The document discusses optimizing power efficiency for edge inferencing in convolutional neural networks (CNNs) through improved memory management techniques. It covers the challenges of balancing compute power and I/O power, introduces methods like tiling and forwarding to optimize memory usage and reduce energy consumption, and highlights a case study on various modern CNN architectures. The conclusion emphasizes the complexity of memory management and the need for compilers to effectively analyze and optimize performance indices.

1. © 2020 Samsung
Improving Power Efficiency for
Edge Inferencing Through
Memory Management
Optimizations
Nathan Levy
Samsung
September 2020

2. © 2020 Samsung
Outline
• Introduction
• Tiling and forwarding
• Filters management
• Case study of different modern convolutional neural networks
• Conclusion
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Counting the I/O in the TOps per Watt KPI
• Power consumption is composed of:
• Compute power: perform the arithmetic computations
• I/O power: bring the data to the compute engine and back
• For convolutional neural networks, I/O power can be higher than compute
• The compiler must optimize the memory management
• Scope: convolutional neural network (CNN) acceleration
• Focus on convolutions
• Assume the network is fixed (and quantized, pruned…)
• Inference only
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Description of the working model
What is a convolution?
• Input: feature map and filters
• Apply: multiply and accumulate
• Output: feature map
CNN can be processed by CPU, DSP, GPU, NPU/TPU
• These often have multiple cores
• These have multiple levels of memories (cache and
scratch-pad)
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Input feature map
Filter
zb
Output feature map
Output activation
Filter
za
Output activation
X
i
Yi
X
o
Yo

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Description of the working model
Simplified toy example:
• Single processing core
• Small local memory with “free”
access and big remote memory
with expensive access
Remote memory contains:
• Input and output feature maps
• Convolution filters
• Intermediate feature maps (if
needed)
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Remote memory Conv 1
Local memory
Processing
unit
Feature Map 1 Feature Map N
Conv M
Filters

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Typical orders of magnitude for NPU
• The memory management problem
depends on the relative size of:
• The local memory of the processor
• The typical feature map of the
network
• Our working point:
Feature map size ~
10 * Local memory size
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Local memory size [B]
Feature map size [B]
10k 100k 1M 10M
10k
100k
1M
10M
100M
IoT
Mobile
Automotive

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Tiling and forwarding

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Tiling and Forwarding
How to overcome the small memory size:
• Split the input feature map into smaller
parts → tiles
• Apply the convolution on the input feature
map tile
→ Creates an output feature map tile
The output feature tile can be either:
• Written in remote memory
• Used for the next convolution
→ This is called forwarding
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Remote memory Conv 1
Local memory
Processing
unit
IFM (not in mem) OFM (not in
mem)
Feature Map 1 Feature Map N
Conv M
Conv i
Input feature
map tile
Output feature
map tile
Filters

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How to tile and forward
How does the compiler tile and forward to minimize power
consumption?
Key optimization parameters:
• Power consumption: bandwidth (I/O) + compute
→ Focus of this talk
• Processing throughput (operations/second)
• Chip area → cost
• System complexity, flexibility and maintenance
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3x3
conv
3x3
conv
From
remote
memory
To
remote
memory
Tiling and increasing receptive fields
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
For i in [1-12]:
• Load tile i of blue feature map from remote memory
• Run 1st convolution → tile i of green feature map
• Run 2nd convolution → tile i of yellow feature map
• Store tile i of yellow feature map to remote memory

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Tiling and increasing receptive fields
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
For i in [1-12]:
• Load tile i of blue feature map from remote memory
• Run 1st convolution → tile i of green feature map
• Run 2nd convolution → tile i of yellow feature map
• Store tile i of yellow feature map to remote memory
3x3
conv
3x3
conv
From
remote
memory
To
remote
memory

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Tiling and increasing receptive fields
3x3
conv
3x3
conv
From
remote
memory
To
remote
memory
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
For i in [1-12]:
• Load tile i of blue feature map from remote memory
• Run 1st convolution → tile i of green feature map
• Run 2nd convolution → tile i of yellow feature map
• Store tile i of yellow feature map to remote memory

