Sparse Convolutional Neural Networks∗
Baoyuan Liu1 , Min Wang1 , Hassan Foroosh1 , Marshall Tappen3 , and Marianna Penksy2
1

Computational Imaging Lab, Computer Science, University of Central Florida, Orlando, FL, USA
2
Department of Mathematics, University of Central Florida, Orlando, FL, USA
3
Amazon.com, Seattle, WA 98109{bliu, mwang, foroosh}@cs.ucf.edu, tappenm@amazon.com, Marianna.Pensky@ucf.edu

Abstract

input	
  feature	
  maps	  input	
  feature	
  maps	  channel	
  
	
  	
  	
  basis	  Deep neural networks have achieved remarkable performance in both image classification and object detection
problems, at the cost of a large number of parameters andcomputational complexity. In this work, we show how to
reduce the redundancy in these parameters using a sparsedecomposition. Maximum sparsity is obtained by exploiting both inter-channel and intra-channel redundancy, witha fine-tuning step that minimize the recognition loss caused
by maximizing sparsity. This procedure zeros out more than
90% of parameters, with a drop of accuracy that is lessthan 1% on the ILSVRC2012 dataset. We also propose
an efficient sparse matrix multiplication algorithm on CPU
for Sparse Convolutional Neural Networks (SCNN) models.
Our CPU implementation demonstrates much higher efficiency than the off-the-shelf sparse matrix libraries, witha significant speedup realized over the original dense network. In addition, we apply the SCNN model to the object detection problem, in conjunction with a cascade modeland sparse fully connected layers, to achieve significant
speedups.

kernel	  	
  basis	  convolu1on	
  kernels	  sparse	  kernel	  matrix	  output	  feature	
  maps	  output	  feature	
  maps	  Figure 1: Overview of our sparse convolutional neural network.
Left: the operation of convolution layer for classical CNN, which
convolves large amount of convolutional kernels with the input
feature maps. Right: our proposed SCNN model. We apply twostage decompositions over the channels and the convolutional kernels, obtaining a remarkably(more than 90%) sparse kernel matrix
and converting the operation of convolutional layer to spare matrix
multiplication.

1. Introduction

network size and the classification accuracy. The ILSRVR
2014 submission from VGG [20] builds a network with up
to 16 convolutional layers that reduces the top-5 classification error to 7.4%, at the expense of approximately one
month of network training with 4 high-end GPUs.

In this paper, we show how expressing the filtering steps
in a convolutional neural network using sparse decomposition can dramatically cut down the cost of computation,
while maintaining the accuracy of the system. Deep neural networks have achieved remarkable performance in both
image classification and object detection problems [14][8].
Results of ImageNet LSVRC [2] competitions in recent
years have demonstrated a strong correlation between the

The structure of these networks makes it reasonable toconjecture that there exists heavy redundancy in these huge
networks. Due to the highly non-convex property of neuralnetworks, over-parameterization, together with random initialization, is necessary to overcome the negative impact of
local minimum in network training. Additionally, the factthat no independence constraint is imposed among the con-∗ This work was supported in part by the National Science Foundation under grants IIS-1212948, IIS-091686, DMS-1106564 and DMS1407475.

1

volutional kernels for each layer in the training phase also
indicates high potential for redundancy.
In this paper, we show that this redundancy makes it possible to notably reduce the amount of computation required
to process images, by sparse decompositions of the convolutional kernels. As Figure 1 illustrate, two-stage decompositions are applied to explore the inter-channel and intrachannel redundancy of convolution kernels. We first perform an initial decomposition based on the reconstruction
error of kernel weights, then fine-tune the network while
imposing the sparsity constraint. In the fine-tuning phase,
we optimize the network training error, the sparsity of convolutional kernels, as well as the number of convolutionalbases simultaneously, by minimizing a sparse group-lasso
object function. Surprisingly high sparsity can be achievedin our model. We are able to zero out more than 90% of the
convolutional kernel parameters of the network in [14] withrelatively small number of bases while keeping the drop of
accuracy to less than 1%.
In our Sparse Convolutional Neural Networks (SCNN)
model, each sparse convolutional layer can be performed
with a few convolution kernels followed by a sparse matrix multiplication. It could be assumed that the sparse matrix formulation naturally leads to highly efficient computation. However, computing sparse matrix multiplication
can involve severe overhead that makes it difficult to actually achieve attractive acceleration. Thus, we also propose
an efficient sparse matrix multiplication algorithm. Basedon the fact that the sparse convolutional kernels are fixed
after training, we avoid the necessity of indirect and discontinuous memory access by encoding the structure of the
input sparse matrix into our program as the index of registers. Our CPU-based implementation demonstrates muchhigher efficiency than off-the-shelf sparse matrix libraries
and a significant speedup over the original dense networks
is realized. While convolutional network systems are dominated by GPU-based approaches, advances in CPU-based
systems are useful because they can be deployed in commodity clusters that do not have specialized GPU nodes.

