C O MN ET : C ORTICAL M ODULES A RE P OWERFUL
Anonymous authors
Paper under double-blind review
A BSTRACT
Existing CNN architectures may achieve efﬁciency in either one or two dimensions:
FLOPs, depth, accuracy, representation power, latency but not in all.
In this work, we present a pragmatically designed novel CNN architecture “CoMNet” which offers multi-dimensional efﬁciency at once such as: simple yet accurate, lower latency and FLOPs, high representation power in limited parameters, low memory consumption, negligible branching, smaller depths, and only a few design hyperparameters.
The key to achieve the multi-dimensional efﬁciency is our use of biological underpinnings into CoMNet which is primarily the organization of cortical modules in the visual cortex.
To realize CoMNet, a few concepts from well understood CNN designs are directly inherited such as residual learning.
Our solid experimental evaluations demonstrate superiority of CoMNet over many state-ofthe-art industry and academia dominant architectures such as ResNet, RepVGG etc.
For instance, CoMNet supersedes ResNet- 50 on ImageNet while being 50% shallower, 22% lesser parameters, 25% lower FLOPs and latency, and in 16% lesser training epochs.
Code will be opensourced post the reviews.
I NTRODUCTION
To date, a wide variety of CNN architectures exists: branchless Krizhevsky et al.
; Simonyan &
Zisserman (2014), single branch (He et al., 2016), multi-branch and variable ﬁlter topology (Szegedy et al., 2015; 2016), feature resue (Huang et al., 2017), mobile series (Howard et al., 2017; Zhang et al., 2018).
Despite their architectural diversity, they all have one characteristic in common; their primary design objective dimensions are limited.
For instance, (Simonyan & Zisserman, 2014; Szegedy et al., 2015; He et al., 2016; Huang et al., 2017) seek to achieve higher accuracy regardless of network size and runtime, while mobile networks (Howard et al., 2017; Zhang et al., 2018) aim for fewer FLOPs at the cost of reduced representation power.
Some neural architecture search based designs (Zoph et al., 2018; Tan & Le, 2019) focus on both higher accuracy and lower FLOPs but are heavily branched, discard latency, and runs slower on GPUs.
More recently, (Ding et al., 2021) focus on accelerating (Simonyan & Zisserman, 2014) during inference phase while leaving training-phase unaddressed.
In contrast, there are many other dimensions which are left unaddressed but are crucial in real-time applications, such as autonomous driving, robotics etc.
Foremost is latency or per-sample runtime which still remains ignored in CNNs in favour of FLOPs or throughput.
Optimizing latency is crucial because real-time applications tend to process as few as one frame at a time but not large batches, and also, fewer FLOPs does not mean lower latency (Ding et al., 2021).
Second, neural networks are replacing their traditional coutnerparts, posing a huge challenge to run multiple networks on a single device (Kumar et al., 2020), bringing in resource efﬁciency constraints.
Third, rising complexity of the tasks (Kumar et al., 2020; Kumar & Behera, 2019) demands large networks to achieve satisfactory results, because mobile networks are insufﬁcient due to their smaller representation power (Sec 2).
In the existing designs, the primary dimensions such as higher accuracy (He et al., 2016; Szegedy et al., 2016), fewer FLOPs (Sandler et al., 2018), and inference time FLOPs (Ding et al., 2021) are explored only individually.
However, considering the above discussion, addressing multi-dimensional efﬁciency is current need of the time.
Consequently, in this work, we aim to achieve multi-dimensional efﬁciency at once, while offering stronger trade-offs in some of them if efﬁciency in all dimensions is not feasible.
To the best of our knowledge, achieving multi-dimensional efﬁciency has not been explored as it is a difﬁcult task, mostly due to a high correlation among dimensions, which makes it possible to be better in one but worse in another.
Towards our objective, we propose a novel architecture “CoMNet”, that can offer multi-dimensional beneﬁts of lower architectural complexity, smaller depth, hardware-accelerators compatible, low memory consumption, low memory access costs on parallel computing hardware, low latency, parameter efﬁciency.
CoMNet is primarily based on our translation of biological underpinnings of cortical modules (Mountcastle, 1997) which predominantly exist in a visual cortex.
We particularly refer to the structure of cortical modules in ventral stream (Tanaka, 1996) which is responsible for object recognition in mammalians.
To realize our mere biological design inspirations, we inherit a few concepts from the well established CNN designs e.g. residual leaning (He et al., 2016).
CoMNet mostly outperforms many key CNN designs, and also offers trade-offs but minimally.
Although
CoMNet has its design inspired from biological studies, we neither investigate nor assert that CoMNet is functionally similar to the visual cortex.
