Consider a cost function which maps a parameter vector to a scalar which we would like to minimize. In machine learning the cost function is typically the average or the expectation of a loss functional:
(this is called the training loss) or
(this is called the generalization loss), where in supervised learning we have and is a prediction of , indexed by the parameters .
The gradient of the function of a single scalar is formally defined as follows:
Hence it is the variation induced by a change , when is very small.
When is a vector, the gradient is a vector with one element per , where we consider the other parameters fixed, we only make the change and we measure the resulting . When is small then becomes .
We want to find a that minimizes . If we are able to solve
then we can find the minima (and maxima and saddle points), but in general we are not able to find the solutions of this equation, so we use numerical optimization methods. Most of these are based on the idea of local descent: iteratively modify so as to decrease , until we cannot anymore, i.e., we have arrived at a local minimum (maybe global if we are lucky).
The simplest of all these gradient-based optimization techniques is gradient descent. There are many variants of gradient descent, so we define here ordinary gradient descent:
where represents our parameters at iteration and is a scalar that is called the learning rate, which can either be chosen fixed, adaptive or according to a fixed decreasing schedule.
We exploit the fact that is an average, generally over i.i.d. (independently and identically distributed) examples, to make updates to much more often, in the extreme (and most common) case after each example:
where is the next example from the training set, or the next example sampled from the training distribution, in the online setting (where we have not a fixed-size training set but instead access to a stream of examples from the data generating process). Stochastic Gradient Descent (SGD) is a more general principle in which the update direction is a random variable whose expectations is the true gradient of interest. The convergence conditions of SGD are similar to those for gradient descent, in spite of the added randomness.
SGD can be much faster than ordinary (also called batch) gradient descent, because it makes updates much more often. This is especially true for large datasets, or in the online setting. In fact, in machine learning tasks, one only uses ordinary gradient descent instead of SGD when the function to minimize cannot be decomposed as above (as a mean).
This is a minor variation on SGD in which we obtain the update direction by taking the average over a small batch (minibatch) of examples (e.g. 10, 20 or 100). The main advantage is that instead of doing Vector x Matrix products one can often do a Matrix x Matrix product where the first matrix has rows, and the latter can be implemented more efficiently (sometimes 2 to 10 times faster, depending on the sizes of the matrices).
Minibatch SGD has the advantage that it works with a slightly less noisy estimate of the gradient (more so as increases). However, as the minibatch size increases, the number of updates done per computation done decreases (eventually it becomes very inefficient, like batch gradient descent). There is an optimal trade-off (in terms of computational efficiency) that may vary depending on the data distribution and the particulars of the class of function considered, as well as how computations are implemented (e.g. parallelism can make a difference).
Another variation that is similar in spirit to minibatch SGD is the use of so-called momentum: the idea is to compute on-the-fly (online) a moving average of the past gradients, and use this moving average instead of the current example’s gradient, in the update equation. The moving average is typically an exponentially decaying moving average, i.e.,
where is a hyper-parameter that controls the how much weight is given in this average to older vs most recent gradients.
If the step size is too large – larger than twice the largest eigenvalue of the second derivative matrix (Hessian) of –, then gradient steps will go upward instead of downward. If the step size is too small, then convergence is slower.
The most common choices of learning rate schedule () are the following:
constant schedule, : this is the most common choice. It in theory gives an exponentially larger weight to recent examples, and is particularly appropriate in a non-stationary environment, where the distribution may change. It is very robust but error will stop improving after a while, where a smaller learning rate could yield a more precise solution (approaching the minimum a bit more).
This schedule is guaranteed to reach asymptotic convergence (as ) because it satisfies the following requirements:
and this is true for any but must be small enough (to avoid divergence, where the error rises instead of decreasing
A disadvantage is that an additional hyper-parameter is introduced. Another is that in spite of its guarantees, a poor choice of can yield very slow convergence.
Consider a function (in our case it is ) of several arguments, and we wish to compute it as well as its derivative (gradient) w.r.t. some of its arguments. We will decompose the computation of the function in terms of elementary computations for which partial derivatives are easy to compute, forming a flow graph (as already discussed there). A flow graph is an acyclic graph where each node represents the result of a computation that is performed using the values associated with connected nodes of the graph. It has input nodes (with no predecessors) and output nodes (with no successors).
Each node of the flow graph is associated with a symbolic expression that defines how its value is computed in terms of the values of its children (the nodes from which it takes its input). We will focus on flow graphs for the purpose of efficiently computing gradients, so we will keep track of gradients with respect to a special output node (denoted here to refer to a loss to be differentiated with respect to parameters, in our case). We will associate with each node
- the node value
- the symbolic expression that specifies how to compute the node value in terms of the value of its predecessors (children)
- the partial derivative of with respect to the node value
- the symbolic expressions that specify to how compute the partial derivative of each node value with respect to the values of its predecessors.
Let be the output scalar node of the flow graph, and consider an arbitrary node whose parents (those nodes taking the value computed at as input). In addition to the value (abuse of notation) associated with node , we will also associate with each node a partial derivative .
The chain rule for derivatives specifies how the partial derivative for a node can be obtained recursively from the partial derivatives for its parents :
Note that which starts the recursion at the root node of the graph (node that in general it is a graph, not a tree, because there may be multiple paths from a given node to the root – output – node). Note also that each is an expression (and a corresponding value, when the inputs are given) that is associated with an arc of the graph (and each arc is associated with one such partial derivative).
Note how the gradient computations involved in this recipe go exactly in the opposite direction compared to those required to compute . In fact we say that gradients are back-propagated, following the arcs backwards. The instantiation of this procedure for computing gradients in the case of feedforward multi-layer neural networks is called the **back-propagation algorithm**.
In the example already shown earlier, and there are two paths from to .
This recipe gives us the following nice guarantee. If the computation of is expressed with computations expressed through nodes (and each node computation requires a constant computation time) and arcs, then computing all the partial derivatives requires (at most) computations, using the above recursion (in general, with a bounded in-degree, this is also ). Furthermore, this is a lower bound, i.e., it is not possible to compute the gradients faster (up to an additive and multiplicative constant).
Note that there are many ways in which to compute these gradients, and whereas the above algorithm is the fastest one, it is easy to write down an apparently simple recursion that would instead be exponentially slower, e.g., in . In general can be written as a sum over all paths in the graph from to of the products of the partial derivatives along each path.
An illustration of this is with a graph with the following structure:
where there are such pairs of node, ending with and with input nodes and . The number of paths from to is . Note by mental construction how the number of paths doubles as we increase by 1.