2. Related Work
Several attempts have been made to study the redundancy of deep neural networks. Denil et al. [3] reduce
the number of parameters in general neural network withlow rank matrix factorization. They obtain 95% parameter
reduction of MLP network on MNIST. Both Jaderberg etal. [12] and Denton et al. [4] use the idea of tensor lowrank expansions technique to speedup convolutional neuralnetworks. Jaderberg et al. [12] obtain 4.5x speedup withless than 1% drop in accuracy of a 4 layer CNN trained ona scene character classification dataset. Denton et al. [4]achieve 2× speedup on the first two convolutional layers of
CNN trained on ILSVRC dataset. Notably, both [12] and[4] only demonstrate speedups on relatively large convolutional kernel size. None of them show that their method canwork on kernels as small as 3 × 3, which are extensively
used in state-of-the-art CNN models.
There are also several works that try to optimize thespeed of CNN from other perspectives. Vanhoucke etal. [22] studies CPU based general neural network speed
optimization. They discuss the usage of SIMD instructions,alignment of memory, as well as fixed point quantizationof the network. Mathieu et al. [17] proposes to utilize FFT
to perform convolution in Fourier domain. They achieve 2x
speedup on Alex net. Their method prefers a relatively large
kernel size due to the overhead of FFT. Farabet et al. [6] implement a large scale CNN based on FPGA infrastructurethat can perform embedded real-time recognition tasks.
Previous works on sparse matrix computation focus on
the sparse matrix dense vector multiplication (SpMV) problem. The sparse matrix is stored with various formats,such as CSR [1] and ESB [15], for efficiency. Blocking
is adopted in register [11] and cache [18] level to improve
the spatial locality of sparse matrix. To further reduce bandwidth requirement, various techniques including matrix reordering [19], value and index compression [23] are proposed. We refer readers to [9] for a more comprehensive
review.

3. Our Method
3.1. Sparse Convolutional Neural Networks

Contribution Relative to Previous WorkAs will be discussed in Section 2, below, previous work,
such as [4][12], have used low-rank approximations to express the network computations in terms of a smaller number of basis filter. The approach presented here gains additional efficiency by expressing the filtering steps using asparse decomposition, in addition to low-rank approximations. As will be shown in Section 6.2, this results in a combination of efficiency and accuracy that cannot be matchedby just a low-rank decomposition.

Consider the input feature maps I in Rh×w×m , where h,w and m are the height, width and number of channels of
the input feature maps, and the convolutional kernel K in
Rs×s×m×n , where s is size of the convolutional kernel andn is the number of output channels. We assume the convolution is performed with no padding zeros and stride equals
to 1. Then, the output feature maps of a convolutional layer
O ∈ R(h−s+1)×(w−s+1)×n = K ∗ I are given bym X
sXO(y,x,j) =K(u, v, i, j)I(y+u−1,x+v−1,i) (1)i=1 u,v=1

Our objective is to replace computationally expensive convolutional operation O = K ∗ I in formula (1) by its fastsparsified version which is based on multiplication of sparse
matrices.
For this purpose, we first transform the tensor I to J ∈Rh×w×m and convolutional kernel K to R ∈ Rs×s×m×n
using a matrix P ∈ Rm×m obtaining O ≈ R ∗ J whereK(u, v, i, j) ≈J(y, x, i) =m
X

R(u, v, k, j)P(k, i)

k=1

m
X

(2)

P(i, k)I(y, x, k)

k=1

Next, for every channel i = 1, · · · , m, we decompose
tensor R(·, ·, i, ·) ∈ Rs×s×n into the product of matrix
Si ∈ Rqi ×n and tensor Qi ∈ Rs×s×qi , where qi is thenumber of bases:qiXR(u, v, i, j) ≈Si (k, j)Qi (u, v, k)
k=1