In a brief, key take-aways of the paper are: First , the notion of multidimensional efﬁciency in CNNs,
Second , the notion of artiﬁcial cortical modules ( ACM ) which helps achieving high representation in fewer parameters, controlled parameter growth, increased computational density, Third , the concept of columnar organization (Mountcastle, 1997) which helps to achieve smaller depths, lower latency and FLOPs, faster convergence, Fourth , long range connections similar to pyramidal neurons (Mountcastle, 1997) which further improves the accuracy of CoMNet.
We also provide a detailed ablations, and a minimal design space of CoMNet to help one choosing a suitable model.
By using
GradCAM (Selvaraju et al., 2017), we also show that CoMNet learns better data representations.
Finally, we brieﬂy discuss BrainScore (Schrimpf et al., 2020) and future prospects of CoMNet.
In the next section, we talk about the most relevant works.
In Sec. 2, we brief our biological insights and their translation into the CoMNet architecture.
In Sec. 5, we present a rigorous experimental analysis.
Finally, in Sec. 6, we provide conclusions about the paper.
R ELATED W ORK
Parameters and Representation Power: The earlier CNN designs (Krizhevsky et al., 2012; Simonyan & Zisserman, 2014; Szegedy et al., 2015; He et al., 2016) possess high representation power.
In these designs, the deeper layers consist of a large number of channels e.g.
(Simonyan &
Zisserman, 2014) to compensate for the reduction in resolution, leading to exponential growth in the parameters, and synaptic connections of a kernel in these layers.
It becomes a predominant cause of overﬁtting (Simonyan & Zisserman, 2014) which is alleviated via dropout (Krizhevsky et al., 2012), but at the cost of increased training time.
ResNet (He et al., 2016) avoids that by reducing and expanding the number of channels via 1 × 1 conv layers and placing them before and after 3 ×convolutions.
Xie et al.
(2017) restructures the residual unit of ResNet in form of groups, however, the issue of large depth, overall parameters still remains in picture.
The mobile CNNs (Howard et al., 2017; Sandler et al., 2018; Zhang et al., 2018; Ma et al., 2018) on the other hand employ depthwise convolutions (Sifre & Mallat) to control the parameter growth and reduce FLOPs.
However representation power decreases quickly.
Moreover, depthwise convs are devoid of cross channel context which is crucial for better performance (Zhang et al., 2018).
Therefore such convolutions are followed by a 1 × 1 conv to intertwine cross channel information.
Depth: In the above networks, the use of 1 × 1 layers increases network depth rapidly.
For example, two 1 × 1 for each 3 × 3 in (He et al., 2016; Sandler et al., 2018; Zhang et al., 2018; Tan & Le, 2019), while one in (Howard et al., 2017).
Despite being beneﬁcial, these layers constitute a signiﬁcant amount of depth e.g. 66% in (He et al., 2016).
Moreover, due to their pointwise nature, 1 × 1 convs do not contribute in the receptive ﬁeld, which in contrast is governed by 3 × 3 layers.
Branching: With time, the CNN architectures have grown from branchless (Krizhevsky et al., 2012; Simonyan & Zisserman, 2014) to single branch (He et al., 2016) to multibranch (Szegedy et al., 2016; Schneider et al., 2017).
Neural architecture search has resulted in even heavily branched designs (Zoph et al., 2018; Tan & Le, 2019).
Although branching helps in improving accuracy, it also raises memory access cost on parallel computing hardware (Ding et al., 2021) which directly impacts latency and memory consumption.
Latency: Both Depth and high branching increase latency despite having fewer FLOPs.
In CNNs, each layer require certain computing time and many such layers are linked serially.
Output of one layer can not be computed until all outputs of the preceding layers are available, despite enough computing power remains.
This dramatically increases the latency despite fewer calculations per layer.
For example, 100 layers each with 1 ms time result in 100 ms of latency while a shallow network of 15 layers with 3 ms per layer runtime will result in 45 ms of latency.
The best illustration of this phenomenon is (Tan & Le, 2019) which despite having fewer FLOPs, runs equivalent to a ﬁve times bigger network (He et al., 2016).
More recently, (Ding et al., 2021) proposes structural reparameterization to accelerate (Simonyan & Zisserman, 2014) during inference phase, however, the training-phase network is still high in parameters, and branches even more than its predecessor (He
et al., 2016), showing no improvement in training time (Table 4).
Convergence: Reduced representation power in (Howard et al., 2017; Sandler et al., 2018; Zhang et al., 2018; Ma et al., 2018; Tan & Le, 2019) results in overly large training schedules on ImageNet, and reduced performance on other downstream tasks.
