Overview on Optimization algorithms in Deep Learning

Gradient Descent
Optimization Algorithms
Van Phu Quang Huy
Ta Duc Tung
Pham Quang Khang 1

Topics of today’s talk
● Function optimization
● Basics optimization algorithms and limitation
● Challenges in gradient descent optimization
● Practical gradient descent algorithms
2

Optimization in Machine learning
● Machine learning cares about performance measure P, that
is defined with respect to the test set and may also be
intractable
● Learning process: optimize P indirectly by optimizing a
cost function J(θ), in the hope that doing so will
improve P
● First problem of machine learning: optimization for cost
function J(θ)
3

What is function optimization
● Optimization = minimizing or maximizing
● Maximizing a function f may be accomplished via
minimizing -f
● f is called an objective function or criterion
● In the case of minimization, f is also called cost
function, loss function, or error function
5

Optimization Problem
● Example: find extrema of function f(x,y)=x2
+y2
6

+y2
● Easy?
7

+y2
● Easy?
● How about f(x,y)=-x2
-y2
8

+y2
● Easy?
-y2
● Still easy?
9

+y2
● Easy?
-y2
● Still easy?
● How about f(x,y)=x2
-y2
10

Maxima, minima, and saddle points
11
f(x,y)=x2
+y2
f(x,y)=-x2
-y2
f(x,y)=x2
-y2

Gradient and the Hessian matrix
● Gradient: vector of first-order
partial derivatives
12
● Hessian: matrix of second-order
partial derivatives

Second Partial Derivative Test
f is a multivariate function
● Stationary points: points that make ∇f = 0
● Second partial derivative test:
A stationary point is
○ Local minimum if all eigenvalues of the Hessian positive
○ Local maximum if all eigenvalues of the Hessian negative
○ Saddle point if the Hessian has both positive and negative
eigenvalues
○ Inconclusive if the Hessian is invertible
13

Since H has 2 eigenvalues 1,3 > 0 then (0,0) is a minimum 14
Example: f(x,y) = x2
-xy+y2

Basic Optimization Algorithms
15

Batch Gradient Descent
Blindfolded Hiker: how to get to the lowest place?
16
θ*
: Position
What we need
to find
J(θ): Height

Blindfolded Hiker: how to get to the lowest place?
17
Go step-by-step
● Which direction?
Left or Right?
● How far should I step?
Gradient
Learning
Rate

Solution: θ := θ - η∇θ
J(θ)
1. Initiate step size η
2. Start with a random point θ
3. Calculate gradient ∇θ
J(θ) at point θ
4. Follow the inversed direction of gradient → get new θ
5. Repeat until reach minima
a. Stop condition? → gradient is small enough
18

Pros
● Stable convergence
Cons
● Need to calculate gradient for whole dataset
● Slow if is not implemented wisely
19

Stochastic Gradient Descent
Principle: Same as Batch Gradient Descent
Difference:
● Updating θ at each example of training dataset
20
θ := θ - η∇θ
J(θ)
θ := θ - η∇θ
J(θ;x(i)
,y(i)
)

Stochastic Gradient Descent
Pros
● Faster than Batch Gradient
● Possible to learn online
Cons
● Unstable convergence
● Not use optimized vector operation
21
Image Credit: Pham Quang Khang
Fluctuation in Stochastic Gradient Descent

Optimization in Machine Learning
22

Example of a cost function
● Example: cross entropy in logistic regression
where xn
is vector input, yn
is label, w is weight matrix, N
is number of training examples, σ is sigmoid function
● The shape of the cost function is poorly understood
23

Convexity problem
● In traditional machine learning, objective functions are
designed carefully to be convex
○ Example: objective function of SVM is convex.
● When training neural networks, we must confront the
non-convex case
○ Many local minima → infeasible to find global minima
○ Dealing with saddle points
○ Flat regions exist
24

Local minima
● In practice, local minima is not a major problem
● [1] gives some theoretical insights about local minima:
○ For large-size networks, most local minima are equivalent and yield
similar performance on a test set
○ The probability of finding a “bad” (high value) local minimum is
non-zero for small-size networks and decreases quickly with network
size
○ Struggling to find the global minimum on the training set (as opposed
to one of the many good local ones) is not useful in practice and may
lead to overfitting
25[1] Choromanska et al. 2014.

Saddle points
● For high-dimensional non-convex functions, saddle points
are much more than local minima (and maxima) [1]
● Saddle points slow down training process
○ Batch Gradient Descent may be stuck at saddle points
○ Stochastic Gradient Descent seems to be able to escape saddle points
in many cases [2]
26
[1] Dauphin et al. 2014.
[2] Goodfellow et al. 2015.

