Reducing Loss: Learning Rate

As noted, the gradient vector has both a direction and a magnitude. Gradient descent algorithms multiply the gradient by a scalar known as the learning rate (also sometimes called step size) to determine the next point. For example, if the gradient magnitude is 2.5 and the learning rate is 0.01, then the gradient descent algorithm will pick the next point 0.025 away from the previous point.

Hyperparameters are the knobs that programmers tweak in machine learning algorithms. Most machine learning programmers spend a fair amount of time tuning the learning rate. If you pick a learning rate that is too small, learning will take too long:

Same U-shaped curve. Lots of points are very close to each other and their trail is making extremely slow progress towards the bottom of the U.

Figure 6. Learning rate is too small.

Conversely, if you specify a learning rate that is too large, the next point will perpetually bounce haphazardly across the bottom of the well like a quantum mechanics experiment gone horribly wrong:

Same U-shaped curve. This one contains very few points. The trail of points jumps clean across the bottom of the U and then jumps back over again.

Figure 7. Learning rate is too large.

There's a Goldilocks learning rate for every regression problem. The Goldilocks value is related to how flat the loss function is. If you know the gradient of the loss function is small then you can safely try a larger learning rate, which compensates for the small gradient and results in a larger step size.

Same U-shaped curve. The trail of points gets to the minimum point in about eight steps.

Figure 8. Learning rate is just right.

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