For instance, (Howard et al., 2017) requires 200 epochs on ImageNet to perform similar to one variant of (Simonyan & Zisserman, 2014) which is trained only for 75 epochs, but performs poorly on object detection.
A similar case (Tan & Le, 2019) which requires 4 × training schedule( 400 epochs), and larger input resolutions in contrast to the earlier ones (He et al., 2016; Simonyan & Zisserman, 2014; Schneider et al., 2017) that are always trained for a smaller resolution 224 × 224 and in a range of 90 − 120 epochs.
B IOLOGICAL V ISUAL C ORTEX
A biological visual cortex is fairly complex but consists of several interesting properties.
We highlight the most relevant ones below.
For more details, kindly refer to the supplementary material.
Columnar Structure.
Cortical modules are present all across a biological cortex (Mountcastle, 1997).
Modules in the shallower layers are referred as ocular dominance columns which respond to simple stimuli such as edges, lines of different orientations (Hubel & Wiesel, 1963).
Whereas modules in the deeper layers are a collection of neurons which respond to a complex stimuli such as face, monkey, human etc, by encoding stimuli information from different viewpoints (Tanaka, 1996).
These modules are mainly present in inferotemporal cortex (IT) which is responsible for object detection, and recognition tasks (Figure 1a).
Shared Input or Input Replication.
Two cortical modules having different stimuli response can have a common input i.e. the input is replicated to feed the modules.
This is crucial because at a given location in the visual ﬁeld, there can be a monkey or a car etc.
Hence, a module with highest similarity with the stimuli ﬁres strongly, and signals to other parts of the cortex (Hubel & Wiesel, 1963).
Limited Synaptic Connections.
Typically, a cortical column contains a small number of neurons ( 70 − 100 ) (Mountcastle, 1997) which results in smaller number of synaptic connections per column.
It is advantageous as this collection of neurons can learn a simple stimuli.
Massive Parallelization.
Regardless of layers being shallower or deeper, a cortical module processes only a small region of retina.
Multiples of such modules having similar stimuli response are replicated to span the whole retinal ﬁeld.
This organization facilitates massive parallelization.
Lateral Connection Inhibition.
Cortical modules do not communicate with each other except at the output (Tanaka, 1996), which is achieved via fewer pyramidal neurons having large number of long range connections.
Such neurons fuse cross module information (Tanaka, 1996).
C O MN ET D ESIGN
While designing CoMNet, we closely pay attention to the ﬁve most crucial objectives: latency, depth, branching, FLOPs, and parameters, which are sufﬁcient to achieve multi-dimensional efﬁciencies of controlled parameter growth, high representation power in fewer parameters, increased computational density, minimal branching, as other objectives such as memory consumption, memory access cost depend on these.
Next, we discuss our proposed biologically inspired designs, their advantage in the context of the above objectives, and their translation into CNN realm.
At the end of this section, we intertwine all the individual designs to develop the fundamental computational unit of CoMNet which is used to build different instances of CoMNet.
4. A RTIFICIAL C ORTICAL M ODULES
Our ﬁrst contribution is to develop IT like structure in CNNs.
We achieve that by developing artiﬁcial cortical modules ACM s, and a mechanism to feed them similar to input replication in the visual cortex.
We realize input replication by deﬁning an operator, called CM_Feeder .
This operator transforms a tensor ∈ R C × H × W into another tensor ∈ R ( M × C ) × H × W , where M denotes the desired number of cortical modules.
This operator consumes a tensor and outputs its M identical replicas (Figure 1b).
Since a cortical module is essentially a group of neurons, we realize its artiﬁcial counterpart ( ACM ) via a k × k convolution layer, k = 3 , having N neurons, Figure 1c.
Following, the limited synaptic connection property, N is kept small.
To serve as M cortical modules, M such convolutions are employed in parallel, each processing one of the replicas obtained from the CM_Feeder .
We refer the collection of M ACM s as a Collective Cortical Module ( CCM ).
The above proposed concept of
ACM helps achieving controlled parameter growth, high representation power in fewer parameters, and improved computational density as discussed below.
CM CM
CM
X ∈ R C × H × W
CM_Feeder
Y ∈ R ( M × C ) × H × W
W 0 ∈ R N × C × k × k
W 1 ∈ R N × C × k × k
W 2 ∈ R N × C × k × k
ACM
ACM ACM
CCM
[Figure]
Controlled Parameter Growth.
In the earlier designs, number of parameters is controlled by either increasing network width or depth which offers only a little control, and leads to exponential growth in parameters, especially in the deeper layers (Sec. 2).
On the other hand, ACM s provide an absolute control over number of synaptic connections which precisely reﬂects the parameters count.
This ﬂexibility is crucial while scaling a CNN as per the needs.