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3x3
conv
3x3
conv
From
remote
memory
To
remote
memory
Tiling and increasing receptive fields
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
For i in [1-12]:
• Load tile i of blue feature map from remote memory
• Run 1st convolution → tile i of green feature map
• Run 2nd convolution → tile i of yellow feature map
• Store tile i of yellow feature map to remote memory

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3x3
conv
3x3
conv
From
remote
memory
To
remote
memory
Tiling and increasing receptive fields
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
For i in [1-12]:
• Load tile i of blue feature map from remote memory
• Run 1st convolution → tile i of green feature map
• Run 2nd convolution → tile i of yellow feature map
• Store tile i of yellow feature map to remote memory

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3x3
conv
3x3
conv
From
remote
memory
To
remote
memory
Tiling and increasing receptive fields
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
For i in [1-12]:
• Load tile i of blue feature map from remote memory
• Run 1st convolution → tile i of green feature map
• Run 2nd convolution → tile i of yellow feature map
• Store tile i of yellow feature map to remote memory

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3x3
conv
3x3
conv
From
remote
memory
To
remote
memory
Tiling and increasing receptive fields
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
Problem: 3x3 convolution requires margins

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3x3
conv
3x3
conv
From
remote
memory
To
remote
memory
Tiling and increasing receptive fields
2 convolutions with 3x4 tiling. Without access to remote memory in the middle (forwarding)
Solution: Bigger blue and green tiles
Problems:
• Load more data for the first (blue) layer
• Compute more data for the intermediate (green) layer
2 pixels margin 1 pixel margin

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Tiling and increasing receptive fields
Consider a long chain of convolutions. How many stages of forwarding do you want?
• Forwarding tiles through as many layers as possible avoids dumping to remote
memory
• This causes the overlaps to grow
• More data to load for the first layer
• Redundant processing
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conv conv conv conv

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2 convolutions
# of Convolutions Feature Map
Tiling
Bandwidth Bandwidth per
convolution
Operations per
convolution
Energy per
convolution
2 3x4 +6% -47% +5% -30%
3 3x4 +12% -63% +10% -39%
4 3x4 +17% -71% +16% -43%
5 4x4 +29% -74% +26% -42%
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All numbers are compared to “no forwarding”
conv conv
To remote
memory
From remote
memory
Load the
overlaps
Compute the
overlaps
Less I/O
More compute
Break the feature map
into 12 to fit in memory

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3 convolutions
# of Convolutions Feature Map
Tiling
Bandwidth Bandwidth per
convolution
Operations per
convolution
Energy per
convolution
2 3x4 +6% -47% +5% -30%
3 3x4 +12% -63% +10% -39%
4 3x4 +17% -71% +16% -43%
5 4x4 +29% -74% +26% -42%
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All numbers are compared to “no forwarding”
conv conv conv
To remote
memory
From remote
memory

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4 convolutions
# of Convolutions Feature Map
Tiling
Bandwidth Bandwidth per
convolution
Operations per
convolution
Energy per
convolution
2 3x4 +6% -47% +5% -30%
3 3x4 +12% -63% +10% -39%
4 3x4 +17% -71% +16% -43%
5 4x4 +29% -74% +26% -42%
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All numbers are compared to “no forwarding”
conv conv conv conv
To remote
memory
From remote
memory

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5 convolutions
# of Convolutions Feature Map
Tiling
Bandwidth Bandwidth per
convolution
Operations per
convolution
Energy per
convolution
2 3x4 +6% -47% +5% -30%
3 3x4 +12% -63% +10% -39%
4 3x4 +17% -71% +16% -43%
5 4x4 +29% -74% +26% -42%
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All numbers are compared to “no forwarding”
conv conv conv conv conv
From remote
memory
To remote
memory
Overlaps are so big we
need smaller tiles to fit
in local memory