Ti (y, x, k) =

s
X

(3)

Qi (u, v, k)J(y+u−1, x+v−1, i).

u,v=1

so thatO(y, x, j) ≈

qi
m X
X
i=1 k=1

Si (k, j)Ti (y, x, k)(4)Note that if we represent the tensor O and Ti as matrices bycombining the first two dimensions, and concatenate both
Si and Ti along the dimension qi , formula (4) can be implemented by a single matrix multiplication.
Here, we shall search for matrices P, Qi and Si , i =1, · · · , m, such that qi are much smaller than s2 , matrices
Si have a large number of zero elements and columns, while
our new sparse convolutional kernel R provides output thatis close to the one obtained with the original kernel K.

3.2. Computational Complexity
We analyze the theoretical complexity of our method bymeasuring the number of multiplications. The multiplications required by original convolution is given by:
m n s2 (h − s + 1)(w − s + 1)

(5)

Our method reduces the complexity by sparsifying the theconvolutional kernel, while introducing overhead of two
matrix decompositions:
!
n
X
γmn +
qi s2 (h − s + 1)(w − s + 1) + m2 hw (6)i=1

where γ is the proportion of non-zeros of the sparse matrix.
The decomposition overhead is small when (1) average ofqi is much smaller than γm and (2) m is much smaller thanγns2 .

3.3. Learning Parameters
The parameters of Sparse Convolutional Neural Networks are learned in two phases: initial decomposition andfine-tuning. The factorization in both formula (2) and formula (3) can be treated as matrix factorization problem if
we transform the convolutional kernel from tensor to matrix
along the decomposition dimension. Sparse matrix decomposition algorithm is an intuitive choice for initialization.
However, there are a couple of concerns regarding current
sparse dictionary learning algorithms: (a) They are not convex and therefore cannot achieve a global optimum solution.
(b) The learning process is a tradeoff between accuracy andsparsity, hence cannot provide exact decomposition. Dueto these concerns, we choose the following decomposition
methods and compare their performance in Section 6.4:
• Decompose K and R using the sparse dictionary
learning algorithm in [16], with P, Qi the bases• Decompose K and R using Principal Component
Analysis (PCA), with P, Qi the principal components• Initialize P, Qi as identity matrices, and keep K and
R unchanged. In this way, sparsity is achieved solely
by fine-tuning.
As initial decompositions, the above methods can only
obtain very limited sparsity, but provide meaningful starting
point with very low or no reconstruction error. Further finetuning is critical for obtaining a highly sparse network. In
the fine-tuning phase, we impose sparsity constraints overthe network parameters, while continuing to train the wholenetwork. The underlying reason of why fine-tuning can increase sparsity is that the original network is trained without any sparsity constraint. Since the network is known to
be over-parameterized to avoid the sensitivity to initialization, it should have some freedom of modification withoutcompromising the classification accuracy. The initialization phase can only obtain sparsity based on the reconstruction error, while the fine-tuning process can fully exploit
the sparsity potential of our model by adopting the network
loss as the direct error measurement. Our experiments justify this point by showing a significant increase in sparsity
due to fine-tuning.
Formally, we minimize the following objective functionin the fine-tuning phaseminimize Lnet +λ1
P,Qi ,Sim
X
i=1

kSi k1 +λ2s.t. kP(·, j)k2 ≤ 1, j = 1, . . . , m

qi
m X
X
i=1 j=1

kSi (j, ·)k2

kQi (·, ·, k)k2 ≤ 1, i = 1, . . . , m, k = 1, . . . , qi

(7)where Lnet stands for the logistic loss function of the output
layer of the network[14], and k · k1 and k · k2 denote theelement-wise l1 and l2 norms of a matrix. The second termin (7) imposes lasso constraint over each of the elementsof Si , and the third term treats each row of Si as a groupvariable in a spirit of group-lasso formulation. The effective
columns of each Qi that need to be calculated is equal to thenumber of non-zero rows of corresponding Si . In this way,
we can further reduce the overhead of convolving Qi overthe feature maps.

