of the hidden linear layers by a constant factor and rounding to the nearest integer.
The ranges of width scales (and data scales) for the main experiments are detailed in Table 1b .

assistant: text: We predict the generalization error and specify the model which attains it across model/data scales.