High Representation Power
We build a hypothesis that multiple neurons with lesser synaptic connections are better than a single neuron with large connections.
For any stimuli, it requires certain amount of representation power or neurons to be learnt.
In the existing CNNs, a kernel operates on a large number of channels, thus having large number of synaptic connections.
During learning phase, it gets penalized for all the visual stimuli even if it is uncorrelated with them.
This is also visible by weight pruning which suggests that post training, eliminating many channels/connections does not impact accuracy, indicating a wastage of synaptic connections.
CoMNet addresses the issue by prioritizing smaller neurons over larger ones by limiting the number of synaptic connections.
In other words, we synthesize more number of neurons out of a larger one.
As a result, CoMNet exhibits higher accuracy (Table 1) in lesser parameters without prolonging the training duration, in contrast to (Sandler et al., 2018; Tan & Le, 2019).
It shows that CoMNet learns better representations in fewer parameters (Figure 3).
Increased Parallelization and Computational density.
Naively, each ACM in a CCM is executed independently, however as all of the ACM s are operating parallely, they can be computed efﬁciently by using NVIDIA’s highly optimized Batched-Matrix-Multiply routines.
To achieve that, we combine all the ACM s of a CCM layer into a single convolution layer having M groups.
This strategy packs computations of all ACM s into a single convolution, which leads to increased computational density, increased GPU utilization, and reduced memory access cost (Ding et al., 2021).
Although group convs are widely explored (Xie et al., 2017; Zhang et al., 2018), there exist two key differences.
First , in this approach, input channels are divided into groups i.e. a slicing operation, whereas in CoMNet, inputs is replicated i.e. replication operation.
Second , group convolutions has to be followed by 1 × 1 conv to avoid loss of accuracy due to lack of inter-group communication (Zhang et al., 2018), increasing network depth, however CoMNet is free from such constraint.
4. C OLUMNAR S TRUCTURE
Our second contribution is to bring columnar organization into ACM .
In a cortical column, several neurons are connected both parallely and serially (Mountcastle, 1997).
The parallel connections get realized implicitly via N neurons in a module.
However, to realize the serial connection, we stack several ACM s one after the another, forming a column, as shown in Figure 2a.
Since, ACM s are housed in CCM s, stacking essentially is performed over CCM s. Moreover to respect lateral connection inhibition, an ACM of a CCM can communicate with exactly one ACM of another CCM .
For example, an
ACM at k th location in a CCM can only feed the ACM at k th location in the subsequent CCM (Figure 2a).
We also follow the concept of pyramidal neurons which fuses the outputs of several cortical modules.
We achieve that by placing a 1 × 1 conv ( P c ) layer after the ﬁnal CCM in the columnar organization.
This conv layer fuses the information of multiple modules or ACM s by operating at each ( h, w ) ∈
R ( M × N ) × H × W .
Each neuron in the P c consists of a large number of connections arising due to combining the output of M ∗ N channels.
It greatly mimics the property of pyramidal neurons.
The proposed artiﬁcial columnar organization facilitates eliminating 1 × 1 layers which results in reduced depth, reduced FLOPs, latency, and faster convergence of CoMNet, as discussed below.
Reduced Depth, FLOPs, and Latency As mentioned in Sec. 2, 1 × 1 layers are the major constituent of depth in state-of-art networks because they are based on unit/blocks which is a stack of 1 × 1 , 3 × 3 , and 1 × 1 layers.
Several such units are connected serially to form a stage.
For instance, in a stage with three units, six 1 × 1 layers are present for three 3 × 3 layers.
Since, receptive ﬁeld is mainly governed by k × k conv layers with k >
(Luo et al., 2016), the columnar organization facilitates elimination of 1 × 1 layers by stacking 3 × 3 CCM layers, achieving equivalent
receptive
[Figure]
CCM
ACM 0 ACM
[Figure]
ACM
CCM
ACM 0 ACM
ACM
CCM×
Pooling 3 × 3 )
Conv 1 × 1 ( L c )
LongRangeConnections( LRC )
S
CM_Feeder CCM 0 CCM 1 + CCM 2 + P c +
LRC
[Figure]
in just three layers.
In other words, columnar organization reduces the three units into one.
Notice that, It is true for any number of units.
Interestingly, for this reason, a CoMNet unit acts as a stage in contrast to earlier networks where a unit serves as a block, and multiple of such blocks forms stage which are sequentially connected to build a network.
The elimination of 1 × 1 results in decreased depth which results in reduced FLOPs and latency because of the reason mentioned in
Sec 2.
As an example, CoMNet performs better than ResNet- 50 at 50% fewer layers, while having lower parameters, FLOPs and latency, indicating a huge achievement.
