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Published as a conference paper at ICLR 2018
MODEL COMPRESSION VIA
DISTILLATION AND QUANTIZATION
Antonio Polino
ETH Z¨urich
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
Razvan Pascanu
Google DeepMind
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
Dan Alistarh
IST Austria
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
ABSTRACT
Deep neural networks (DNNs) continue to make significant advances, solving
tasks from image classification to translation or reinforcement learning. One
aspect of the field receiving considerable attention is efficiently executing deep
models in resource-constrained environments, such as mobile or embedded devices.
This paper focuses on this problem, and proposes two new compression
methods, which jointly leverage weight quantization and distillation of larger networks,
called “teachers,” into compressed “student” networks. The first method
we propose is called quantized distillation and leverages distillation during the
training process, by incorporating distillation loss, expressed with respect to the
teacher network, into the training of a smaller student network whose weights are
quantized to a limited set of levels. The second method, differentiable quantization,
optimizes the location of quantization points through stochastic gradient
descent, to better fit the behavior of the teacher model. We validate both methods
through experiments on convolutional and recurrent architectures. We show that
quantized shallow students can reach similar accuracy levels to state-of-the-art
full-precision teacher models, while providing up to order of magnitude compression,
and inference speedup that is almost linear in the depth reduction. In sum,
our results enable DNNs for resource-constrained environments to leverage architecture
and accuracy advances developed on more powerful devices.
1 INTRODUCTION
Background. Neural networks are extremely effective for solving several real world problems, like
image classification (Krizhevsky et al., 2012; He et al., 2016a), translation (Vaswani et al., 2017),
voice synthesis (Oord et al., 2016) or reinforcement learning (Mnih et al., 2013; Silver et al., 2016).
At the same time, modern neural network architectures are often compute, space and power hungry,
typically requiring powerful GPUs to train and evaluate. The debate is still ongoing on whether
large models are necessary for good accuracy. It is known that individual network weights can be
redundant, and may not carry significant information, e.g. Han et al. (2015). At the same time, large
models often have the ability to completely memorize datasets (Zhang et al., 2016), yet they do
not, but instead appear to learn generic task solutions. A standing hypothesis for why overcomplete
representations are necessary is that they make learning possible by transforming local minima into
saddle points (Dauphin et al., 2014) or to discover robust solutions, which do not rely on precise
weight values (Hochreiter & Schmidhuber, 1997; Keskar et al., 2016).
If large models are only needed for robustness during training, then significant compression of these
models should be achievable, without impacting accuracy. This intuition is strengthened by two
related, but slightly different research directions. The first direction is the work on training quantized
neural networks, e.g. Courbariaux et al. (2015); Rastegari et al. (2016); Hubara et al. (2016); Wu
et al. (2016a); Mellempudi et al. (2017); Ott et al. (2016); Zhu et al. (2016), which showed that
neural networks can converge to good task solutions even when weights are constrained to having
values from a set of integer levels. The second direction aims to compress already-trained models,
while preserving their accuracy. To this end, various elegant compression techniques have been
1 arXiv:1802.05668v1 [
cs.NE] 15 Feb 2018
Published as a conference paper at ICLR 2018
proposed, e.g. Han et al. (2015); Iandola et al. (2016);Wen et al. (2016); Gysel et al. (2016); Mishra
et al. (2017), which combine quantization, weight sharing, and careful coding of network weights,
to reduce the size of state-of-the-art deep models by orders of magnitude, while at the same time
speeding up inference.
Both these research directions are extremely active, and have been shown to yield significant compression
and accuracy improvements, which can be crucial when making such models available on
embedded devices or phones. However, the literature on compressing deep networks focuses almost
exclusively on finding good compression schemes for a given model, without significantly altering
the structure of the model. On the other hand, recent parallel work (Ba & Caruana, 2013; Hinton
et al., 2015) introduces the process of distillation, which can be used for transferring the behaviour
of a given model to any other structure. This can be used for compression, e.g. to obtain compact
representations of ensembles (Hinton et al., 2015). However the size of the student model needs to
be large enough for allowing learning to succeed. A model that is too shallow, too narrow, or which
misses necessary units, can result in considerable loss of accuracy (Urban et al., 2016).
In this work, we examine whether distillation and quantization can be jointly leveraged for better
compression. We start from the intuition that 1) the existence of highly-accurate, full-precision
teacher models should be leveraged to improve the performance of quantized models, while 2)
quantizing a model can provide better compression than a distillation process attempting the same
space gains by purely decreasing the number of layers or layer width. While our approach is very
natural, interesting research questions arise when these two ideas are combined.
Contribution. We present two methods which allow the user to compound compression in terms of
depth, by distilling a shallower student network with similar accuracy to a deeper teacher network,
with compression in terms of width, by quantizing the weights of the student to a limited set of
integer levels, and using less weights per layer. The basic idea is that quantized models can leverage
distillation loss (Hinton et al., 2015), the weighted average between the correct targets (represented
by the labels) and soft targets (represented by the teacher’s outputs).
We implement this intuition via two different methods. The first, called quantized distillation, aims
to leverage distillation loss during the training process, by incorporating it into the training of a student
network whose weights are constrained to a limited set of levels. The second method, which we
call differentiable quantization, takes a different approach, by attempting to converge to the optimal
location of quantization points through stochastic gradient descent. We validate both methods empirically
through a range of experiments on convolutional and recurrent network architectures. We
show that quantized shallow students can reach similar accuracy levels to full-precision and deeper
teacher models on datasets such as CIFAR and ImageNet (for image classification) and OpenNMT
and WMT (for machine translation), while providing up to order of magnitude compression, and
inference speedup that is linear in the depth. 1
Related Work. Our work is a special case of knowledge distillation (Ba & Caruana, 2013; Hinton
et al., 2015), in which we focus on techniques to obtain high-accuracy students that are both quantized
and shallower. More generally, it can be seen as a special instance of learning with privileged
information, e.g. Vapnik & Izmailov (2015); Xu et al. (2016), in which the student is provided additional
information in the form of outputs from a larger, pre-trained model. The idea of optimizing
the locations of quantization points during the learning process, which we use in differentiable quantization,
has been used previously in Lan et al. (2014); Koren & Sill (2011); Zhang et al. (2017),
although in the different context of matrix completion and recommender systems.
Using distillation for size reduction is mentioned in Hinton et al. (2015), for distilling ensembles.
To our knowledge, the only other work using distillation in the context of quantization is Wu et al.
(2016b), which uses it to improve the accuracy of binary neural networks on ImageNet. We significantly
refine this idea, as we match or even improve the accuracy of the original full-precision
model: for example, our 4-bit quantized version of ResNet18 has higher accuracy than full-precision
ResNet18 (matching the accuracy of the ResNet34 teacher): it has higher top-1 accuracy (by >15%)
and top-5 accuracy (by >7%) compared to the most accurate model in Wu et al. (2016b).
1Source code available at https://github.com/antspy/quantized_distillation
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Published as a conference paper at ICLR 2018
2 PRELIMINARIES
2.1 THE QUANTIZATION PROCESS
We start by defining a scaling function sc : Rn ! [0; 1], which normalizes vectors whose values
come from an arbitrary range, to vectors whose values are in [0; 1]. Given such a function, the
general structure of the quantization functions is as follows:
Q(v) = sc􀀀1

^Q
(sc(v))