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Tiling and increasing receptive fields
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Minimal energy
Energy
[J]
• The trade-off between I/O and compute
power leads to a U-shaped behavior
• The trade-off requires an optimization
mechanism in the compiler
• In our toy example, all the convolutions
are identical
• In a real CNN, the convolutions are
different, so the optimization problem is
not one-dimensional
Number of convolutions

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Tiling and increasing receptive fields
More constraints to take into account:
• Some hardware will prefer vertical tiling for memory access
• Increases overlap sizes
• Some hardware will impose constraints on tile size (multiple of a given number)
Out-of-scope ways to deal with overlaps:
• Multi-core processing → hardware considerations and system complexity
• Keep overlap data instead of loading/computing again: takes up memory and requires memory
fragmentation management → system complexity
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Tile 4
Tile 3
Tile 2
Tile 1
Tile 3 Tile 4
Tile 1 Tile 2

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Filters management

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Filters management
Option 1: reload the filters from remote to
local memory for each tile
→ wasted bandwidth
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Option 2: keep the filters of all convolutions
in local memory while processing all the tiles
→wasted space in the local memory
→ may increase the tiles number
Local memory
Filters for 1
convolution
Reload #tiles
times
Local memory
Filters for
convolution 1
Load once
Filters for
convolution n

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Filters management
• No single strategy is the best all the time
• Choosing the right strategy can save up to
20% energy
• In practice, the problem is even more
complicated due to different convolutions
through the network
• The compiler should take the decision to
keep/reload the filters for each
convolution independently
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0 100 200 300 400 500
Memory size [kB]
Energy
[J]
Local Memory Size [kB]

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Case study of different modern
convolutional neural networks
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Skip connections – residual block
Skip connections were introduced in the ResNet paper as
a way to facilitate back propagation
• Forwarding becomes more challenging: one strives
to keep both main path and skip connection in local
memory
Intensive usage in DenseNet
• Up to 6 skip connections alive simultaneously
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He 2016

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Skip connections – segmentation networks
U-net adds skip connections to encoder-decoder structure to
improve quality of segmentation
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Ronneberger 2015.

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Skip connections – segmentation networks
It can be mixed with residual blocks
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Singla 2019

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Inverted bottleneck
• Inverted bottleneck introduced in MobileNet
family
• Splitting the 3x3 convolution into 3x3 depthwise
convolution + 1x1 convolution reduces the
amount of parameters
• Can more easily be kept in memory and/or
reloaded
• 1x1 convolutions don’t create overlaps
• Skip connection on thinner feature map to
minimize memory impact
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Sandler 2018

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Conclusion
• Memory management is a complicated optimization problem
• The compiler affects many performance indices, in particular power and runtime
• The tradeoff is not intuitive and requires some evaluation mechanism
How do we do it?
• Map all the performance indices (power, runtime…) to a single cost
• The mapping may depend on the use case
• The compiler is able to analyze its decisions and evaluate the cost
• The compiler optimizes memory management to minimize the cost
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Conclusion
What we didn’t cover:
• Pre-fetching for runtime reduction
• Multicore edge devices
• We described fixed hardware and fixed network. We actually co-design:
• Memory size, processing throughput, hardware limitations
• Network design to avoid access to remote memory
• Use of network architecture search
• Quantization and pruning to avoid access to remote memory
• Choice of bit-width and pruning rate per layer
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Resources
He, Kaiming, et al. "Deep residual learning for image recognition." Proceedings of the IEEE
conference on computer vision and pattern recognition. 2016.
Huang, Gao, et al. "Densely connected convolutional networks." Proceedings of the IEEE
conference on computer vision and pattern recognition. 2017.
Ronneberger, Olaf, Philipp Fischer, and Thomas Brox. "U-net: Convolutional networks for
biomedical image segmentation." International Conference on Medical image computing and
computer-assisted intervention. Springer, Cham, 2015.
https://github.com/Nishanksingla/UNet-with-ResBlock
Sandler, Mark, et al. "Mobilenetv2: Inverted residuals and linear bottlenecks." Proceedings of the
IEEE conference on computer vision and pattern recognition. 2018.
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