Faster Convergence In just half epochs i.e. 60 , CoMNet achieves 99 .
17% ( 76 . 16% ) of its accuracy obtained at 120 epochs ( 76 .
76% ), as compared to ResNet- 50 which achieves only 97% ( 74 . 15% vs 76 .
Primary reason behind these improvements is our belief that the 3 × 3 layers are more important than 1 × 1 because they are solely responsible for extracting spatial context, thereby governing receptive ﬁeld.
Hence, we can obtain equivalent receptive ﬁeld without 1 × 1 by keeping the number of 3 × 3 layers same.
However, 3 × 3 suffers with overly large parameters for which 1 ×convolutions were introduced (He et al., 2016).
Our CoMNet tackles this issue by employing ACM s.
4. A RTIFICAL L ONG R ANGE C ONNECTIONS
Our third contribution is to develop artiﬁcial long range connections.
In a visual cortex, these are realized via pyramidal neurons which apart from communicating with the modules, also projects their input to many layers (Mountcastle, 1997).
This helps in taking advantage of multi-layer information.
We achieve the same by employing 1 × 1 conv ( L c ) layer which is fed by pooling the input of the CoMNet unit (discussed next).
L c projects this input directly toward the output of the unit while fusing it with the output of P c .
This simulates the behavior of combining the cortical module information and multi-layer information.
There are many CCM layers between the input and output of a CoMNet unit, however, L c bypasses all of them, and directly connect input and output, thereby realizing the effect of long range connections.
Although they improve accuracy signiﬁcantly, they can be traded for parameters, FLOPs at the cost of reduced accuracy.
See Figure 2b for visualization.
The long range connections ﬁnd their structural similarity with projections in (He et al., 2016), however there are notable differences.
First, projections in (He et al., 2016) are used only in the ﬁrst block of a stage, while projection between stages does not exist.
Second, projection conv operates with a stride of 2 in (He et al., 2016), whereas in our case, L c is preceded by a pooling operation.
Pooling serves two objectives: First, it helps gathering spatial context because L c is a pointwise convolution, and aligns the spatial size of the input with output of the ﬁnal CCM .
4. T HE C O MN ET U NIT .
The above proposed CoMNet design ideas are combined to develop the fundamental computational unit of CoMNet, as shown in Figure 2c.
In this unit, an input tensor T i is ﬁrst passed through a 1 × 1 conv ( S ) which squeezes the number of channels by a factor R .
Its output T s is then passed to
CM_Feeder which is followed by a stack of CCM s. Each CCM is connected with the previous via residual connections.
The output of ﬁnal CMM is fed to the P c which fuses the output of all ACM s in the ﬁnal CCM .
Now, T i is fed to L c after pooling ( p ), which projects the information from previous stage i.e. T i to the P c layer.
The output of P c and L c are summed to produce an output tensor T o .
Mathematically, the whole computational unit can be written as follows:
[Equation]
where, (cid:12) represents function-of-function operator.
It can be seen that CoMNet is uni-branched.
Although CoMNets are faster even with branch, but it can be eliminated using structural reparameterization (Ding et al., 2021) during inference, if desired.
Fewer branching reduce the memory consumption and memory access cost regardless of training or testing.
As CoMNet unit is mostly 3 × 3 convs, it is fully compatible with hardware accelerators because they have dedicated support for them 3 × 3 .
So far, we have shed light on all of the objectives which we envisaged to achieve.
[Table]
4 5 201 1 1 (cid:51) (cid:51) 13 .
4 M 2 .
12 B 75 .
4. C O MN ET INSTANTIATION
A CoMNet variant can be instantiated by connecting many CoMNet units serially which also serve as stages.
Without complicating, we follow earlier models (He et al., 2016; Simonyan & Zisserman, 2014) to keep the tradition of ﬁve stages among which ﬁrst is a plain 3 × 3 convolution with stride 2 , while remaining are the CoMNet units.
Following (He et al., 2016), the channel dimension of S starts with 64 which gets multiplied by 2 at each stage, while channel dimension of P c and L c always equals to R times of the channels in S .
We set R = 4 , following (He et al., 2016).
To further simplify the instantiation process, the number of CCM layers i.e. l in k th CoMNet unit is set equal to the number of blocks in the k th stage of ResNet- 50 (He et al., 2016).
A CoMNet unit mainly has three hyperparameters: M, N, l
In this work, we do not explore every permutation and combination of them, instead, the presented CoMNet models in this paper, are derived such that, they can outperform most useful and deployed state-of-the-art models.
E XPERIMENTS
We present in-silico testing of CoMNet on ImageNet (Deng et al., 2009) which is de-facto standard to benchmark novel architectures.
