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Title: Minimal Random Code Learning: Getting Bits Back from Compressed Model Parameters

Paper content:
While deep neural networks are a highly successful model class, their large memory footprint puts considerable strain on energy consumption, communication bandwidth, and storage requirements.
Consequently, model size reduction has become an utmost goal in deep learning.
Following the classical bits-back argument, we encode the network weights using a random sample, requiring only a number of bits corresponding to the Kullback-Leibler divergence between the sampled variational distribution and the encoding distribution.
By imposing a constraint on the Kullback-Leibler divergence, we are able to explicitly control the compression rate, while optimizing the expected loss on the training set.
The employed encoding scheme can be shown to be close to the optimal information-theoretical lower bound, with respect to the employed variational family.
On benchmarks LeNet-5/MNIST and VGG-16/CIFAR-10, our approach yields the best test performance for a fixed memory budget, and vice versa, it achieves the highest compression rates for a fixed test performance.
Traditional approaches to model compression usually rely on three main techniques: pruning, quantization and coding.
For example, Deep Compression BID3 proposes a pipeline employing all three of these techniques in a systematic manner.
From an information-theoretic perspective, the central routine is coding, while pruning and quantization can be seen as helper heuristics to reduce the entropy of the empirical weight-distribution, leading to shorter encoding lengths BID11 .
Also, the recently proposed Bayesian Compression BID9 falls into this scheme, despite being motivated by the so-called bits-back argument BID7 which theoretically allows for higher compression rates.1
While the bits-back argument certainly motivated the use of variational inference in Bayesian Compression, the downstream encoding is still akin to Deep Compression (and other approaches).
In particular, the variational distribution is merely used to derive a deterministic set of weights, which is subsequently encoded with Shannonstyle coding.
This approach, however, does not fully exploit the coding efficiency postulated by the bits-back argument.1
Recall that the bits-back argument states that, assuming a large dataset and a neural network equipped with a weight-prior p, the effective coding cost of the network weights is KL(q||p) = Eq[log q p ], where q is a variational posterior.
However, in order to realize this effective cost, one needs to encode both the network weights and the training targets, while it remains unclear whether it can also be achieved for network weights alone.In this paper, we step aside from the pruning-quantization pipeline and propose a novel coding method which approximately realizes bits-back efficiency.
In particular, we refrain from constructing a deterministic weight-set but rather encode a random weight-set from the full variational posterior.
This is fundamentally different from first drawing a weight-set and subsequently encoding it -this would be no more efficient than previous approaches.
Rather, the coding scheme developed here is allowed to pick a random weight-set which can be cheaply encoded.
By using results from BID4 , we show that such a coding scheme always exists and that the bits-back argument indeed represents a theoretical lower bound for its coding efficiency.
Moreover, we propose a practical scheme which produces an approximate sample from the variational distribution and which can indeed be encoded with this efficiency.
Since our algorithm learns a distribution over weightsets and derives a random message from it, while minimizing the resulting code length, we dub it Minimal Random Code Learning (MIRACLE).
In this paper we followed through the philosophy of the bits-back argument for the goal of coding model parameters.
Our algorithm is backed by solid recent information-theoretic insights, yet it is simple to implement.
We demonstrated that it outperforms the previous state-of-the-art.An important question remaining for future work is how efficient MIRACLE can be made in terms of memory accesses and consequently for energy consumption and inference time.
There lies clear potential in this direction, as any single weight can be recovered by its group-index and relative index within each group.
By smartly keeping track of these addresses, and using pseudo-random generators as algorithmic lookup-tables, we could design an inference machine which is able to directly run our compressed models, which might lead to considerable savings in memory accesses.

assistant: text: This paper proposes an effective coding scheme for neural networks that encodes a random set of weights from a variational distribution.