; (1)
where sc􀀀1 is the inverse of the scaling function, and ^Q
is the actual quantization function that
only accepts values in [0; 1]. We always assume v to be a vector; in practice, of course, the weight
vectors can be multi-dimensional, but we can reshape them to one dimensional vectors and restore
the original dimensions after the quantization.
Scaling. There are various specifications for the scaling function; in this paper, we will use linear
scaling, e.g. He et al. (2016b), that is sc(v) = v􀀀
 ; with  = maxi vi 􀀀 mini vi and  = mini vi
which results in the target values being in [0; 1], and the quantization function
Q(v) = ^Q

v 􀀀 


+ : (2)
Bucketing. One problem with this formulation is that an identical scaling factor is used for the
whole vector, whose dimension might be huge. Magnitude imbalance can result in a significant loss
of precision, where most of the elements of the scaled vector are pushed to zero. To avoid this, we
will use bucketing, e.g. Alistarh et al. (2016), that is, we will apply the scaling function separately
to buckets of consecutive values of a certain fixed size. The trade-off here is that we obtain better
quantization accuracy for each bucket, but will have to store two floating-point scaling factors for
each bucket. We characterize the compression comparison in Section 5. The function ^Q
can also be
defined in several ways. We will consider both uniform and non-uniform placement of quantization
points.
Uniform Quantization. We fix a parameter s  1, describing the number of quantization levels
employed. Intuitively, uniform quantization considers s + 1 equally spaced points between 0 and 1
(including these endpoints). The deterministic version will assign each (scaled) vector coordinate vi
to the closest quantization point, while in the stochastic version we perform rounding probabilistically,
such that the resulting value is an unbiased estimator of vi, of minimal variance.
Formally, the uniform quantization function with s + 1 levels is defined as
^ Q(v; s)i =
bvisc
s
+
i
s
; (3)
where i is the rounding function. For the deterministic version, we define ki = svi 􀀀 bvisc and set
i =

1; if ki > 1
2
0; otherwise;
(4)
while for the stochastic version we will set i  Bernoulli(ki). Note that ki is the normalized
distance between the original point vi and the closest quantization point that is smaller than vi and
that the vector components are quantized independently.
Non-Uniform Quantization. Non-uniform quantization takes as input a set of s quantization points
fp1; : : : ; psg and quantizes each element vi to the closest of these points. For simplicity, we only
define the deterministic version of this function.
2.2 STOCHASTIC QUANTIZATION IS EQUIVALENT TO ADDING GAUSSIAN NOISE
In this section we list some interesting mathematical properties of the uniform quantization function.
Clearly, stochastic uniform quantization is an unbiased estimator of its input, i.e. E[Q(v)] = v.
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Published as a conference paper at ICLR 2018
What interests us is applying this function to neural networks; as the scalar product is the most
common operation performed by neural networks, we would like to study the properties of Q(v)T x,
where v is the weight vector of a certain layer in the network and x are the inputs. We are able to
show that
Q(v)T x = vT x + " (5)
where " is a random variable that is asymptotically normally distributed, i.e. 1
n
"
D
􀀀! N(0; 1):
Convergence occurs with the dimension n. For a formal statement and proof, see Section B.1 in the
Appendix.
This means that quantizing the weights is equivalent to adding to the output of each layer (before
the activation function) a zero-mean error term that is asymptotically normally distributed. The
variance of this error term depends on s. This connects quantization to work advocating adding
noise to intermediary activations of neural networks as a regularizer, e.g. Gulcehre et al. (2016).
3 QUANTIZED DISTILLATION
The context is the following: given a task, we consider a trained state-of-the-art deep model solving
it–the teacher, and a compressed student model. The student is compressed in the sense that 1) it
is shallower than the teacher; and 2) it is quantized, in the sense that its weights are expressed at
limited bit width. The strategy, as for standard distillation (Ba & Caruana, 2013; Hinton et al., 2015)
is for the student to leverage the converged teacher model to reach similar accuracy. We note that
distillation has been used previously to obtain compact high-accuracy encodings of ensembles (Hinton
et al., 2015); however, we believe this is the first time it is used for model compression via
quantization.
Given this setup, there are two questions we need to address. The first is how to transfer knowledge
from the teacher to the student. For this, the student will use the distillation loss, as defined by Hinton
et al. (2015), as the weighted average between two objective functions: cross entropy with soft
targets, controlled by the temperature parameter T, and the cross entropy with the correct labels.
We refer the reader to Hinton et al. (2015) for the precise definition of distillation loss.
The second question is how to employ distillation loss in the context of a quantized neural network.
An intuitive approach is to rely on projected gradient descent, where a gradient step is taken as
in full-precision training, and then the new parameters are projected to the set of valid solutions.
Critically, we accumulate the error at each projection step into the gradient for the next step. One
can think of this process as if collecting evidence for whether each weight needs to move to the
next quantization point or not. Crucially, the error accumulation prevents the algorithm from getting
stuck in the current solution if gradients are small, which would occur in a naive projected gradient
approach. This is simillar to the approach taken by BinaryConnect technique, with some differences.
Li et al. (2017) also examines these dynamics in detail. Compared to BinnaryConnect, we use
distillation rather than learning from scratch, hence learning more effeciently. We also do not restrict
ourselves to binary representation, but rather use variable bit-width quantization frunctions and
bucketing, as defined in Section 2.
An alternative view of this process, illustrated in Figure 1, is that we perform the SGD step on the
full-precision model, but computing the gradient on the quantized model, expressed with respect to
the distillation loss. With all this in mind, the algorithm we propose is:
4 DIFFERENTIABLE QUANTIZATION
4.1 GENERAL DESCRIPTION
We introduce differentiable quantization as a general method of improving the accuracy of a quantized
neural network, by exploiting non-uniform quantization point placement. In particular, we are
going to use the non-uniform quantization function defined in Section 2.1. Experimentally, we have
found little difference between stochastic and deterministic quantization in this case, and therefore
will focus on the simpler deterministic quantization function here.
Let p = (p1; : : : ; ps) be the vector of quantization points, and let Q(v; p) be our quantization function,
as defined previously. Ideally, we would like to find a set of quantization points p which
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Published as a conference paper at ICLR 2018
Algorithm 1 Quantized Distillation
1: procedure QUANTIZED DISTILLATION
2: Let w be the network weights
3: loop
4: wq   quant function(w; s)
5: Run forward pass and compute distillation loss l(wq)
6: Run backward pass and compute@l(wq)
@wq
7: Update original weights using SGD in full precision w = w 􀀀   @l(wq)
@wq
8: Finally quantize the weights before returning: wq   quant function(w; s)
9: return wq
quantize quantize quantize
SGD step
distil
SGD step
distil
SGD step
distil
model quantized
model
teacher model quantized
model
teacher model quantized
model
teacher
Figure 1: Depiction of the steps of quantized distillation. Note the accumulation over multiple steps
of gradients in the unquantized model leads to a switch in quantization (e.g. top layer left most
square).
minimizes the accuracy loss when quantizing the model using Q(v; p). The key observation is that
to find this set p, we can just use stochastic gradient descent, because we are able to compute the
gradient of Q with respect to p.
A major problem in quantizing neural networks is the fact that the decision of which pi should
replace a given weight is discrete, hence the gradient is zero:
@Q(v; p)
@v
= 0; almost everywhere:
This implies that we cannot backpropagate the gradients through the quantization function. To
solve this problem, typically a variant of the straight-through estimator is used, see e.g. Bengio
et al. (2013); Hubara et al. (2016). On the other hand, the model as a function of the chosen pi
is continuous and can be differentiated; the gradient of Q(v; p)i with respect to pj is well defined
almost everywhere, and it is simply
@Q(v; p)i
@pj
=