3.4. Comparison between our method and low-rank
decomposition
Like [12] [4], our model uses a low-rank decomposition,
but goes beyond previous work by encouraging sparsity inthe weights used to express the filter to further reduce theredundancy in convolutional layers. We impose both sparse
constraints and low-rank constraints with a combination ofan l1 norm and group lasso penalty
Consider the decomposing matrix M ∈ Rm×n as themultiplication of S ∈ Rm×n and P ∈ Rn×n . Using asparse decomposition, the speedup is proportional to thepercentage of non-zero elements in S, while the reduction
in complexity using a decomposition is proportional to thepercentage of non-zero columns. The sparsity constraint isable to target specific entries in S, while a low-rank constraint must eliminate entire columns. This makes it possible for our approach to target redundancy more precisely.
As the results in Section 6.2 will show, this leads to remarkable performance improvement using the multiplication algorithm.

Figure 2: Matrix Multiplication Algorithm in OpenBLAS. The input matrices are first divided into blocks which can fit in L2 cache.
Each block is then divided into 8 element wide strips. The multiplication outputs of two strips are held in 8 AVX registers during
calculation.

with input images. Therefore, the location of non-zeroelements are known and can be encoded directly in thecompiled multiplication code.
• Only the convolutional kernels are treated as sparse
matrices. The input feature maps do possess moderate sparsity for some layers, but will be treated as adense matrix.

While the sparsity penalties can target redundancy more
precisely, the performance benefits can only be realizedwith a sparse matrix multiplication algorithm that does notincur overhead that overwhelms the benefit of the sparse
matrix. In this section, we show how the multiplication canbe implemented efficiently.

We implemented our method on x86 64 CPU microarchitecture with the Advanced Vector Extension (AVX),
which is available on both Intel and AMD’s CPUs after
2011, although we expect that this approach could be extended to GPU architectures. Our method is based on
OpenBLAS[24], which is an open source dense linear algebra library. In the following sections, we first briefly describe the matrix multiplication algorithm in OpenBLAS,
then introduce our method on sparse matrices.

4.1. Motivation

4.2. Dense Matrix Multiplication in OpenBLAS

To avoid extra storage and calculation of zero values, thenon-zero elements in a sparse matrix are typically storedcontinuously, with their locations indexed in some specificstructure. This leads to indirect jumping memory accesswhen traversing the matrix, which is much slower than thecontinuous direct access used in the dense case. In addition,
the irregular pattern of input matrices also makes it difficult
to fully utilize the capacity of Single Instruction Multiple
Data (SIMD) micro architectures, which is the key in highperformance dense matrix algorithms.
We propose an efficient, sparse-dense matrix multiplication algorithm for executing the sparse convolutional kernels. Our idea is based on the following two key observations:

There are two main considerations on designing an efficient matrix multiplication algorithm: (a) Taking advantage of SIMD instructions for higher computing throughput; (b) Maximally utilizing the caches to reduce memory
access latency. In latest OpenBLAS library, matrix multiplication is implemented with AVX instruction sets, in which
8 float numbers can be stored in one 256bit AVX register.
Namely, 8 pairs of float point numbers can be multipliedand added simultaneously in one cycle of each CPU core.
To maximally reduce the memory latency, the input matrices are first divided into blocks that can fit into the L2 cacheof CPU. Each block of one input matrix is then multipliedwith each block of the other input matrix in the following
way: One block is divided to 8-elements wide row strips
and the other block is divided to 8-elements wide column
strips. Then every two strips are multiplied to generate an
8 × 8 tiny square which can be stored in 8 AVX registers.

4. Sparse Matrix Multiplication Algorithm• Once the network is fully trained, the convolutional
kernels are constant while the input feature maps vary

two types of data loading. The first one is loading
loading each strips from the L2 cache to the regist1
10 of the total running time, while the second one
pipeline design of modern CPU, the second loadin
the overhead of loading is very small for the whole
However, if one of the input matrix is sparse,
the second loading time becomes the dominant tim
Input:
A: 8 × 12 dense matrix
B: 12 × 8 sparse matrix
Output:
C=A×B
Operations:c7 + = a1 × b1,7
c3 + = a2 × b2,3
c6 + = a3 × b3,6
c2 + = a5 × b5,2
c5 + = a5 × b5,5
c4 + = a7 × b7,4
c5 + = a7 × b7,5
c3 + = a10 × b10,3
c5 + = a10 × b10,5
c4 + = a11 × b11,4

Figure 2 gives a graphical illustration of the matrix multiplication algorithm in OpenBLAS.

