#001B Deep Learning, wait but why now?

#001B Deep Learning, wait but why now?

Deep Learning, wait but why now?


If the basic technical idea behind deep learning neural networks has been around for decades, why are they only now taking off?

To answer this question, we plot a figure where on the x-axis we plot the amount of labeled data we have for a task, and on the y-axis we plot the performance of our learning algorithm (accuracy).

For example, we want to measure accuracy of our spam classifier or the accuracy of our neural net for figuring out the position of other cars for our self-driving car.

If we plot the performance of a traditional learning algorithms such as Support Vector Machine or Logistic Regression as a function of the amount of data we will get the following curve:

How to overcome performance plateau problem?

In this regime of smaller training sets, the relative ordering of the algorithms is not very well defined.

Often, skill at hand engineering features is what determines performance!

Over the last 20 years we accumulated a lot more data than traditional learning algorithms were able to effectively take advantage of. Enormous amount of human activity is now in the digital realm where we spend so much time on internet.

With neural networks it turns out that if you train a very large neural network then its performance often keeps getting better and better.

In the early days in the modern rise of deep learning it was a scale of data and a scale of computation constraints which limited our ability to train very large neural networks on either a CPU or a GPU. But relatively recently, there have been tremendous algorithmic innovations that make neural networks run much faster.

For example, switching from a sigmoid function to a ReLU function has been one of the huge breakthroughs in neural networks with large impact on their performance.

Let’s look in more detail why switching from sigmoid to ReLU function did so well on neural network performance.

One of the problems of using sigmoid functions in machine learning arises in these regions: >5 and <-5. Here, the slope of the function of a gradient is nearly zero. That is, learning becomes slow when gradient descent is implemented.

When the gradient is nearly zero, the parameters change very slowly, yielding to a very slow learning. If a gradient is zero parmeters won’t change.

For the Rectified Linear Unit function, the gradient is equal to 1 for all positive values of input. The gradient is much less likely to gradually shrink to 0, and the slope of the line on the left is 0. At the end, just by switching from the sigmoid function to the ReLU function has made an algorithm called gradient descent to work much faster.

To conclude, often you have an idea for a neural network architecture and you want to implement it in code. Fast computation is important because the process of training neural network is very iterative and can be time consuming. Implementing our idea then lets us run an experiment which tells us how well our neural network does. Then, by looking at it, you go back to change the details of our neural network and then you go around this circle over and over, until we get the desired performance. 

An entire deep learning research community has been incredible at  inventing new algorithms and making nonstop progress on that front. An ability of the very large neural networks, from computation point of view, will keep on getting better, as the society will continue throwing off more and more digital data.

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