i; if vi has been quantized to pj
0; otherwise;
(6)
where i is i-th element of the scaling factor, assuming we are using a bucketing scheme. If no
bucketing is used, then i =  for every i. Otherwise it changes depending on which bucket the
weight vi belongs to.
Therefore, we can use the same loss function we used when training the original model, and with
Equation (6) and the usual backpropagation algorithm we are able to compute its gradient with
respect to the quantization points p. Then we can minimize the loss function with respect to p with
the standard SGD algorithm. The algorithm then becomes the following:
Note on Efficiency. Optimizing the points p can be slower than training the original network, since
we have to perform the normal forward and backward pass, and in addition we need to quantize the
weights of the model and perform the backward pass to get to the gradients w.r.t. p. However, in our
experience differential quantization requires an order of magnitude less iterations to converge to a
good solution, and can be implemented efficiently.
Weight Sharing. Upon close inspection, this method can be related to weight sharing Han et al.
(2015). Weight sharing uses a k-mean clustering algorithm to find good clusters for the weights,
adopting the centroids as quantization points for a cluster. The network is trained modifying the
values of the centroids, aggregating the gradient in a similar fashion. The difference is in the initial
assignment of points to centroids, but also, more importantly, in the fact that the assignment of
5
Published as a conference paper at ICLR 2018
Algorithm 2 Differentiable Quantization
1: procedure DIFFERENTIABLE QUANTIZATION
2: Let w be the networks weights and p the initial quantization points
3: loop
4: wq   quant function(w; p)
5: Run forward pass and compute loss l(wq)
6: Run backward pass and compute@l(wq)
@wq
7: Use equation above to compute@l(wq)
@p
8: Update quantization points using SGD or similar: p = p 􀀀   @l(wq)
@p
9: return p
weights to centroids never changes. By contrast, at every iteration we re-assign weights to the
closest quantization point, and use a different initialization.
4.2 DISCUSSION AND ADDITIONAL HEURISTICS
While the loss is continuous w.r.t. p, there are indirect effects when changing the way each weight
gets quantized. This can have drastic effect on the learning process. As an extreme example, we
could have degeneracies, where all weights get represented by the same quantization point, making
learning impossible. Or diversity of pi gets reduced, resulting in very few weights being represented
at a really high precision while the rest are forced to be represented in a much lower resolution.
To avoid such issues, we rely on the following set of heuristics. Future work will look at adding a
reinforcement learning loss for how the pi are assigned to weights.
Choose good starting points. One way to initialize the starting quantization points is to make
them uniformly spaced, which would correspond to use as a starting point the uniform quantization
function. The differentiable quantization algorithm needs to be able to use a quantization point in
order to update it; therefore, to make sure every quantization point is used we initialize the points to
be the quantiles of the weight values. This ensures that every quantization point is associated with
the same number of values and we are able to update it.
Redistribute bits where it matters. Not all layers in the network need the same accuracy. A
measure of how important each weight is to the final prediction is the norm of the gradient of each
weight vector. So in an initial phase we run the forward and backward pass a certain number of times
to estimate the gradient of the weight vectors in each layer, we compute the average gradient across
multiple minibatches and compute the norm; we then allocate the number of points associated with
each weight according to a simple linear proportion. In short we estimate





E

@l
@v





2
(7)
where l is the loss function,v is the vector of weights in a particular layer and
􀀀 @l
@v