We begin with ablations where we speciﬁcally study the signiﬁcance of ACM s, variation in M, N, l , effect of long range connection, and residual connections in CCM .
We train each CoMNet variant for 120 epochs using SGD , Nesterov momentum, base_lr = 0 .
1 with cosine-annealing (Loshchilov & Hutter, 2016).
We also study 200 epochs regime to demonstrate efﬁcacy of CoMNet against state-of-the-art CNNs which are evaluated for 200 epcohs.
Forepochs, we use RandomResized crop (Paszke et al., 2019), random ﬂip only, while for 200 epochs,
MixUp (Zhang et al., 2017), AutoAugment(Cubuk et al., 2019), LabelSmoothing (Szegedy et al., 2016) are also employed.
We also show faster convergence by training CoMNet for 60 epochs.
5. A BLATION S TUDY Variation in M, N
Table 1 demonstrate the effect of varying N and M (R 0 -R 5 ).
We ﬁrst ﬁx the values of N and vary M
(R 0 -R 5 ), and then vary M while ﬁxing N (R 0 ↔ R 3 , R 1 ↔ R 4 , R 2 ↔ R 5 ).
It can be seen that for ﬁxed N , accuracy improves by increasing M , and the same effect is seen by ﬁxing M and varying N .
The purpose of this ablation is to show how accuracy, parameters and FLOPs changes with these hyper-parameters.
It can be noticed that parameters, FLOPs can be precisely controlled by changing the M (R 1 ↔ R 2 , R 4 ↔ R 5 ) which directly reﬂects accuracy.
Effect of ACM To analyze the signiﬁcance of ACM , we compare instances having different N, M , but have similar parameters and FLOPs budget, for instance, R 1 ↔ R 2 , R 1 ↔ R 3 , Table 1.
It is noticeable that, R 2 with 5 ACM is better by 0 .
36% in accuracy, only at 1 .
1 M more parameters relative to R 1 .
Similarly, R 1 is better by 0 .
28% in accuracy, only at 0 .
8 M more parameters relative to R 3 .
It shows that multiple ACM s facilitates improved accuracy in just a fraction of parameters, and FLOPs.
Moreover, if comparing R 9 (a deeper model) with R 2 , we see that R 2 with multiple cortical modules achieves 0 .
13% more accuracy in 0 .
2 M fewer parameters, and 0 .
17 B fewer FLOPs.
It shows the advantage of having multiple cortical modules while being shallower.
Varying “ l ” The impact of varying l is shown in R 9 , Table 1.
It can be seen that accuracy deﬁnitely improves relative to shallower, but depth increases simultaneously.
The increased depth causes increased latency, therefore, we stick to 20 − 40 layers of depth.
We also carry out additional experiment where each CCM is followed by a 1 × 1 as done in group convolutions, while keeping depth and parameters constant.
We observe a drop in accuracy 1% .
Residuals in CCM R 6 -R 9 , Table 1 shows this analysis for a shallower and a deeper variant.
For shallower model, residual connection shows only minor improvement ( 0 . 09% ) however for deeper model the effect of residual connections is signiﬁcant ( 0 . 7% ).
[Table]
[Table]
Effect of Long Range Connections ( LRC )
As mentioned previously
[Table]
LRC module can be traded for parameters and FLOPs at the cost of reduced accuracy.
However, it is important to analyze its signiﬁcance.
To achieve that
[Table]
we train a CoMNet instance in three ways: First , remove LRC entirely,
Second , use LRC without pool
[Table]
and Third , use LRC with pool.
See Table 2 for the analysis.
It can be noticed that without LRC (R 0 )
[Table]
the model suffers with heavy accuracy loss of ∼ 0 .
54% relative to when LRC is used without pool (R 1 ).
Moreover, when using LRC with pool, accuracy improves signiﬁcantly i.e. 1 .
22% and 1 .
76% relative to R 1 , and R 0 respectively.
It happens because pooling operation can provide more spatial context to the 1 × 1 L c layer by summarizing the neighborhood.
5.1. C O MN ET D ESIGN S PACE
Training many instances by varing hyperparameters of a network is a very time consuming process which requires a lot of computing resources and months of duration, also visible in (Schneider et al., 2017).
Even the earlier (He et al., 2016; Simonyan & Zisserman, 2014) and newer ones (Ding et al., 2021) avoids exploring the whole design space due to this reason.
ResNet being half a decade older, its design space has only recently been explored (Schneider et al., 2017).
For this reason, despite having only three hyperparameters, we could train only a few CoMNet models that shows superiority over the existing models.
However, the accomplishments of CoMNet design encourages us to push ourself to provide a minimal design space of CoMNet (See Table 3).