4.3. Sparse Matrix Multiplication
We focus on the sparse-dense matrix multiplication
problem C = A × B. A ∈ Rm×k is a dense matrix
and B ∈ Rk×n is a fixed sparse matrix. A and B are divided into blocks and strips in the same way as the dense
case described above. Now, we consider the multiplicationof one row strip and one column strip C̄ = Ā × B̄, Ā ∈R8×k , B̄ ∈ Rk×8 , C̄ ∈ R8×8 . For any matrix M, let mi,∗be the ith row of M and m∗,j be the j th column of M. The
matrix multiplication can be represented asc̄∗,j =kXi=1ā∗,i b̄i,j , 1 ≤ j ≤ 8(8)

In which every c̄∗,j and ā∗,i are held in one AVX vector.
Since B̄ is sparse in our case, we need to have the knowledge of the locations of non-zero elements and skip the zeros. To avoid indirect memory access, we propose to encode
the structure of B̄ into the program. For each non-zero valueb̄i,j , i indicates which ā∗,i to multiply with and j indicates
which c̄∗,j to save to. Since they both correspond to single AVX registers, we can simply encode i and j into ourprogram as the index of registers. Figure 3 shows a toy example of how our method generates code from given sparse
matrix.

5. Application to Object Detection
We now apply SCNN to the object detection problem.
Girshick et al. [8] first proposed to use CNN to solve the object detection problem. They warp each candidate window
generated by the Selective Search[21] method to a fixed
size and use CNN to generate high level discriminative features, with whick linear SVM with hard negative mining is
adopted to train object detection model. He et al. [10] significantly improves the speed of [8] by utilizing a Spatial
Pyramid Pooling (SPP) scheme, in which the convolutional
layers only need to be performed once (single scale) or several times (multiple scales) on the whole image and only
the fully connected layers are performed on each candidate
window. Due to large amount of candidate windows in each
image (∼ 2000), the total running time of fully connected
layers in [10] is comparable to or dominant over the one ofconvolutional layers.
We apply SCNN to accelerate the convolutional layersof SPP model. Although our method can accelerate theconvolutional layers by a high factor, the total running time
would not reduce much due to the relatively time consuming fully connected layers. In this section, we propose two
schemes to reduce the complexity of fully connected layers
to achieve an overall high efficiency.

(a) An example sparse matrix
B. The shadowed squares represent non-zero elements andthe blank squares represent zeroelements.

(b) Generated Pseudo code forcalculating C = A × B. ci isthe ith column of C and aj is
the j th column of A. bi,j is theelement of B at ith column andj th row

Figure 3: An example that illustrates how our algorithm generates
code for multiplying a dense matrix and a sparse matrix

First, we propose to apply a cascade scheme over the
network in a similar fashion to [7]. Since the output of thelast convolutional layer is already highly discriminative, directly applying a linear SVM classifier over it achieves good
performance [8][10]. Therefore, we can use the last convolutional layer as the first stage of our cascade model to prune
large portion of candidate windows, and then use the outputof the final fully connected layer as a second stage classifier
to generate the final detection results. The decision of detection threshold of first stage is a tradeoff between efficiency
and accuracy. A lower threshold retains high recall so thatthe overall accuracy is not affected while a higher threshold removes more candidates to achieve higher efficiency.
In our case, we found that a threshold with a corresponding precision equals 0.05 is a balanced tradeoff. Previous
work on cascade models were applied only to single classdetection. Multiple class detection, in general, achieve lessgain, since the number of candidates rapidly increases with
the number of classes. However in our model we can still
remove considerably large portion of candidates with multiple classes.
Second, we decompose the fully connected layer in asimilar fashion as we did with the convolutional layers. The
operation of fully connected layer can be represented bymatrix-vector multiplication followed by the neuron function. We decompose the weight matrix into the product of
one sparse matrix and one dense matrix as M ≈ PS, where
M ∈ Rm×n , P ∈ Rm×k and S ∈ Rk×n . We choose to
enforce sparsity on P if m > n and on S otherwise, sothat the dense matrix has a smaller dimension. We impose

layerkernel size
input channels
output channels
complexity%
sparsity%
average qi
theoretical speedup
Actual speedup

conv1
11
3
96
15.8
0.927
29
2.61
2.47

conv2
5
96
256
33.6
0.950
7.91
7.14
4.52

conv3
3
256
384
22.5
0.951
5.23
16.12
6.88

conv4
3
384
384
16.8
0.942
4.32
12.42
5.18

conv5
3
384
256
11.2
0.938
3.95
10.77
3.92

Table 1: Sparsity, Average number of bases ,theoretical and actual
speedup corresponding to each convolutional layer for our SCNN
model. qi is the average number of bases in each channel. Results
demonstrates that our highly sparse model could lead to remarkably acceleration for computation in both theory and practice.

both l1 norm and group lasso cost function so that k is also
reduced.