i = @l
@vi
and we
use this value to determine which layers are most sensitive to quantization.
When using this process, we will use more than the indicated number of bits in some layers, and less
in others. We can reduce the impact of this effect with the use of Huffman encoding, see Section
5; in any case, note that while the total number of points stays constant, allocating more points to a
layer will increase bit complexity overall if the layer has a larger proportion of the weights.
Use the distillation loss. In the algorithm delineated above, the loss refers to the loss we used
to train the original model with. Another possible specification is to treat the unquantized model
as the teacher model, the quantized model as the student, and to use as loss the distillation loss
between the outputs of the unquantized and quantized model. In this case, then, we are optimizing
our quantized model not to perform best with respect to the original loss, but to mimic the results of
the unquantized model, which should be easier to learn for the model and provide better results.
Hyperparameter optimization. The algorithm above is an optimization problem very similar to
the original one. As usual, to obtain the best results one should experiment with hyperparameters
optimization, and different variants of gradient descent.
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Published as a conference paper at ICLR 2018
5 COMPRESSION
We now analyze the space savings when using b bits and bucket size of k. Let f be the size of full
precision weights (32 bit) and let N be the size of the “vector” we are quantizing. Full precision
requires fN bits, while the quantized vector requires bN + 2fN
k . (We use b bits per weight, plus the
scaling factors  and  for every bucket). The size gain is therefore g(b; k; f) = kf
kb+2f :
For differentiable quantization, we also have to store the values of the quantization points. Since
this number does not depend on N, the amount of space required is negligible and we ignore it for
simplicity. For example, at 256 bucket size, using 2 bits per component yields 14:2 space savings
w.r.t. full precision, while 4 bits yields 7:52 space savings. At 512 bucket size, the 2 bit savings
are 15:05, while 4 bits yields 7:75 compression.
Huffman encoding. To save additional space, we can use Huffman encoding to represent the quantized
values. In fact, each quantized value can be thought as the pointer to a full precision value; in
the case of non uniform quantization is pk, in the case of uniform quantization is k=s. We can then
compute the frequency for every index across all the weights of the model and compute the optimal
Huffman encoding. The mean bit length of the optimal encoding is the amount of bits we actually
use to encode the values. This explains the presence of fractional bits in some of our size gain tables
from the Appendix.
We emphasize that we only use these compression numbers as a ballpark figure, since additional
implementation costs might mean that these savings are not always easy to translate to practice Han
et al. (2015).
6 EXPERIMENTAL RESULTS
6.1 SMALL DATASETS
Methods. We will begin with a set of experiments on smaller datasets, which allow us to more
carefully cover the parameter space. We compare the performance of the methods described in the
following way: we consider as baseline the teacher model, the distilled model and a smaller model:
the distilled and smaller models have the same architecture, but the distilled model is trained using
distillation loss on the teacher, while the smaller model is trained directly on targets. Further, we
compare the performance of Quantized Distillation and Differentiable Quantization. In addition,
we will also use PM (“post-mortem”) quantization, which uniformly quantizes the weights after
training without any additional operation, with and without bucketing. All the results are obtained
with a bucket size of 256, which we found to empirically provide a good compression-accuracy
trade-off. We refer the reader to Appendix A for details of the datasets and models.
CIFAR-10 Experiments. For image classification on CIFAR-10, we tested the impact of different
training techniques on the accuracy of the distilled model, while varying the parameters of a CNN architecture,
such as quantization levels and model size. Table 1 contains the results for full-precision
training, PM quantization with and without bucketing, as well as our methods. The percentages on
the left below the student models definition are the accuracy of the normal and the distilled model
respectively (trained with full precision). More details are reported in table 11 in the appendix. We
also tried an additional model where the student is deeper than the teacher, where we obtained that
the student quantized to 4 bits is able to achieve significantly better accuracy than the teacher, with
a compression factor of more than 7.
We performed additional experiments for differentiable quantization using a wide residual network
(Zagoruyko & Komodakis, 2016) that gets to higher accuracies; see table 3.
Overall, quantized distillation appears to be the method with best accuracy across the whole range of
bit widths and architectures. It outperforms PM significantly for 2bit and 4bit quantization, achieves
accuracy within 0:2% of the teacher at 8 bits on the larger student model, and relatively minor
accuracy loss at 4bit quantization. Differentiable quantization is a close second on all experiments,
but it has much faster convergence. Further, we highlight the good accuracy of the much simpler
PM quantization method with bucketing at higher bit width (4 and 8 bits).
CIFAR-100 Experiments. Next, we perform image classification with the full 100 classes. Here,
we focus on 2bit and 4bit quantization, and on a single student architecture. The baseline architecture
is a wide residual network with 28 layers, and 36.5M parameters, which is state-of-the-art for its
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Table 1: CIFAR10 accuracy. Teacher model: 5.3M param, 21 MB, accuracy 89.71 %.Details about
the resulting size of the models are reported in table 11 in the appendix.
2 bits 4 bits 8 bits
Student model 1
1M param - 4 MB
84.5% - 88.8%
PM Quant.(No bucket) 9.30 % 67.99 % 88.91 %
PM Quant. (with bucket) 10.53 % 87.18 % 88.80 %
Quantized Distill. 82.4 % 88.00 % 88.82 %
Differentiable Quant. 80.43% 88.31 % ——
Student model 2
0.3M param - 1.27 MB
80.3% - 84.3%
PM Quant. (No bucket) 10.15 % 68.05 % 84.38 %
PM Quant. (with bucket) 11.89 % 81.96 % 84.38 %
Quantized Distill. 74.22 % 83.92 % 84.22 %
Differentiable Quant. 72.79 % 83.49 % ——
Student model 3
0.1M param - 0.45 MB
71.6% - 78.2%
PM Quant. (No bucket) 10.15 % 61.30 % 78.04 %
PM Quant. (with bucket) 10.38 % 72.44 % 78.10 %
Quantized Distill. 67.02 % 77.75 % 77.92 %
Differentiable Quant. 57.84 % 77.36 % ——
Table 2: CIFAR10 accuracy. Teacher model: 5.3M param, 21 MB, accuracy 89.71%. Details about
the resulting size of the models are reported in table 14 in the appendix.
2 bits 4 bits
Deeper student
5.8M param - 23.2 MB
93.22% - 92.6%
PM Quant.(No bucket) 12.60 % 91.11 %
PM (with bucketing) 45.82 % 92.30 %
Quantized Distilled 89.33 % 92.17 %
Table 3: CIFAR10 accuracy (Wide Residual Network). Teacher model: 145M param, 580 MB,
accuracy 95.7 %. Details about the resulting size of the models are reported in table 17 in the
appendix.
Student model 2 bits 4 bits
82.7M param - 330 MB
95.3% - 94.19%
PM Quant. (with bucket) 15.35 % 81.1 %
Quantized Distill. 94.23 % 94.73 %
depth on this dataset. The student has depth and width reduced by 20%, and half the parameters.
It is chosen so that reaches the same accuracy as the teacher model when distilled at full precision.
Accuracy results are given in Table 4. More details are reported in Table 20, in the appendix.
Table 4: CIFAR100 accuracy and model size. Teacher: 36.5M param, 146 MB, acc. 77.21 %.
2 bits 4 bits
Student model
17.2M param - 68.8 MB
77.08% - 77.24%
PM Quant. (No bucket) 1.38 % - 3.18 MB 1.29 % - 5.77 MB
PM Quant. (with bucket) 1.00 % - 3.9 MB 73.5 % - 8.2 MB
Quantized Distill. 27.84 % - 4.3 MB 76.31 % - 8.2 MB
Differentiable Quant. 49.32 % - 7.9 MB 77.07 % - 12.4 MB
The results confirm the trend from the previous dataset, with distilled and differential quantization
preserving accuracy within less than 1% at 4bit precision. However, we note that accuracy loss is
catastrophic at 2bit precision, probably because of reduced model capacity. We note that differentiable
