# KeOps Lazy tensors

Current state of the art. The level of performance provided by KeOps may surprise readers who grew accustomed to the limitations of tensor-centric frameworks. As discussed in previous sections, common knowledge in the machine learning community asserts that “kernel” computations can not scale to large point clouds with the CUDA backends of modern libraries: $$\mathrm{N}$$-by-$$\mathrm{N}$$ kernel matrices stop fitting contiguously on the Device memory as soon as $$\mathrm{N}$$ exceeds some threshold in the 10,000–50,000 range that depends on the GPU chip.

Focusing on the key operation involved, the kernel dot product or discrete convolution:

\begin{split}\begin{aligned} \begin{array}{ccccl} \text{KP} & : & \mathbb{R}^{\mathrm{M}\times \mathrm{D}}\times \mathbb{R}^{\mathrm{N}\times \mathrm{D}} \times \mathbb{R}^{\mathrm{N}\times\mathrm{E}} & \to & \mathbb{R}^{\mathrm{M}\times\mathrm{E}} \\ & & \big( (x_i), (y_j), (b_j)\big) & \mapsto & (a_i) ~~\text{with}~~ a_i = \sum_{j=1}^\mathrm{N} k(x_i, y_j)\,b_j~, \end{array} \end{aligned}\end{split}

most authors are tempted to introduce an $$\mathrm{M}$$-by-$$\mathrm{N}$$ kernel matrix $$K_{ij} = k(x_i,y_j)$$ and implement the operation above as a matrix dot product

\begin{aligned} \text{KP}\big((x_i), (y_j), (b_j)\big) ~~=~~ (K_{ij}) \cdot (b_j)~. \end{aligned}

To accelerate computations, a flourishing literature has then focused on the construction of low-rank approximations of the linear operator $$(K_{ij})$$. Common methods rely on random sampling schemes, multiscale decompositions of the input data or take advantage of specific properties of the kernel function $$k$$ – in our case, a convenient Gaussian blob.

## Our focus: exact Map-Reduce computations

As discussed in detail in our conclusion, such approximation strategies have a long history and a clear intrinsic value. Nevertheless, acknowledging the fact that progresses can also be made through low-level software engineering, we decide to tackle this problem in a completely different way. Brushing aside the elegant but inefficient matrix decomposition written above, the KeOps package directly optimizes the kernel sum by understanding it as a Map-Reduce composition of the operators:

\begin{split}\begin{aligned} &\begin{array}{ccccc} \quad\quad\text{Map} & : & \mathbb{R}^{\mathrm{D}}\times \mathbb{R}^{\mathrm{D}} \times \mathbb{R}^{\mathrm{E}} & \to & \mathbb{R}^{\mathrm{E}} \\ & & ( x, y, b ) & \mapsto & k(x, y)\,b \end{array} \\[.5cm] & \begin{array}{ccccc} \text{and}\quad\text{Reduce} & : & \mathbb{R}^{\mathrm{E}} \times \mathbb{R}^{\mathrm{E}} & \to & \mathbb{R}^{\mathrm{E}} \\ & & ( a, a' ) & \mapsto & a + a' \end{array} \label{eq:KP_map_reduce}\end{aligned}\end{split}

over the indexing indices $$i \in \left[\!\left[ 1,\mathrm{M}\right]\!\right]$$ and reduction indices $$j\in \left[\!\left[ 1,\mathrm{N}\right]\!\right]$$.

Parenthesis: are we re-packaging the wheel? This approach is common in the computer graphics literature but tends to be strictly limited to C++/CUDA programming guides: with its emphasis on real-time rendering and explicit models, the graphics community never felt the need to develop high-level libraries that would be suited to machine learning research.

In this context, our scientific contribution does not lie in any new theorem or algorithm. Described in the next few sections, the tools on which KeOps relies (the backpropagation algorithm, online Map-Reduce CUDA schemes and symbolic variadic templating) are all very well-known in their respective communities. But as we combine them in a versatile framework, endowed with a transparent interface and a comprehensive documentation, we allow them to reach a much wider audience and have, hopefully, a positive and fertilizing impact.