6. Experimental Results
6.1. Setup
We trained our model on the ImageNet LSVRC 2012[2] dataset. We start from a pre-trained Caffe[13] reference CNN model, which is almost identical to the model
described in [14]. The model consists of 5 convolutional
layers and two fully connected layers, interlaced with subsampling layers, local normalizing layers, max pooling layers, rectified linear unit layers and dropout layers. The first
convolutional layer has relatively large 11 × 11 kernels andonly 3 input channels; the second convolutional layer has
5 × 5 kernels; The third, fourth and fifth convolutional layers have very small 3 × 3 kernels. The difference of kernel
sizes as well as the number of input kernels affects the possible sparsity that can be achieved.
All 5 convolutional layers are optimized simultaneously
according to Equation 7 using stochastic gradient decent
with momentum. The base learning rate is initially set to0.001, while sparsifying the network parameters. To stabilize the training process, we adopt a thresholding functionthat sets parameters smaller than 1e−4 to zero during training. Once the training process converges, we remove thesparsity constraint, but keep the thresholding function. Finally, we gradually decrease the base learning rate to finetune the network for best accuracy.

6.2. Results on ILSVRC12
Table 1 shows the results on ILSVRC12. For all 5 convolutional layers, we obtain more than 90% sparsity. The average number of bases in each layer is significantly smaller
than s2 (square of kernel size), which corresponds to the fullrank decomposition. The theoretical speedup are the ratio
between the running time of our SCNN layer, and the original convolutional layer. The final column of Table 1 shows
the actual acceleration factor achieved. Because of the over-layerkernel size
sparsity%
average qi
theoretical speedup
low-rank[4]

conv1
7
0.840
21
2.62
2.4

conv2
5
0.956
9.06
7.06
2.5

conv3
3
0.893
6.76
8.03
-

conv4
3
0.904
6.86
8.78
-

conv5
3
0.890
6.98
7.29
-

Table 2: Comparison between a model, similar to[10], trainedwith sparsity and the speedup factors reported in [4].

head in sparse matrix multiplication, it is expected that actual performance improvements will not match theoreticalresults.
The speedup factor in conv1 is not as significant as otherlayers due to limited redundancy that is caused by large kernel size and the small number of input and output channels.
The number of bases in conv1, although substantially reduced, still accounts for a large portion of running time because of small number of output channels. This issue isalso present in previous low-rank approaches [12]. We experimented with decomposing the basis filters with combination of separable filters, but the bases showed too muchvariation to be expressed with separable filters.

6.3. Comparison with Only Using Low-Rank Approximations
To more directly compare with previous work, we also
trained a modified version of the model used in [10], which
is similar to that used in [4]. The primary difference lies ina spatial pyramid layer that will not affect the sparsity properties of the convolutional layers. Table 2 shows the comparison of theoretical speedup factors between [4] and our
method trained based on [10]. We have a similar speedup as[4] in conv1 due to the above mentioned reason, while obtaining much higher speedup for conv2. It should be notedthat [4] did not attempt to accelerate the later layers, likely
due to the tiny size of filters (3×3), while our method is able
to achieve significant speedup.

6.4. Comparison of Initialization Methods
Figure 4 shows a comparison of the performance of different initialization methods that we adopted. Both PCA
and sparse coding obtain more than 90% sparsity, while random initialization shows inferior sparsity by a large margin.
Notably, all the methods obtain very limited sparsity with
only initialization. These results demonstrate the importance of both initialization and fine-tuning. All of the methods show very small amount of accuracy drop during thefine-tuning process. The accuracy numbers shown in Figure
4 are generally lower than final accuracy since the learning
rate we set during sparsifying the network is higher than final learning rate for faster convergence. Among all the threemethods, the accuracy of PCA is slightly better than the others. The sparse coding method, although seems to make

pca
sparseidentity

accuracy

0.57
0.56

org	
  
rec	
  
Conv1(sim	
  =	
  0.85)	
  