All the
CoMNet instances are diverse, and have their ﬁve most crucial attributes reported i.e. parameters,
FLOPs, Latency, epochs and accuracy.
These instances are crafted such that they can easily outperform the most popular deployable networks, and can be used by the community as pretrained ones.
Deﬁnitely there are always better ways to train i.e. longer training ( 300 epochs), more complex augmentation (Yun et al., 2019), (Huang et al., 2016) which can be used to further improve the accuracy of these instances.
5.1. I MAGE N ET C LASSIFICATION
Next we put CoMNet against existing and popular models which are quite dominant in the academia, industry and real-time applications.
We shed light on the multi-dimensional efﬁciencies achieved by
CoMNet.
If efﬁciency is not possible in all dimensions, CoMNet offers stronger trade-offs relative the rival network.
We show that CoMNet is simple during both training and inference while can also take advantage of recent structural reparameterization (Ding et al., 2021) during inference phase.
When comparing CoMNet with existing CNNs, if efﬁciency in every dimension is not feasible, we prioritize a few dimensions over the others by deﬁning a dimension precedence:
Latency = Depth > Branching > FLOPs > Parameters .
Although parameters efﬁciency is important, it can be sacriﬁced if CoMNet is better in other dimensions, while latency is kept at the highest.
Table 4 shows the comparison of CoMNet with existing models over multi-dimensional metrics.
Latency is reported for RTX- 2070 GPU which may vary on a different GPU device, therefore the numbers are only for reference.
Below we discuss the observations in a structured manner.
[Table]
R
[Table]
CoMNet is 3 .
29% accurate, has 25% fewer parameters, 31% fewer FLOPs than ResNetwhile offers similar runtime speed.
Although it has 6 more layers, CoMNet offers higher representation power in fewer parameters at equivalent latency.
Similarly, in comparison to ResNet- 34 , it is accurate by 0 .
28% , has 59% fewer parameters, 66% fewer FLOPs, 23% less deeper while runs faster by 37% .
In terms of state-of-the-art RepVGG (Ding et al., 2021) models, CoMNet is more accurate by 2 .
04% while having marginally fewer FLOPs, parameters and equivalent speed.
R 1 .
CoMNet is a strong competitor to RepVGG i.e. ∼ 13% fewer parameters, 32% fewer FLOPs while offering similar speeds but higher accuracy by 1 .
19% for RepVGG-A 1 .
Although we do not target mobile regime in this paper, we still show that having fewer parameters and FLOPs do not guarantee faster speeds.
EfﬁcientNet-B 0 has 50% fewer parameters and 77% fewer FLOPs, but is 50% more deeper, and runs 37% slower.
We believe that by exploring the design space of CoMNet for mobile regime, CoMNet can offer trade-offs to EfﬁcientNet, which we leave for future work.
R 2 .
ResNet- 50 is the most widely employed backbone for many areas such as (Ren et al., 2015;
He et al., 2017; Goyal et al., 2017) due to its affordability in terms of representation power, FLOPs, depth and accuracy.
CoMNet easily surpasses ResNet- 50 while being 50% shallower, 22% fewer parameters, 25% fewer FLOPs and 40% faster.
CoMNet also outperforms RepVGG-A 2 .
R 3 -R 4 .
The corresponding experiments shows that CoMNet is better than two larger variants of
ResNet which despite being older, still serves as backbones for many cutting-edge works (Carion et al., 2020; Li et al., 2022).
Our CoMNet outperforms these two baselines in every aspect while surprisingly being 72% and 82% less deeper relative to ResNet- 101 and ResNet- 152 respectively.
CoMNet also runs signiﬁcantly faster by almost 50% in roughly 50% fewer parameters and FLOPs.
This is quite promising in short training epochs while being shallower.
From the table, it can be also be seen that CoMNet despite being smaller then ResNeXt, outperforms in all the dimensions with marginal accuracy difference, which can be improved by matching the parameters of CoMNet with ResNeXt.
Overall CoMNet is 50% less deeper then ResNeXt- 50 while runs 50% faster at 6% fewer FLOPs, 2% fewer parameters and roughly same accuracy which is huge.
Moreover, CoMNet is also 75% less deeper , 11% less parameters, 12% fewer FLOPs and 35% faster at a marginal accuracy difference which can be matched by changing the CoMNet hyperparameters.
R 5 .
CoMNet is as accurate as RegNetX- 12 GF (Schneider et al., 2017) at 10% fewer FLOPs, 36% less depth, and runs 15% faster while offering competitive tradeoffs in the parameters.
For such a shallow model, achieving beyond 80% with merely 36 layers in multi-dimensional efﬁciency setting has not been seen previously.