Flat regions
● Flat regions: regions of constant value, where the
gradient and the Hessian are both 0
● Big problem when those regions have high value of the
objective function
● Escaping from those regions is extremely difficult
27y = x5

● At (0,0) the gradient and the
Hessian are both 0
→ f is super flat at (0,0)
● Second partial derivative test
can’t determine whether (0,0)
is a local minimum, a local
maximum or a saddle point
● In this case, (0,0) is a
global maximum
28
Flat regions: example of cost function f(x,y) = -x2
y2

Flat regions: example of cost function f(x,y) = xy(x+y)(1+y)
● At (0,0) the gradient and the
Hessian are both 0 too
● But in this case, (0,0) is a
saddle point
29

Practical Gradient Descent Algorithms
30

Gradient descent optimization problem
● Objective: to find the parameters θ that minimize the
lost function J(θ)
● Approach: iteratively update the params θ by utilizing
the gradient ∇J(θ)
● Gradient descent conventional method: θt
= θt-1
- η∇J(θ)
31

Momentum
● Add momentum to params updater:
vt
= αvt-1
- η∇J(θt-1
)
θt
= θt-1
+ vt
● Essential meaning of Momentum:
○ Accelerate the learning rate during the training process
○ In physic: add the momentum to the ball that rolling down the hill
○ The momentum parameter α is less than 1 as there is always resistance
force to slow the ball down and usually picked as 0.9
32

Adaptive learning rate
● Previous algorithm always use the fixed learning rate
throughout the learning process
○ The learning rate has to be either set to be very small at the
beginning or periodically decrease the learning rate
● Adaptive learning rate: learning rate is automatically
decreased in the learning process
● Adaptive learning rate algo: AdaGrad, RMSprop, Adam
33

Adagrad
● Essential meaning: the larger the params change the
slower it get updated
● Algorithms:
Accumulated sum-square ht
= ht-1
+ ∇J(θt-1
)・∇J(θt-1
)
Params updater θt
= θt-1
- η(1/sqrt(ht
))・∇J(θ)
● The learning rate is decreased as the number of update
step increases
34

RMSprop
● Adagrad decreases the learning rate too fast as it adds
all previous square gradients
● Consider partially previous added up sum square gradient:
Accumulated sum-square ht
= γht-1
+ (1-γ)∇J(θt-1
)・∇J(θt-1
)
Params updater θt
= θt-1
- η(1/sqrt(ht
))・∇J(θt-1
)
● Good practice γ = 0.9
35

Adam (Adaptive momentum estimation)
● Utilizing the advantage of both Momentum and Adagrad
● Algorithm
Momentum: vt
= β1
vt-1
+ (1 - β1
)∇J(θt-1
)
Learning rate: ht
= β2
ht-1
+ (1 - β2
)∇J(θt-1
)・∇J(θt-1
)
To avoid momentum and learning rate decay to be too
small, zero-bias counteract is calculated:
Vt
’ = vt
/(1 - β1
t
), ht
’ = ht
/(1 - β2
t
)
Param update:
θt
= θt-1
- ηVt
’/(sqrt(ht
’) + ε)
36

Compare all 4 algorithms +SGD on benchmark data
● Data: part of MNIST
● Input size: 28x28x1, output size: 10
● Model: NN with 4 hidden layers, 100 units each layer
● Train data number: 800, test data number: 200
● Batch size: 100
● Number of epoch: 1000
● Initial weight: Ɲ(0, 0.1)
37

Result 1: Change of cost function during process
● Learning rate: η = 0.001
● Momentum: α = 0.9
● RMSprop: γ = 0.9
● Adam: β1
= 0.9, β2
= 0.999
38

Result 2: Accuracy of training
Training set
39
Test set

Simple CNN test
● Conv - pool - fully connected - softmax
● Conv params:
○ Filter number: 30
○ Filter size: 5x5x1
○ Pad: 0
○ Stride: 1
● Input size: 28x28x1, output size: 10
● Train data: 800, test: 200
● Number epoch: 50
● FC layer size: 100
40

Simple CNN result1: cost function
● Learning rate: η = 0.001
● Momentum: α = 0.9
● RMSProp: γ = 0.9
● Adam: β1
= 0.9, β2
= 0.999
41

Simple CNN result 2: accuracy
Train
42
Test

Machine Learning Definition
"A computer program is said to learn from experience E with
respect to some class of tasks T and performance measure P
if its performance at tasks in T, as measured by P, improves
with experience E." [1]
[1] Mitchell, T. (1997). Machine Learning. McGraw Hill. p2.
44

Exploding Gradients
● Cliffs: where the
gradient is super big
● One update step of
gradient descent can
move the parameters
extremely far, usually
over the minimum point
● Solution: gradient
clipping
45
Goodfellow et al, Deep Learning book, p289

Vanishing Gradients
● Is a major problem when training deep networks
● The gradient tends to get smaller as we move backward
through the hidden layers when running backpropagation
● Weights in the earlier layers may not be learned
● Solutions:
○ Use good activation functions (e.g. ReLU) instead of sigmoid
○ Good initialization
○ Better network architectures (LSTMs/GRUs instead of basic RNNs)
46

Sharp and Wide Minima [1]
● Large-batch Gradient Descent tends to converge to sharp minima
→ poorer generalization
● Small-batch Gradient Descent consistently converges to wide minima
→ better generalization
47[1] Keskar et al. 2017.

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