quantization is able to best recover accuracy for this harder task.
OpenNMT Experiments. The OpenNMT integration test dataset (Ope) consists of 200K train
sentences and 10K test sentences for a German-English translation task. To train and test models we
use the OpenNMT PyTorch codebase (Klein et al., 2017). We modified the code, in particular by
adding the quantization algorithms and the distillation loss. As measure of fit we will use perplexity
and the BLEU score, the latter computed using the multi-bleu.perl code from the moses
project (mos).
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Our target models consist of an embedding layer, an encoder consisting of n layers of LSTM, a decoder
consisting of n layers of LSTM, and a linear layer. The decoder also uses the global attention
mechanism described in Luong et al. (2015). For the teacher network we set n = 2, for a total of
4 LSTM layers with LSTM size 500. For the student networks we choose n = 1, for a total of 2
LSTM layers. We vary the LSTM size of the student networks and for each one, we compute the
distilled model and the quantized versions for varying bit width. Results are summarized in Table 5.
The BLEU scores below the student model refer to the BLEU scores of the normal and distilled
model respectively (trained with full precision). Details about the resulting size of the models are
reported in table 23 in the appendix.
Table 5: OpenNMT dataset BLEU score and perplexity (ppl). Teacher model: 84.8M param, 340
MB, 26.1 ppl, 15.88 BLEU. Details about the resulting size of the models are reported in table 23 in
the appendix.
2 bits 4 bits
Student model 1
81.6M param - 326 MB
14.97 - 16.13 BLEU
PM Quant.(No bucket) 0.00 - 2  1017 ppl 0.24 - 2  106 ppl
PM Quant. (with bucket) 4.12 - 125.1 ppl 16.29 - 26.2 ppl
Quantized Distill. 0.00 - 6645 ppl 15.73 - 25.43 ppl
Differentiable Quant. 0.7 - 249 ppl 15.01 - 28.8 ppl
Student model 2
64.8M param - 249 MB
14.22 - 15.48 BLEU
PM Quant. (No bucket) 0.00 - 5  108 ppl 6.65 - 71.78 ppl
PM Quant. (with bucket) 1.72 - 286.98 ppl 15.19 - 28.95 ppl
Quantized Distill. 0.00 - 4035 ppl 15.26 - 29.1 ppl
Differentiable Quant. 0.28 - 306 ppl 13.86 - 31.33 ppl
Student model 3
57.2M param - 228 MB
12.45 - 13.8 BLEU
PM Quant. (No bucket) 0.00 - 3  108 ppl 5.47 - 106.5 ppl
PM Quant. (with bucket) 0.24 - 1984 ppl 12.64 - 36.56 ppl
Quantized Distill. 0.14 - 731 ppl 12 - 37 ppl
Differentiable Quant. 0.26 - 306 ppl 12.06 - 38.44 ppl
A reasonable intuition would be that recurrent neural networks should be harder to quantize than
convolutional neural networks, as quantization errors do not average out when executing repeatedly
through the same cell, but accumulate. Results contradict this intuition. In particular, medium and
large-sized students are able to essentially recover the same scores as the teacher model on this
dataset. Perhaps surprisingly, bucketing PM and quantized distillation perform equally well for 4bit
quantization. As expected, cell size is an important indicator for accuracy, although halving both
cell size and the number of layers can be done without significant loss.
6.2 LARGER DATASETS
WMT13 Experiments. We run a similar LSTM architecture as above for the WMT13
dataset (Koehn, 2005) (1.7M sentences train, 190K sentences test) and we provide additional experiments
for quantized distillation technique, see Table 6. We note that, on this large dataset, PM
quantization does not perform well, even with bucketing. On the other hand, quantized distillation
with 4bits of precision has higher BLEU score than the teacher, and similar perplexity.
Table 6: WMT13 dataset BLEU score and perplexity (ppl). Teacher model: 84.8M param, 340 MB,
5.8 ppl, 34.7 BLEU. Details about model size are reported in Table 26.
4 bits
Student Model
81.6M param - 326 MB
30.22 - 30.21 BLEU
PM Quant. (No bucket) 21.38 BLEU - 12.61 ppl
PM Quant. (with bucket) 27.73 BLEU - 7.4 ppl
Quantized Distill. 35.32 BLEU - 6.48 ppl
The ImageNet Dataset. We also experiment with ImageNet using the ResNet architecture He et al.
(2016a). In the first experiment, we use a ResNet34 teacher, and a student ResNet18 student model.
Experiments quantizing the standard version of this student resulted in an accuracy loss of around
4%, and hence we experiment with a wider model, which doubles the number of filters for each
convolutional layer. We call this 2xResNet18. This is in line with previous work on wide ResNet
architectures (Zagoruyko & Komodakis, 2016), wide students for distillation (Ba & Caruana, 2013),
and wider quantized networks (Mishra et al., 2017). We also note that, in line with previous work
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on this dataset (Zhu et al., 2016; Mishra et al., 2017), we do not quantize the first and last layers of
the models, as this can hurt accuracy.
After 62 epochs of training, the quantized distilled 2xResNet18 with 4 bits reaches a validation
accuracy of 73.31%. Surprisingly, this is higher than the unquantized ResNet18 model (69.75%),
and has virtually the same accuracy as the ResNet34 teacher. In terms of size, this model is more
than 2 smaller than ResNet18 (but has higher accuracy), and is 4 smaller than ResNet34, and
about 1:5 faster on inference, as it has fewer layers. This is state-of-the-art for 4bit models with
18 layers; to our knowledge, no such model has been able to surpass the accuracy of ResNet18.
We re-iterated this experiment using a 4-bit quantized 2xResNet34 student transferring from a
ResNet50 full-precision teacher. We obtain a 4-bit quantized student of almost the same accuracy,
which is 50% shallower and has a 2:5 smaller size.
Table 7: Imagenet accuracy and model size. Bucket size = 256.
Model name Top-1 Accuracy Top-5 Accuracy # of parameters Size (MB)
Teacher model: ResNet34 73.31 % 91.42 % 21.79 millions 87.16 MB
ResNet18 normal 69.75 % 89.07 % 11.69 millions 46.76 MB
2xResNet18 QD 4 bits 73.10 % 91.17 % 45.69 millions 21.98 MB
Teacher model: ResNet50 76.13 % 92.86 % 25.55 millions 102.2 MB
2xResNet34 QD 4 bits 76.07 % 92.71 % 86.11 millions 41.53 MB
6.3 ADDITIONAL EXPERIMENTS
Distillation Loss versus Normal Loss. One key question we are interested in is whether distillation
loss is a consistently better metric when quantizing, compared to standard loss. We tested this for
CIFAR-10, comparing the performance of quantized training with respect to each loss. At 2bit precision,
the student converges to 67:22% accuracy with normal loss, and to 82:40% with distillation
loss. At 4bit precision, the student converges to 86:01% accuracy with normal loss, and to 88:00%
with distillation loss. On OpenNMT, we observe a similar gap: the 4bit quantized student converges
to 32:67 perplexity and 15:03 BLEU when trained with normal loss, and to 25:43 perplexity (better
than the teacher) and 15:73 BLEU when trained with distillation loss. This strongly suggests that
distillation loss is superior when quantizing. For details, see Section A.4.1 in the Appendix.
Impact of Heuristics on Differentiable Quantization. We also performed an in-depth study of how
the various heuristics impact accuracy. We found that, for differentiable quantization, redistributing
bits according to the gradient norm of the layers is absolutely essential for good accuracy; quantiles
and distillation loss also seem to provide an improvement, albeit smaller. Due to space constraints,
we defer the results and their discussion to Section A.4.2 of the Appendix.
Inference Speed. In general, shallower students lead to an almost-linear decrease in inference cost,
w.r.t. the depth reduction. For instance, in the CIFAR-10 experiments with the wide ResNet models,
the teacher forward pass takes 67:4 seconds, while the student takes 43.7 seconds; roughly a 1.5x
speedup, for 1.75x reduction in depth. On the ImageNet test set using 4 GPUs (data-parallel), a
forward pass takes 263 seconds for ResNet34, 169 seconds for ResNet18, and 169 seconds for our
2xResNet18. (So, while having more parameters than ResNet18, it has the same speed because it
has the same number of layers, and is not wide enough to saturate the GPU. We note that we did
not exploit 4bit weights, due to the lack of hardware support.) Inference on our model is 1.5 times
faster, while being 1.8 times shallower, so here the speedup is again almost linear.
7 DISCUSSION
We have examined the impact of combining distillation and quantization when compressing deep