## A generic framework

The seminal Theano library combined the flexibility of high-level Python frameworks with a first-class support of convolutional architectures on the GPU. In the same vein, the KeOps package puts the spotlight on Map-Reduce schemes for (off-grid) sampled data, an algorithmic structure that we deem to be relevant in many fields that are related to data sciences and shape analysis.

Removing all the Python sugar coating, the workhorse of our library is a Generic Reduction (Genred) operator that supports a wide family of formulas. Let us assume that we have:

1. A collection $$p^1, p^2, \dots, p^P$$ of vectors.

2. A collection $$x^1_i, x^2_i, \dots, x^X_i$$ of vector sequences, indexed by an integer $$i$$ in $$\left[\!\left[ 1,\mathrm{M}\right]\!\right]$$.

3. A collection $$y^1_j, y^2_j, \dots, y^Y_j$$ of vector sequences, indexed by an integer $$j$$ in $$\left[\!\left[ 1,\mathrm{N}\right]\!\right]$$.

4. A vector-valued formula $$F(p^1, p^2, \dots, x^1_i, x^2_i, \dots, y^1_j, y^2_j, \dots)$$ on these input vectors.

5. A $$\operatorname{Reduction}$$ operation that may be a sum, an arg-min, a log-sum-exp, etc.

Then, referring to the $$p$$’s as parameters, the $$x_i$$’s as $$i$$-variables and the $$y_j$$’s as $$j$$-variables, a single KeOps Genred call allows users to compute efficiently the expression

\begin{aligned} a_i ~&=~ \operatorname{Reduction}_{j=1,\dots,\mathrm{N}}\limits \big[ F(p^1, p^2, \dots, x^1_i, x^2_i, \dots, y^1_j, y^2_j, \dots) \big] \qquad \text{for}~~ i=1,\dots,\mathrm{M}, \label{eq:keops_genred}\end{aligned}

alongside its derivatives with respect to all variables and parameters. As showcased in our gallery of tutorials, this level of generality allows KeOps to handle K-Nearest-Neighbors classification, K-Means clustering, Gaussian Mixture Model-fitting and many other tasks.

## The LazyTensor abstraction

Implementation details are covered in the next few sections but probably won’t interest most mathematicians. Wary of making users step outside of the convenient tensor-centric paradigm, we give a matrix-like interface to the generic reduction above. Through a new LazyTensor wrapper for NumPy arrays and PyTorch tensors, users may specify formulas $$F$$ without ever leaving the comfort of a NumPy-like interface.

KeOps LazyTensors embody the concept of “symbolic” tensors that are not sparse in the traditional sense, but can nevertheless be handled more efficiently than large $$\mathrm{M}$$-by-$$\mathrm{N}$$ arrays by using:

1. A symbolic mathematical formula $$F$$, the “.formula” attribute that is encoded as a well-formed string, manipulated with Python operations and parsed at reduction time.

2. A collection of “small” data arrays $$p$$, $$x$$ and $$y$$, the “.variables” list of parameters, $$i$$- and $$j$$-variables that are needed to evaluate the formula $$F$$.

Coming back to the example of the previous section, we may display the LazyTensor K_ij using:

>>> print(K_ij)
... KeOps LazyTensor
...     formula: Exp((Minus(Sum(Square((Var(0,3,0) - Var(1,3,1))))) / Var(2,1,2)))
...     shape: (1000, 1000)


Here, the Var(index, dimension, [ i | j | parameter ] ) placeholders refer to the data arrays q_i, q_j and 1/(2*s*s) that are stored in the list of K_ij.variables. As we call a supported reduction operator such as the matrix dot-product “@” on K_ij, this information is fed to the Genred engine and a result is returned as a genuine, differentiable PyTorch tensor: things just work smoothly, with full support of operator broadcasting and batch dimensions.