RMSprop: In-depth Explanation

Neha Kumawat

8 months ago

In my previous article “Optimizers in Machine Learning and Deep Learning.” I gave a brief introduction about RMSprop optimizers. In this article, I will try to give an in-depth explanation of the optimizer’s algorithm.
If you didn’t read my previous articles. I recommend you to first go through my previous articles on optimizers mentioned below and then come back to this article for better understanding:
Optimizers in Machine Learning and Deep Learning
Gradient descent Algorithm: In-Depth explanation
Adagrad and Adadelta optimizers
Let’s start with a brief idea behind Rmsprop
RMSProp stands for Root Mean Square Prop, which is an adaptive learning rate optimization algorithm proposed by Geoff Hinton in lecture 6 of the online course “Neural Networks for Machine Learning”.
The core idea behind RMSprop is to keep the moving average of the squared gradients for each weight. And then divide the gradient by square root of the mean square. That’s why it’s called RMSprop (root mean square). 
RMSProp tries to resolve Adagrad’s radically diminishing learning rates by using a moving average of the squared gradient. It utilizes the magnitude of the recent gradient descents to normalize the gradient.
Adagrad will accumulate all previous gradient squares, and RMSprop just calculates the corresponding average value, so it can alleviate the problem that the learning rate of the Adagrad algorithm drops quickly.
The main difference is that RMSProp calculates the differential squared weighted average of the gradient. This method is beneficial to eliminate the direction of large swing amplitude and is used to correct the swing amplitude so that the swing amplitude in each dimension is smaller. On the other hand, it also makes the network function converge faster.
In RMSProp learning rate gets adjusted automatically and it chooses a different learning rate for each parameter.
RMSProp divides the learning rate by the average of the exponential decay of squared gradients.
Now, as you got a brief idea about the RMSprop optimizer. Let me now explain you the intuition behind RMSprop.
Let’s try to understand in a simple way. We can say that the RMSprop optimizer is similar to the gradient descent algorithm with momentum.
In the RMSprop optimizer, it tries to restrict the oscillations in the vertical direction, which in turn helps us to increase our learning rate and so that our algorithm could take larger steps in the horizontal direction and converge fast. The main difference between RMSprop and gradient descent is how we calculate the gradients for them. From the below-mentioned equations we can see how the gradients can be calculated for the RMSprop and gradient descent with momentum. Here, the value of momentum is denoted by beta and which is usually set to 0.9 most of the time.
Gradient descent with momentum update rule
Gradient descent with momentum update rule
RMSprop optimizer update rule
RMSprop optimizer update rule
So, from the above equation, we can see how both the equation is almost similar, only the difference between them is how we calculate gradient for both of them and how we update the weights and bias for them.
Momentum (blue) and RMSprop (green) convergence. Observe RMSprop is faster
Momentum (blue) and RMSprop (green) convergence. Observe RMSprop is faster

RMSProp implementation in Python

We can simply define a function for RMSProp as shown below:
def rmsprop():

w, b, eta = init_w, init_b, 0.1   
vw, vb, beta, eps = 0, 0, 0.9, 1e-9
for i in range(max_epochs):       
   dw, db = 0, 0
   for x,y in zip(X,Y):
            dw += grad_w(w, b, x, y)
            db += grad_b(w, b, x, y)       
vw = beta * vw + (1 - beta) * dw**2       
vb = beta * vb + (1 - beta) * db**2
w = w - (eta/np.sqrt(v_w + eps)) * dw
b = b - (eta/np.sqrt(v_b + eps)) * db
RMSProp (green) vs AdaGrad (white) showing with or without the sum of gradient shown by the squares
RMSProp (green) vs AdaGrad (white) showing with or without the sum of gradient shown by the squares
To see the effect of the decaying. Let’s compare them AdaGrad (white) keeps up with RMSProp (green) initially, as expected with the tuned learning rate and decay rate. But we can see from the above animation that the sums of gradient squared for AdaGrad get accumulate so fast that they soon become humongous (shown by the sizes of the squares in the animation). They harm and eventually AdaGrad practically stops moving.
But RMSProp, on the other hand, tries to keep the squares under a manageable size whole time with the help of the decay rate. This makes RMSProp faster than AdaGrad.
Thanks for reading…

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