neural networks. Our main finding is that, when quantizing, one can (and should) leverage large,
accurate models via distillation loss, if such models are available. We have given two methods to
do just that, namely quantized distillation, and differentiable quantization. The former acts directly
on the training process of the student model, while the latter provides a way of optimizing the
quantization of the student so as to best fit the teacher model.
Our experimental results suggest that these methods can compress existing models by up to an
order of magnitude in terms of size, on small image classification and NMT tasks, while preserving
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accuracy. At the same time, we note that distillation also provides an automatic improvement in
inference speed, since it generates shallower models. One of our more surprising findings is that
naive uniform quantization with bucketing appears to perform well in a wide range of scenarios. Our
analysis in Section 2.2 suggests that this may be because bucketing provides a way to parametrize
the Gaussian-like noise induced by quantization. Given its simplicity, it could be used consistently
as a baseline method.
In our experimental results, we performed manual architecture search for the depth and bit width of
the student model, which is time-consuming and error-prone. In future work, we plan to examine
the potential of reinforcement learning or evolution strategies to discover the structure of the student
for best performance given a set of space and latency constraints. The second, and more immediate
direction, is to examine the practical speedup potential of these methods, and use them together and
in conjunction with existing compression methods such as weight sharing Han et al. (2015) and with
existing low-precision computation frameworks, such as NVIDIA TensorRT, or FPGA platforms.
ACKNOWLEDGEMENTS
We would like to thank Ce Zhang (ETH Z´urich), Hantian Zhang (ETH Z´urich) and Martin Jaggi
(EPFL) for their support with experiments and valuable feedback.
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A FULL EXPERIMENTAL RESULTS
A.1 CIFAR10
The model used to train CIFAR10 is the one described in Urban et al. (2016) with some minor modifications.
We use standard data augmentation techniques, including random cropping and random
flipping. The learning rate schedule follows the one detailed in the paper. The structure of the models
we experiment with consists of some convolutional layers, mixed with dropout layers and max
pooling layers, followed by one or more linear layers.
The model used are defined in Table 8. The c indicates a convolutional layer, mp a max pooling
layer, dp a dropout layer, fc a linear (fully connected) layer. The exponent indicates how many
consecutive layers of the same type are there, while the number in front of the letter determines the
size of the layer. In the case of convolutional layers is the number of filters. All convolutional layers
of the teacher are 3x3, while the convolutional layers in the smaller models are 5x5.
Table 8: CIFAR10: model specifications
Teacher model 76c2-mp-dp-126c2-mp-dp-148c4-mp-dp-1200fc-dp-1200fc
Smaller model 1 75c-mp-dp-50c2-mp-dp-25c-mp-dp-500fc-dp
Smaller model 2 50c-mp-dp-25c2-mp-dp-10c-mp-dp-400fc-dp
Smaller model 3 25c-mp-dp-10c2-mp-dp-5c-mp-dp-300fc-dp
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Following the authors of the paper, we don’t use dropout layers when training the models using
distillation loss. Distillation loss is computed with a temperature of T = 5.
Table 9 reports the accuracy of the models trained (in full precision) and their size. Table 10 reports
the accuracy achieved with each method, and table 11 reports the optimal mean bit length using
Huffman encoding and resulting model size.
Table 9: CIFAR10: Teacher and distilled model accuracy, full precision
Model name Test accuracy # of parameters Size (MB)
Teacher model 89.7 % 5.3 millions 21.3 MB
Smaller model 1 84.5 % 1.0 millions 4.00 MB
Distilled model 1 88.8 % 1.0 millions 4.00 MB
Smaller model 2 80.3 % 0.3 millions 1.27 MB
Distilled model 2 84.3 % 0.3 millions 1.27 MB
Smaller model 3 71.6 % 0.1 millions 0.45 MB
Distilled model 3 78.2 % 0.1 millions 0.45 MB
Table 10: CIFAR10: Test accuracy for quantized models. Results computed with bucket size = 256
2 bits 4 bits 8 bits
PM Quant. 1 (No bucket) 9.30 % 67.99 % 88.91 %
PM Quant. 1 (with bucket) 10.53 % 87.18 % 88.80 %
Quantized Distill. 1 82.4 % 88.00 % 88.82 %
Differentiable Quant. 1 80.43% 88.31 % —
PM Quant. 2 (No bucket) 10.15 % 68.05 % 84.38 %
PM Quant. 2 (with bucket) 11.89 % 81.96 % 84.38 %
Quantized Distill. 2 74.22 % 83.92 % 84.22 %
Differentiable Quant. 2 72.79 % 83.49 % —
PM Quant. 3 (No bucket) 10.15 % 61.30 % 78.04 %
PM Quant. 3 (with bucket) 10.38 % 72.44 % 78.10 %
Quantized Distill. 3 67.02 % 77.75 % 77.92 %
Differentiable Quant. 3 57.84 % 77.36 % —
Table 11: CIFAR10: Optimal length Huffman encoding and resulting model size. Bucket size = 256
2 bits 4 bits 8 bits
PM Quant. 1 (No bucket) 1.34 bits - 0.17 MB 2.43 bits - 0.3 MB 6.48 bits - 0.81 MB
PM Quant. 1 (with bucket) 1.58 bits - 0.22 MB 3.52 - 0.47 MB 7.58 bits - 0.98 MB
Quantized Distill. 1 1.7 bits - 0.24 MB 3.64 bits - 0.48 MB 7.70 bits - 1 MB
Differentiable Quant. 1 3.18 bits - 0.43 MB 5.34 bits - 0.7 MB ——
PM Quant. 2 (No bucket) 1.43 bits - 0.05 MB 2.6 bits - 0.1 MB 6.65 bits - 0.26 MB
PM Quant. 2 (with bucket) 1.6 bits - 0.07 MB 3.58 bits - 0.15 MB 7.64 bits - 0.31 MB
Quantized Distill. 2 1.7 bits - 0.08 MB 3.55 bits - 0.15 MB 7.64 bits - 0.31 MB
Differentiable Quant. 2 3.16 bits - 0.13 MB 5.34 bits - 0.22 MB ——
PM Quant. 3 (No bucket) 1.46 bits - 0.02 MB 2.62 bits - 0.03 MB 6.66 bits - 0.09 MB
PM Quant. 3 (with bucket) 1.58 bits - 0.026 MB 3.51 bits - 0.053 MB 7.56 bits - 0.1 MB
Quantized Distill. 3 1.64 bits - 0.027 MB 3.53 bits - 0.053 MB 7.59 bits - 0.11 MB
Differentiable Quant. 3 3.12 bits - 0.04 MB 5.41 bits - 0.08 MB ——
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We also performed an experiment with a deeper student model. The architecture is 76c3-mp-dp-
126c3-mp-dp-148c5-mp-dp-1000fc-dp-1000fc-dp-1000fc (following the same notation as in table
8). We use the same teacher as in the previous experiments. Results are in table 13.
Table 12: CIFAR10: Teacher and distilled model accuracy, full precision
Model name Test accuracy # of parameters Size (MB)
Teacher model 89.7 % 5.3 millions 21.3 MB
Deeper normal model 1 93.22 % 5.8 millions 23.40 MB
Deeper distilled model 1 92.60 % 5.8 millions 23.40 MB
Table 13: CIFAR10: Test accuracy for quantized deeper student model. Results computed with
bucket size = 256
2 bits 4 bits
PM Quant. (No bucket) 12.60 % 91.11 %
PM Quant. (with bucket) 45.82 % 92.30 %
Quantized Distill. 89.33 % 92.17 %
Table 14: CIFAR10: Optimal length Huffman encoding and resulting model size for deeper student
model. Bucket size = 256
2 bits 4 bits
PM Quant. (No bucket) 1.50 bits - 1.13 MB 3.21 bits - 2.38 MB
PM Quant. (with bucket) 1.82 bits - 1.55 MB 3.92 bits - 3.07 MB
Quantized Distill. 1.84 bits - 1.56 MB 3.92 bits - 3.08 MB
A.1.1 CIFAR10 - WIDERESNET ARCHITECTURE
For our second set of experiments on CIFAR10 with the WideResNet architecture, see table 15.
Note that we increase the number of filters but reduce the depth of the model. The implementation
of WideResNet used can be found on GitHub 2. Results of quantized methods are in table 16 while
the size of the resulting models is detailed in table 17.
Table 15: CIFAR10: Teacher and distilled model accuracy, full precision, wide resnet
Model name Structure Test accuracy # of parameters Size (MB)
Teacher model depth = 28, wide factor = 20 95.74 % 145 millions 580 MB
Smaller model depth = 22, wide factor = 16 95.19 % 82.7 millions 330 MB
Distilled model depth = 22, wide factor = 16 94.19 % 82.7 millions 330 MB
Table 16: CIFAR10: Test accuracy for quantized models. Results computed with bucket size = 256
2 bits 4 bits
PM Quant. (with bucket) 15.35 % 81.09 %
Quantized Distill. 94.23 % 94.73 %
A.2 CIFAR100
For our CIFAR100 experiments, we use the same implementation of wide residual networks as in
our CIFAR10 experiments. The wide factor is a multiplicative factor controlling the amount of filters
2https://github.com/meliketoy/wide-resnet.pytorch
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Table 17: CIFAR10: Optimal length Huffman encoding and resulting model size. Bucket size = 256
2 bits 4 bits