0.55

org	
  

0.54
0.53

rec	
  

1

Conv2(sim	
  =	
  0.60)	
  sparsity(% of zeros)

0.9

org	
  

0.8

rec	
  

0.7
pca
sparseidentity

0.6
0.5
0.4
0

2

4

6
iterations

8

10

12
4
x 10

Figure 4: Comparison of initial decomposition methods. We show
the variation of both the accuracy and the average sparsity of oursparse CNN during the training process.

more sense theoretically, is inferior practically. We argue
that the main reason is its non-convexness and non-accurate
reconstruction. Although PCA is also a non-convex problem, global unique optimum can be obtained with SVD.

6.5. Bases Visualization
Figure 6 shows the average number of non-zero elements
in our sparse convolution kernels corresponding to each basis of our decomposition over both input channels and kernels inside each channel. The bases are sorted according
their eigenvalues of PCA initialization. High correlation
can be found between the sparseness and the eigenvalues of
PCA, which justifies the importance of PCA initialization.
For a significant portion of the bases, the sparse kernels arealmost all zero. The all zero bases are equivalent to the ones
that can be eliminated by low-rank decomposition. For relatively large-size kernels, like conv1 and conv2, the percentage of all zero bases is significant enough to achieve moderate level of speedup. However, for small-size kernels likeconv3, conv4 and conv5, the percentage of all zero bases isvery limited. Even a speedup factor of 2 will significantly
sacrifice the accuracy with low-rank decomposition. The
advantages of our method are clearly shown by the sparsity
obtained for the non-zero bases, shown in Figure 6.
To justify the necessity of fine-tuning, we compare theconvolution kernels in the original model and the ones thatare reconstructed from our fine-tuned sparse model in Figure 5. We only show the kernels of conv1, conv2 and conv3
layers due to limited space. We ignore the kernels that are
all zeros, which is very common from conv3 to conv5. We
also measure the average similarity between the original andreconstructed kernels by first deduct the mean values fromboth kernels, and then calculate the cosine similarity meax·ysurement, which is defined as Simcos (x, y) = kxkkyk. FromConv3(sim	
  =	
  0.34)	
  Figure 5: Comparison between the original convolution kernels
and the convolution kernels reconstructed from our sparse kernels. Here we show randomly sampled kernels conv1, conv2 andconv3 layers. conv4 and conv5 are very similar to conv3. For
each layer, the first row shows the original kernels and the second
row shows the reconstructed ones. The average cosine similarity
between them are displayed under.

Figure 5, we can see that the average similarity decreases
rapidly from conv1 to conv3. The kernels in conv3 lookvery different from the original ones. Considering how little
the accuracy drops in our model, this justifies our argumentthat the original network is extremely over-parameterized
and significant regularization can be imposed without affecting the performance. Thus, one can hardly exploit thefull sparsity potential of the network by only attempting
to approximate the original kernel instead of network loss
based fine-tuning adopted in our method.

6.6. Evaluation of Sparse Matrix Multiplication Algorithm
We first analyze the performance of our sparse dense
matrix multiplication algorithm with a randomly generated
matrix. We randomly generate one 1024 × 1024 dense matrix and one sparse 1024 × 1024 matrix, and measure therunning time of multiplying them with our algorithm. Our
experiments are performed on an Intel i7-3930k CPU. We
measure the single-thread performance in this case for simplicity. The multi-thread performance should be consistent.
Figure 7 shows the result of our evaluation. The following
conclusions can be drawn from this figure: (a)The arithmetic time is strictly proportional to the density of the input
matrix, and very close to the theoretical limit; (b) The I/Otime decreases with the increase in sparsity, but in a sublinear speed. The time of loading from memory to cacheand storing results are constant regardless of the variationof sparsity, and the time of loading the dense matrix fromcache to CPU depends on the percentage of consecutive 8
zeros in the sparse matrix; (c) The I/O time is increasingly
dominant when the sparsity increases, when the density isless than 10%, the I/O operations takes more than 80% of
the total running time. (d) Note that the sum of arithmetic
time and I/O time is significantly higher than the total time.

