Exploding gradient problem

See also: Machine learning terms

Exploding Gradient Problem

The exploding gradient problem is a phenomenon encountered in the training of certain types of artificial neural networks, particularly deep networks and recurrent neural networks (RNNs). This problem occurs when the gradients of the loss function with respect to the model's parameters grow exponentially during the backpropagation process, leading to unstable learning dynamics and suboptimal model performance. This article discusses the underlying causes of the exploding gradient problem, its consequences, and potential solutions.

Causes of Exploding Gradients

The main cause of the exploding gradient problem can be traced back to the process of backpropagation used in training artificial neural networks. In backpropagation, gradients of the loss function are computed with respect to each parameter in the network, starting from the output layer and moving backward through the network's layers. During this process, the gradients are multiplied by the weights of the connections between the layers. If these weights are consistently large, the gradients can grow exponentially, leading to the exploding gradient problem.

Deep networks and RNNs are particularly susceptible to this issue because of the increased number of layers and recurrent connections, which allow gradients to accumulate and grow rapidly. In RNNs, the problem can be exacerbated by long sequences of input data, as the gradients can propagate through many time steps, amplifying the effect.

Consequences of Exploding Gradients

The exploding gradient problem can have several detrimental consequences on the training process and the performance of the resulting model:

Instability in training: When gradients grow too large, the updates to the model's parameters during the optimization process can become erratic, causing the model to oscillate between good and bad solutions instead of converging to an optimal solution.
Slow convergence: The presence of large gradients can lead to slow convergence, as the optimizer is forced to use small learning rates to prevent overshooting the optimal solution.
Poor generalization: Large gradients can cause the model to overfit to the training data by focusing on a few dominant features, leading to poor generalization performance on unseen data.

Solutions to the Exploding Gradient Problem

Several techniques have been proposed to mitigate the exploding gradient problem in artificial neural networks:

Gradient clipping: This technique involves limiting the size of the gradients during backpropagation by setting an upper bound on their values. If a gradient exceeds the threshold, it is rescaled to fit within the limit, preventing it from growing too large.
Weight initialization: Properly initializing the weights of the network can help prevent large gradients from forming during training. Techniques like Glorot initialization or He initialization can be used to set the initial weights in a way that encourages stable gradients.
Batch normalization: Batch normalization is a technique that normalizes the input to each layer in the network, reducing the internal covariate shift and helping stabilize the training process. This can help mitigate the exploding gradient problem by ensuring that the inputs to each layer have roughly the same scale.

Explain Like I'm 5 (ELI5)

Imagine you're trying to learn how to stack blocks one on top of the other. Each time you stack a block, you try to learn how much force you need to use to make sure the block stays in place. If you use too much force, the blocks will topple over, and if you use too little, the block won't stay in place. You want to find the perfect balance.

Now imagine that you have a big tower of blocks, and you're trying to learn how much force to use at each level. If you're not careful, the force you use at the bottom can affect the top, and if you use too much force at any level, the whole tower can