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Title: Rethinking the Value of Network Pruning

Paper content:
Network pruning is widely used for reducing the heavy inference cost of deep models in low-resource settings.
A typical pruning algorithm is a three-stage pipeline, i.e., training (a large model), pruning and fine-tuning.
During pruning, according to a certain criterion, redundant weights are pruned and important weights are kept to best preserve the accuracy.
In this work, we make several surprising observations which contradict common beliefs.
For all state-of-the-art structured pruning algorithms we examined, fine-tuning a pruned model only gives comparable or worse performance than training that model with randomly initialized weights.
For pruning algorithms which assume a predefined target network architecture, one can get rid of the full pipeline and directly train the target network from scratch.
Our observations are consistent for multiple network architectures, datasets, and tasks, which imply that: 1) training a large, over-parameterized model is often not necessary to obtain an efficient final model, 2) learned ``important'' weights of the large model are typically not useful for the small pruned model, 3) the pruned architecture itself, rather than a set of inherited ``important'' weights, is more crucial to the efficiency in the final model, which suggests that in some cases pruning can be useful as an architecture search paradigm.
Our results suggest the need for more careful baseline evaluations in future research on structured pruning methods.
We also compare with the "Lottery Ticket Hypothesis" (Frankle & Carbin 2019), and find that with optimal learning rate, the "winning ticket" initialization as used in Frankle & Carbin (2019) does not bring improvement over random initialization.
Over-parameterization is a widely-recognized property of deep neural networks (Denton et al., 2014; Ba & Caruana, 2014) , which leads to high computational cost and high memory footprint for inference.
As a remedy, network pruning BID12 Hassibi & Stork, 1993; Han et al., 2015; Molchanov et al., 2016; BID14 has been identified as an effective technique to improve the efficiency of deep networks for applications with limited computational budget.
A typical procedure of network pruning consists of three stages:
1) train a large, over-parameterized model (sometimes there are pretrained models available),
2) prune the trained large model according to a certain criterion, and
3) fine-tune the pruned model to regain the lost performance.
2015).
Thus most existing pruning techniques choose to fine-tune a pruned model instead of training it from scratch.
The preserved weights after pruning are usually considered to be critical, as how to accurately select the set of important weights is a very active research topic in the literature (Molchanov et al., 2016; BID14 Luo et al., 2017; He et al., 2017b; BID4 Suau et al., 2018) .In
this work, we show that both of the beliefs mentioned above are not necessarily true for structured pruning methods, which prune at the levels of convolution channels or larger. Based
on an extensive empirical evaluation of state-of-the-art pruning algorithms on multiple datasets with multiple network architectures, we make two surprising observations. First
, for structured pruning methods with predefined target network architectures (Figure 2) ,
directly training the small target model from random initialization can achieve the same, if not better, performance, as the model obtained from the three-stage pipeline. In this
case, starting with a large model is not necessary and one could instead directly train the target model from scratch. Second,
for structured pruning methods with autodiscovered target networks, training the pruned model from scratch can also achieve comparable or even better performance than fine-tuning. This observation
shows that for these pruning methods, what matters more may be the obtained architecture, instead of the preserved weights, despite training the large model is needed to find that target architecture. Interestingly, for
a unstructured pruning method (Han et al., 2015) that prunes individual parameters, we found that training from scratch can mostly achieve comparable accuracy with pruning and fine-tuning on smaller-scale datasets, but fails to do so on the large-scale ImageNet benchmark. Note that in some
cases, if a pretrained large model is already available, pruning and fine-tuning from it can save the training time required to obtain the efficient model. The contradiction
between some of our results and those reported in the literature might be explained by less carefully chosen hyper-parameters, data augmentation schemes and unfair computation budget for evaluating baseline approaches.Predefined: prune x% channels in each layer Automatic: prune a%, b%, c%, d% channels in each layer A 4-layer model Figure 2 : Difference between predefined and automatically discovered target architectures, in channel pruning as an example. The pruning ratio
x is userspecified, while a, b, c, d are determined by the pruning algorithm. Unstructured sparse
pruning can also be viewed as automatic.Our results advocate a rethinking of existing structured network pruning algorithms. It seems that the over-parameterization
during the first-stage training is not as beneficial as previously thought. Also, inheriting weights from a large model
is not necessarily optimal, and might trap the pruned model into a bad local minimum, even if the weights are considered "important" by the pruning criterion. Instead, our results suggest that the value
of automatic structured pruning algorithms sometimes lie in identifying efficient structures and performing implicit architecture search, rather than selecting "important" weights. For most structured pruning methods which prune
channels/filters, this corresponds to searching the number of channels in each layer. In section 5, we discuss this viewpoint through
carefully designed experiments, and show the patterns in the pruned model could provide design guidelines for efficient architectures.The rest of the paper is organized as follows: in Section 2, we introduce background and some related works on network pruning; in Section 3, we describe our methodology for training the pruned model from scratch; in Section 4 we experiment on various pruning methods and show our main results for both pruning methods with predefined or automatically discovered target architectures; in Section 5, we discuss the value of automatic pruning methods in searching efficient network architectures; in Section 6 we discuss some implications and conclude the paper.
Our results encourage more careful and fair baseline evaluations of structured pruning methods.
In addition to high accuracy, training predefined target models from scratch has the following benefits over conventional network pruning procedures:
a) since the model is smaller, we can train the model using less GPU memory and possibly faster than training the original large model;
b) there is no need to implement the pruning criterion and procedure, which sometimes requires fine-tuning layer by layer (Luo et al., 2017) and/or needs to be customized for different network architectures BID14 BID4 ;
c) we avoid tuning additional hyper-parameters involved in the pruning procedure.Our results do support the viewpoint that automatic structured pruning finds efficient architectures in some cases.
However, if the accuracy of pruning and fine-tuning is achievable by training the pruned model from scratch, it is also important to evaluate the pruned architectures against uniformly pruned baselines (both training from scratch), to demonstrate the method's value in identifying efficient architectures.
If the uniformly pruned models are not worse, one could also skip the pipeline and train them from scratch.Even if pruning and fine-tuning fails to outperform the mentioned baselines in terms of accuracy, there are still some cases where using this conventional wisdom can be much faster than training from scratch:
a) when a pre-trained large model is already given and little or no training budget is available; we also note that pre-trained models can only be used when the method does not require modifications to the large model training process;
b) there is a need to obtain multiple models of different sizes, or one does not know what the desirable size is, in which situations one can train a large model and then prune it by different ratios.

assistant: text: In structured network pruning, fine-tuning a pruned model only gives comparable performance with training it from scratch.