Even RepVGG is dramatically less efﬁcient i.e. 53% fewer parameters, 62% fewer FLOPs while running signiﬁcantly faster.
This shows promising nature of CoMNet in large network regime.
However, our target in this paper is the real-time domain only.
R 6 .
This experiment shows the faster convergence of CoMNet in half of the original training epochs i.e. 60 .
It can be seen that CoMNet attains 99 .
17% ( 76 . 16% ) of its accuracy obtained atepochs ( 76 .
76% ), as compared to ResNet- 50 which achieves only 97% ( 74 . 15% vs 76 .
I m a g e
R e s N e t
[Figure]
C o M N e t - B
[Figure]
CoMNetResNetRepVGGResNeXtEfﬁcientNet
[Figure]
Structural Reparameterization.
RepVGG uses structural reparameterization ( SR ) during inference phase which converts it into a VGG-like (Simonyan & Zisserman, 2014) structure.
CoMNet is also compatible with SR and can achieve lower latency but marginally.
Interestingly, RepVGG offers lower architectural complexity only during inference phase, but training-phase complexity is still very high i.e. very larger parameters, three branches at each layer (Ding et al., 2021) which leads to increased training time (Table 4).
Whereas CoMNet has seamlessly lower complexity during both training and testing, because CoMNet have only one residual connection in CCM layers which can be eliminated via SR .
This leads to reduction in FLOPs of element-wise summation layers which is only marginal, however, memory access cost gets reduced due to the branch elimination (Ding et al., 2021).
It must be noticed that CoMNet are faster even without SR .
Multi-dimensional Efﬁciency.
Figure 4 shows different plots for accuracy vs an objective dimension.
It can noticed that CoMNet is always better regardless of the objective, except in depth relative to RepVGG, which is only a difference of 3 − 4 layers.
Despite that, CoMNet is accurate, has lower latency, lower FLOPs while has low architectural complexity, justifying the objective of this paper.
5. A DDITIONAL Q UALITATIVE E VALUATIONS
GradCAM:
Gradient based Class Activation Map.
To comprehend why CoMNet performs better in multi-dimension, we investigate the class activation maps on ImageNet (Deng et al., 2009) validation set.
We use GradCAM (Selvaraju et al., 2017) for this purpose, which computes class activation maps for a class label, indicating the attended regions by the network.
GradCAM visualizations of ResNet- 50 and CoMNet-B 1 (based on R 2 , Table 4 are shown in Figure 3).
It can be seen that CoMNet signiﬁcantly improves the attended regions of target class relative to the baseline.
It indicates high generalizability of CoMNet by emphasizing on class speciﬁc parts in the image.
BrainScore.
Although CoMNet follows the structure of biological visual cortex, we neither claim on CoMNet’s functional similarity with brain nor do we aim to build models that are more brain-like (Kubilius et al., 2019).
However, as the structure of IT cortex is our major inspiration, we ran
BrainScore test on CoMNet-B 1 out of curiosity.
The BrainScore (Schrimpf et al., 2020) provides a degree to which the intermediate layers of a network behave similar to the layers of a visual cortex.
For more details, please refer to (Schrimpf et al., 2020).
The BrainScore performance largely depends on the training method.
However, in our test, we found that CoMNet ranks 39 (in more thanentries) in the IT score without any specialized training.
The models ResNet- 101 and ResNet- 152 are ranked 119 and 143 respectively in the IT score.
Notice that our BrainScore analysis merely provides a new perspective on CoMNet, not to assert the functional similarity of CoMNet with visual cortex.
5. F UTURE D IRECTIONS
Achieving multi-dimensional efﬁciency is crucial for real-time applications and CoMNet is the ﬁrst effort in this direction by incorporating biological underpinnings.
Despite the signiﬁcant achievements,
CoMNet is open for improvement.
First , a comprehensive design space of CoMNet can be explored, similar to (Schneider et al., 2017).
Second , mobile regime of CoMNet is yet to be explored.
Third ,
BrainScore can serve as an additional efﬁciency dimension to improve CoMNet’s brain-like behavior.
C ONCLUSION
In this work, we propose a notion of multidimensional efﬁciency in CNNs, and a novel CNN architecture “CoMNet” to achieve that.
CoMNet inherits certain properties of a biological visual cortex such as cortical modules, columnar organization to achieve multi-dimensional efﬁciency in parameters, FLOPs, accuracy, latency, training duration while having simple architecture.
We provide a minimal design space of CoMNet instead of only a few models.
CoMNet outperforms many popular networks such as RepVGG, ResNet, RegNet, ResNeXt, while being shallower, signiﬁcantly faster, and also offering competitive trade-offs when multi-dimensional efﬁciency is not possible.