PM Quant. (with bucket) 1.44 bits - 17.56 MB 2.62 bits - 29.75 MB
Quantized Distill. 1.54 bits - 17.81 MB 3.48 bits - 38.65 MB
in each layer; for more details please refer to the original paper Zagoruyko & Komodakis (2016).
We train for 200 epochs with an initial learning rate of 0.1.
For the CIFAR100 experiments we focused on one student model. Distillation loss is computed with
a temperature of T = 5.
Table 18: CIFAR100: Teacher and distilled model accuracy, full precision
Model name Structure Test accuracy # of parameters Size (MB)
Teacher model depth = 28, wide factor = 10 77.21 % 36.5 millions 146 MB
Smaller model depth = 22, wide factor = 8 77.08 % 17.2 millions 68.8 MB
Distilled model depth = 22, wide factor = 8 77.24 % 17.2 millions 68.8 MB
Table 19: CIFAR100: Test accuracy for quantized models. Results computed with bucket size = 256
2 bits 4 bits
PM Quant. (No bucket) 1.38% 1.29%
PM Quant. (with bucket) 1.00 % 73.47%
Quantized Distill. 27.84% 76.31%
Differentiable Quant. 49.32% 77.07%
Table 20: CIFAR100: Optimal length Huffman encoding and resulting model size. Bucket size =
256
2 bits 4 bits
PM Quant. (No bucket) 1.47 bits - 3.18 MB 2.68 bits - 5.77 MB
PM Quant. (with bucket) 1.56 bits - 3.90 MB 3.55 bits - 8.18 MB
Quantized Distill. 1.73 bits - 4.27 MB 3.54 bits - 8.16 MB
Differentiable Quant. 3.23 bits - 7.84 MB 5.53 bits - 12.44 MB
A.3 OPENTNMT INTEGRATION TEST DATASET
As mentioned in the main text, we use the openNMT-py codebase. We slightly modify it to add
distillation loss and the quantization methods proposed. We mostly use standard options to train
the model; in particular, the learning rate starts at 1 and is halved every epoch starting from the first
epoch where perplexity doesn’t drop on the test set. We train every model for 15 epochs. Distillation
loss is computed with a temperature of T = 1.
A.4 WMT13 DATASET
For the WMT13 datasets, we run a similar architecture. We ran all models for 15 epochs; the smaller
model overfit with 15 epochs, so we ran it for 5 epochs instead.
A.4.1 DISTILLATION VERSUS STANDARD LOSS FOR QUANTIZATION
In this section we highlight the positive effects of using distillation loss during quantization. We
take models with the same architecture and we train them with the same number of bits; one of the
models is trained with normal loss, the other with the distillation loss with equal weighting between
soft cross entropy and normal cross entropy (that is, it is the quantized distilled model).
16
Published as a conference paper at ICLR 2018
Table 21: openNMT integ: Teacher and distilled models perplexity and BLEU, full precision
Model name Structure Perplexity BLEU # of parameters Size (MB)
Teacher model 4 LSTM layer, 500 cell size 26.21 15.88 84.8 millions 339.28 MB
Smaller model 1 2 LSTM layer, 512 cell size 33.03 14.97 81.6 millions 326.57 MB
Distilled model 1 2 LSTM layer, 512 cell size 25.55 16.13 81.6 millions 326.57 MB
Smaller model 2 2 LSTM layer, 256 cell size 34.5 14.22 64.8 millions 249.56 MB
Distilled model 2 2 LSTM layer, 256 cell size 27.7 15.48 64.8 millions 249.56 MB
Smaller model 3 2 LSTM layer, 128 cell size 39.5 12.45 57.2 millions 228.85 MB
Distilled model 3 2 LSTM layer, 128 cell size 33.78 13.8 57.2 millions 228.85 MB
Table 22: openNMT integ: Test accuracy for quantized models. Results computed with bucket size
= 256
2 bits 4 bits
PM Quant. 1 (No bucket) 2  1017 ppl - 0.00 BLEU 2:7  106 ppl - 0.24 BLEU
PM Quant. 1 (with bucket) 125.1 ppl - 4.12 BLEU 26.21 ppl - 16.29 BLEU
Quantized Distill. 1 6645 ppl - 0.00 BLEU 25.43 ppl - 15.73 BLEU
Differentiable Quant. 1 249 ppl - 0.7 BLEU 28.8 ppl - 15.01 BLEU
PM Quant. 2 (No bucket) 5  108 ppl - 0.00 BLEU 71.78 ppl - 6.65 BLEU
PM Quant. 2 (with bucket) 286.98 ppl - 1.72 BLEU 28.95 ppl - 15.19 BLEU
Quantized Distill. 2 4035 ppl - 0.00 BLEU 29.1 ppl - 15.26 BLEU
Differentiable Quant. 2 306 ppl - 0.28 BLEU 31.33 ppl - 13.86 BLEU
PM Quant. 3 (No bucket) 3  108 ppl - 0.00 BLEU 106.5 ppl - 5.47 BLEU
PM Quant. 3 (with bucket) 1984 ppl - 0.24 BLEU 36.56 ppl - 12.64 BLEU
Quantized Distill. 3 731 ppl - 0.14 BLEU 37 ppl - 12 BLEU
Differentiable Quant. 3 306 ppl - 0.26 BLEU 38.4 ppl - 12.06 BLEU
Table 23: openNMT integ: Optimal length Huffman encoding and resulting model size. Bucket size
= 256
2 bits 4 bits
PM Quant. 1 (No bucket) 1.36 bits - 13.93 MB 1.77 bits - 18.10 MB
PM Quant. 1 (with bucket) 1.65 bits - 19.47 MB 3.69 bits - 40.26 MB
Quantized Distill. 1 1.75 bits - 20.4 MB 3.66 bits - 39.97 MB
Differentiable Quant. 1 1.72 bits - 20.1 MB 4.38 bits - 47.32 MB
PM Quant. 2 (No bucket) 1.34 bits - 10.89 MB 1.86 bits - 15.09 MB
PM Quant. 2 (with bucket) 1.65 bits - 15.4 MB 3.68 bits - 31.91 MB
Quantized Distill. 2 1.85 bits - 17.05 MB 3.68 bits - 31.91 MB
Differentiable Quant. 2 1.93 bits - 17.67 MB 4.17 bits - 35.83 MB
PM Quant. 3 (No bucket) 1.47 bits - 10.54 MB 2.13 bits - 15.24 MB
PM Quant. 3 (with bucket) 1.65 bits - 13.6 MB 3.68 bits - 28.14 MB
Quantized Distill. 3 1.86 bits - 15.13 MB 3.68 bits - 28.18 MB
Differentiable Quant. 3 1.99 bits - 16.04 MB 4.25 bits - 31.18 MB
Table 27 shows the results on the CIFAR10 dataset; the models we train have the same structure as
the Smaller model 1, see Section A.1.
Table 28 shows the results on the openNMT integration test dataset; the models trained have the
same structure of Smaller model 1, see Section A.3. Notice that distillation loss can significantly
improve the accuracy of the quantized models.
17
Published as a conference paper at ICLR 2018
Table 24: WMT13: Teacher and distilled models perplexity and BLEU, full precision
Model name Structure Perplexity BLEU # of parameters Size (MB)
Teacher model 4 LSTM layer, 500 cell size 5.83 34.77 84.8 millions 339.28 MB
Smaller model 1 2 LSTM layer, 500 size 7.98 30.22 80.8 millions 323.25 MB
Distilled model 1 2 LSTM layer, 550 cell size 7.18 30.21 84.3 millions 337.21 MB
Table 25: WMT13: Test accuracy for quantized models. Results computed with bucket size = 256
4 bits
PM Quant. (No bucket) 12.17 ppl - 22.79 BLEU
PM Quant. (with bucket) 7.34 ppl - 26.18 BLEU
Quantized Distill. 6.48 ppl - 35.32 BLEU
Table 26: WMT13: Optimal length Huffman encoding and resulting model size. Bucket size = 256
4 bits
PM Quant. 1 (No bucket) 1.98 bits - 20.92 MB
PM Quant. 1 (with bucket) 3.63 bits - 41.02 MB
Quantized Distill. 1 3.65 bits - 41.16 MB
Table 27: CIFAR10: Distillation loss vs normal loss when quantizing
2 bits 4 bits
Normal loss 67.22 % 86.01 %
Distillation loss 82.40 % 88.00 %
Table 28: openNMT integ: Distillation loss vs normal loss when quantizing
4 bits
Normal loss 32.67 ppl 15.03 BLEU
Distillation loss 25.43 ppl 15.73 BLEU
These results suggest that quantization works better when combined with distillation, and that we
should try to take advantage of this whenever we are quantizing a neural network.
A.4.2 DIFFERENT HEURISTICS FOR DIFFERENTIABLE QUANTIZATION
To test the different heuristics presented in Section 4.2, we train with differentiable quantization the
Smaller model 1 architecture specified in Section A.1 on the cifar10 dataset. The same model is
trained with different heuristics to provide a sense of how important they are; the experiments is
performed with 2 and 4 bits.
Results suggests that when using 4 bits, the method is robust and works regardless. When using 2
bits, redistributing bits according to the gradient norm of the layers is absolutely essential for this
method to work ; quantiles starting point also seem to provide an small improvement, while using
distillation loss in this case does not seem to be crucial.
B QUANTIZATION IS EQUIVALENT TO ASYMPTOTICALLY NORMALLY
DISTRIBUTED NOISE
In this section we will prove some results about the uniform quantization function, including the
fact that is asymptotically normally distributed, see subsection B.1 below. Clearly, we refer to the
stochastic version, see Section 2.1.
18
Published as a conference paper at ICLR 2018
Table 29: Results with automatically redistributed bits
Quantiles Uniform
2 bits Distillation loss 82.94 % 78.76 %
Normal loss 83.67 % 76.60 %
4 bits Distillation loss 88.93 % 88.50 %
Normal loss 88.80 % 88.74 %
Table 30: Results without automatically redistributed bits
Quantiles Uniform
2 bits Distillation loss 19.69 % 22.81 %
Normal loss 25.28 % 22.11 %
4 bits Distillation loss 88.39 % 88.67 %
Normal loss 88.43 % 88.44 %
Unbiasedness
We first start proving the unbiasedness of ^ Q;
E[^ Q(^v)i] =
b^visc
s
+
1
s
E[i] =
b^visc
s
+
1
s
(s^vi 􀀀 b^visc) = ^vi (8)
Then it is immediate that
E[Q(v)] = E