0.18

0.26

0.56

0.34

0.3

0.26

0.4

0.09

0.13

0.28

0.17

0.15

0.13

0.2

0
1

2

(a)conv1	
  

3

0
1

25

(b)conv2	
  

49

0
1

61

(c)conv1	
  

121

0
1

13

(d)conv2	
  

25

0
1

5

(e)conv3	
  

9

0
1

5

(f)conv4	
  

9

0
1

5

(g)conv5	
  9

Figure 6: Average ratio of non-zero elements in our sparse convolution kernels corresponding to the bases of decompositions over channels
and filters. (a) (b) show the bases over channels and (c) to (g) show the bases over filters.The bases in each figure are sorted in descendingorder of their eigenvalues in PCA initialization.
0.5

Running time%

0.4

total timearithmetic time
IO timetheoretical limit

spp[10]ours

0.3

fc7bb(1s)
54.19
52.64

fc7(5s)
54.75
52.58

fc7bb(5s)
57.19
55.13

Table 4: Mean average precision of object detection with our
method compared with [10]. “bb” stands for bounding box regression, “1s” means 1 scale and “5s” means 5 scales. Our sparsemodel is inferior to [10] by approximately 2%.

0.2

0.1

0
0

fc7(1s)
52.47
50.16

0.05

0.1

0.15
0.2
Density(% of non−zeros)

0.25

0.3

0.35

Figure 7: Running time analysis of our sparse-dense matrix multiplication algorithm. The horizontal axis stands for the percentage of non-zero elements in the input sparse matrix, and the vertical axis is the relative running time comparing to the dense matrix multiplication code in OpenBLAS. The arithmetic time is therunning time of multiplication and addition, and the I/O time includes loading the input matrix from memory to cache, loadingfrom cache to CPU and writing result to memory. The theoretical time is the best possible speedup one can achieve, which is
identical to the density of the input matrix.
layer
Channel Decomp
Basis Convolution
Matrix Mult
Original

conv1
0.06
0.78
0.16
2.47

conv2
0.14
0.41
0.45
4.52

conv3
0.21
0.79
6.88

conv4
0.30
0.70
5.18

conv5
0.44
0.56
3.92

Table 3: Running time analysis and comparison with original
dense networks. All the numbers for each layer are normalizedwith the layer’s total running time with our method. The last row
is the speed-up factor of our method.

This is due to the parallelism introduced by the pipeline
strategy of CPU. For this reason, the arithmetic operations
and I/Os can be executed simultaneously, thus providing asignificantly better efficiency.

6.7. Running Time Analysis
Table 3 shows the actual speed of our code as well as
the proportion of each component. Significant speedups areachieved for all 5 layers. The sparse matrix multiplication
time is dominant for the last three layers while the basis
convolution time takes a large portion for the first two layers, which is consistent with theoretical analysis. The gap
between actual speedup and theoretical one comes mainly

from two factors: (1) Overhead of sparse matrix multiplication; (2) Low efficiency of basis convolution. Caffe implements convolution as matrix multiplication, which is relatively inefficient for small number of filters as in our case.
We implement a faster version but is still not fully optimized. In addition, joint cache optimization with both convolution and sparse matrix multiplication will further improve the efficiency.

6.8. Results on Object Detection
We used the fine-tuned model in Table 2 to perform object detection on PASCAL VOC2007 [5] dataset. The accuracy of our method compared with the original one in [10]is shown in Table 4. We are not able to reproduce the accuracy using the code published by [10], so we put the number we get instead. Our method is approximately 2% worse
than the original SPP model, while obtaining several times
faster speed. We speculate that the higher accuracy dropthan classification problem is probably due to the fact thatthe convolutional layer is trained on ILSVRC dataset, while
only the fully connected layer are fine-tuned to adapt to the
PASCAL data as in [10]. Thus, although we are sparsifying
the convolutional layers with fine-tuning, the loss of the network is not equivalent to the loss of the detection problem.
A whole network based fine-tuning should further reduce
the accuracy drop of our method.
In our cascade model, we set the thresholds of first stage
so that the precision for each class equals 0.05. Approximately 80% of candidate windows are pruned for each image, thus bringing approximately 5× speedup of fully connected layers with almost no drop in accuracy. In addition,
85% and 68% sparsity are achieved for the first and second
fully connected layer, further providing over 2× speedup.
The running time of fully connected layer is thus reduced to
be much smaller than the convolutional layers.

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