^Q

v 􀀀 


+  = 
v 􀀀 

+  = v (9)
Bounds on second and third moment
We will write out bounds on ^ Q; the analogous bounds on Q are then straightforward. For convenience,
let us call^l
i = b^visc
E[^ Q(^v)2i
] =
^l
2
i
s2 +
1
s2E[2
i ] + 2
^l
i
s2E[i] = (10)
=
^l
2
i
s2 +
1
s2 (s^vi 􀀀^l
i) + 2
^l
i
s2 (s^vi 􀀀^l
i) = (11)
=
1
s2
h
^vis(1 + 2^l
i) 􀀀^l
i(^l
i + 1)
i
(12)
And given that^l
i  ^vis  ^li + 1, we readily find
^l
2
i
s2
 E[^ Q(^v)2i
] 
(^l
i + 1)2
s2 (13)
For the third moment, we have
E[^ Q(^v)3i
] =
^l
3
i
s3 +
1
s3E[3
i ] + 3
^l
i
s3E[2
i ] + 3
^l
2
i
s3E[i] = (14)
=
^l
3
i
s3 +
1
s3 (s^vi 􀀀^l
i) + 3
^l
i
s3 (s^vi 􀀀^l
i) + 3
^l
2
i
s3 (s^vi 􀀀^l
i) = (15)
=
1
s3
h
^vis(3^l
2
i + 3^l
i + 1) 􀀀^l
i(2^l
2
i + 3^l
i + 1)
i
(16)
19
Published as a conference paper at ICLR 2018
And as before, the bounds are
^l
3
i
s3
 E[^ Q(^v)3i
] 
(^l
i + 1)3
s3 (17)
B.1 ASYMPTOTIC NORMALITY
Most of neural networks operations are scalar product computation. Therefore, the scalar product of
the quantized weights and the inputs is an important quantity:
Q(v)T x =
Xn
i=1
Q(vi)xi
We already know from section B that the quantization function is unbiased; hence we know that
Xn
i=1
Q(vi)xi =
Xn
i=1
vixi + "n (18)
with "n is a zero-mean random variable. We will show that "n tends in distribution to a normal
random variable. To prove asymptotic normality, we will use a generalized version of the central
limit theorem due to Lyapunov:
Theorem B.1 (Lyapunov Central Limit Theorem). Let fX1;X2; : : : g be a sequence of independent
random variables, each with finite expected value i and variance 2
i . Define s2
n =
Pn
i=1 2
i . If, for
some  > 0, the Lyapunov condition
lim
n!1
1
s2+
n
Xn
i=1
E

jXi 􀀀 ij2+
= 0 (19)
is satisfied, then
1
sn
Xn
i=1
(Xi 􀀀 i) D 􀀀! N(0; 1) (20)
We can now state the theorem:
Theorem B.2. Let v; x be two vectors with n elements. Let Q be the uniform quantization function
with s levels defined in 2.1 and define s2
n =
Pn
i=1 V ar[Q(vi)xi]. If the elements of v; x are
uniformly bounded by M 3 and limn!1 sn = 1, then
Xn
i=1
Q(vi)xi =
Xn
i=1
vixi + "n (21)
with E["n] = 0 and
lim
n!1
1
sn
"n
D 􀀀! N(0; 1) (22)
Proof. Using the same notation as theorem B.1, let Xi = Q(vi)xi, i = E[Xi] = vixi. We already
mentioned in 2.1 that these are independent random variables. We will show that the Lyapunov
condition holds with  = 1.
We know that
E

jXi 􀀀 ij3
= E

(Xi 􀀀 i)2jXi 􀀀 ij


M2
s
E

(Xi 􀀀 i)2
(23)
3i.e. there exists a constant M such that for all n, jvij  M, jxij  M for all i 2 f1; : : : ; ng
20
Published as a conference paper at ICLR 2018
In fact,
jXi 􀀀 ij = jxijjQ(vi) 􀀀 vij = jxij

i ^Q

vi 􀀀 i
i

+ i 􀀀 vi

 (24)
 jxij

i

vi 􀀀 i
i
+
1
s

+ i 􀀀 vi

 (25)
 jxij
M
s

M2
s
(26)
since during quantization we have bins of size 1
s , so that is the largest error we can make. Also, by
hypothesis M  i; xi for every i.
Hence
0 
1
s3
n
Xn
i=1
E

jXi 􀀀 ij3

1
s3
n
M2
s
Xn
i=1
E

(Xi 􀀀 i)2)

=
M2
s

1
sn
(27)
and since limn!1 sn = 1, we have that the Lyapunov condition is satisfied. Hence
1
sn
Xn
i=1
(Xi 􀀀 i) =
1
sn
Xn
i=1
(Q(vi)xi 􀀀 vixi) =
1
sn
"n
D 􀀀! N(0; 1) (28)
Note about the hypothesis The two hypothesis that were used to prove the theorem are reasonable
and should be satisfied by any practical dataset. Typically we know or we can estimate the range
of the values of inputs and weights, so the assumption that they don’t get arbitrarily large with n
is satisfied. The assumption on the variance is also reasonable; in fact, s2
n =
Pn
i=1 V ar[Q(vi)xi]
consists of a sum of n values. While it is possible for all these values to be 0 (if all vi are in the
form k=s, for example, then s2
n = 0) it is unlikely that a real world dataset would present this
characteristic. In fact, it suffices that there exist 
 > 0 and 0 <   1 such that at least -percent of
2
i  
. This implies s2
n  
n ! 1.
Asymptitc normality when quantizing inputs Theorem B.2 can be easily extended to the case
when also xi are quantized. The proof is almost identical; we simply have to set Xi = Q(vi)Q(xi)
and use the independence of Q(xi) and Q(vi). For completeness, we report the statement of the
theorem :
Theorem B.3. Let v; x be two vectors with n elements. Let Q be the uniform quantization function
with s levels defined in 2.1 and define s2
n =
Pn
i=1 V ar[Q(vi)Q(xi)]. If the elements of v; x are
uniformly bounded by M 4 and limn!1 sn = 1, then
Xn
i=1
Q(vi)Q(xi) =
Xn
i=1
vixi + "n (29)
with E["n] = 0 and
lim
n!1
1
sn
"n
D 􀀀! N(0; 1) (30)
4i.e. there exists a constant M such that for all n, jvij  M, jxij  M for all i 2 f1; : : : ; ng
21